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Thursday 28 November 2013

Q.No.13.9: - What is Wheatstone bridge? How it be used to determine an unknown resistance?

Q.No.13.9: - What is Wheatstone bridge? How it be used to determine an unknown resistance?
Ans: - Wheatstone bridge is used to determine unknown resistance of a wire. The circuit is consists of four resistances R1, R2, R3, R4 connected in such a way as to form a loop or mesh ABCDA. A battery of emf E is connected between points A and C. A galvanometer of resistance Rg is connected between the points B and D.
          When the bridge is balanced, it satisfied the following relation
                                        R1/R2      =       R3/R4
Or,                                   R4           =        R2.R3/R1
If the value of R1, R2, and R3 are known then R4 can be calculated. The galvanometer should not show any deflection by adjusting the potential between terminals B and D. (Sorry for Diagram. Pleas concert with Text Book for diagram.)

Q.No.13.8: - Explain why the terminal potential of a battery decreases when the current drawn from it is increased?

Q.No.13.8: - Explain why the terminal potential of a battery decreases when the current drawn from it is increased?
Ans: - The terminal potential difference of a batter is
                                        IR      =        E – I.r
                                        Vt       =        E – I.r
Where,  E is the emf of the battery, r is the internal resistance of battery, and I.r is the potential difference across internal resistance.
When I increased then I.r becomes larger and terminal potential becomes small. Thus, we can say that when we draw more current from battery then its terminal potential difference will decrease.

Here is a rough example for this concept. The example of current flow is just like a water flow. Let suppose the pressure of water is just like the potential in the electrical system. The dam have a large capacity of water and suppose you are drawing more and more water from the dam and as a result water in the dam becomes low level and now it has low pressure on water to flow. Similarly when we draw more current from the battery then the terminal potential of the battery decreases just like dam pressure on water flow. I just tried to make this answer clear this is not a fit example for it but in my view you can understand something from it.

Q.No.13.7: - Describe a circuit which will give a continuously varying potential?

Q.No.13.7: - Describe a circuit which will give a continuously varying potential?
Ans: - A potential divider or potentiometer is a circuit which can give a continuously varying potential. Consider a resistance R in the form of a wire on which a terminal C can slide. The resistance between A and C can be varied from 0 to R, as C slides from A to B.
          If we connect a battery of emf E across a resistance R. The current flowing thought it is
                                                  I        =        E/R
If we represent the resistance between A and C by r, the potential drop between these points will be
                                                  V        =        rI
Now,
                                                  V        =        r x E/R

Thus as C slides from A to B, r varies from 0 to R, and the potential drop between A and C changes from zero to E. This arrangement by which potential can be varied continuously from 0 to E is known as a potential divider.   (Sorry for Diagram. Pleas concert with Text Book for diagram.)

Q.No.13.6: - Is the filament resistance lower or higher in a 500W, 220V light bulb than in a 100W, 220V bulb?

Q.No.13.6: - Is the filament resistance lower or higher in a 500W, 220V light bulb than in a 100W, 220V bulb?
Ans: -
Power of light bulb                    =        P1       =        500W
Potential difference        =        V1       =        220V
Now apply,
                                        P1       =        V12/R1
                                        R1       =        V12/P1
                                        R1       =        (220)2/500
                                        R1       =        48400/500
                                        R1       =        96.8 Ω
For second Bulb
Power           P2       =        100W
Voltage        V2       =        220V
Now, 
                    R2       =        V12/P2
                    R2       =        (220)2/100
                    R2       =        48400/100
                    R2       =        484 Ω

It means that the filament resistance of 100W, 220V light bulb is higher than that of other.

Q.No.13.5: - What are the difficulties in testing whether the filament of a lighted bulb obeys Ohm’s law?

Q.No.13.5: - What are the difficulties in testing whether the filament of a lighted bulb obeys Ohm’s law?

Ans: - According to Ohm when we increase voltage then current will also increases in direct proportion but resistance of the conductor must remain same and hence temperature also must remain same or constant. But when we test the filament of a lighted bulb that either it obeys Ohm’s law or not then with the passage of time its temperature increases and resistance also increases. These are the difficulties in testing whether filament of a lighted bulb obeys Ohm’s law or not.

Q.No.13.4: - Why does the resistance of a conductor rise with temperature?

Q.No.13.4: - Why does the resistance of a conductor rise with temperature?

Ans: - Electrical resistance is due to the collision of free electrons with atoms of the conductors. Atoms in the conductors are in vibratory motion when electrons collide with atoms they transfer some of their energy to the atoms and atoms starts vibrating with larger amplitude as the amplitude of vibration increases the probability of collision of electrons also increases. As a result increases resistance. When temperature increases then also amplitude of vibration increases and then probability of electrons collision also increases and then increases resistance.

Q.No.13.3: - What are the resistances of the resistors given in the figure A and B? What is the tolerance of each? Explain what is meant by tolerance?

Q.No.13.3: - What are the resistances of the resistors given in the figure A and B? What is the tolerance of each? Explain what is meant by tolerance?
(Sorry for Diagram but I’m trying to describe this figure here)
1st band is Brown 2nd is Green 3rd is Red 4th is Gold. In B fig. 1st band is Yellow 2nd is White 3rd is Orange and 4th is Silver.
Ans: - For Figure A
First band brown   =        1
            Second band Green =     5
             Third band Red (have number two so)= 00
             Now,       R   =   1500Ω
             Fourth band Gold which shows tolerance = ±5%
So,                       R   =   1500Ω±5%
For Figure B
                    First Band Yellow  =        4
                    Second band White=        9
                    Third band orange =        000 (no. of zeros)
                              R     =     49000 Ω
                    Fourth band silver =        ±10%
Now resistance is
                              R     =     49000±10% Ω
Tolerance: -

       Tolerance means the possible variation from the marked value.

Q.No.13.2: - Do bends in a wire affect its electrical resistance?

Q.No.13.2: - Do bends in a wire affect its electrical resistance?
Ans: - The resistance of wire is
                                       R   =  ρ.L/A

Where “ρ” is the resistivity of the conductor, L is the length and A is the area of cross section of the wire. So according to this equation we can say that bends in a wire will never affect the electrical resistance because we bending the wire its ρ, L and A remain same so its resistance remain same. 

Q.No.13.1: - A potential difference is applied across the ends of a copper wire. What is the effect on the drift velocity of free electrons by (i) Increasing the potential difference. (ii) Decreasing the length and the temperature of the wire.

Q.No.13.1: - A potential difference is applied across the ends of a copper wire. What is the effect on the drift velocity of free electrons by
(i)    Increasing the potential difference.
(ii)   Decreasing the length and the temperature of the wire.
Ans: - (i) It is obvious that drift velocity of free electrons in the copper wire will increase do to increasing potential difference.

(ii) If the drift velocity of free electrons is increasing then it means that resistance in the conductor is low at that time and when we decrease the length of the conductor then basically we are trying to decrease the number of atoms and decreasing resistance. As a result the drift velocity of free electrons increases. We know that resistance depends upon temperature change and if we decrease temperature of wire then it mean we are decreasing resistance and as a result increasing drift velocity. So, we can say that by increasing potential difference, decreasing length and temperature of conductor we are increasing drift velocity of electrons.

Friday 22 November 2013

Q.No.12.9: - Do electrons tend to go to region of high potential or of low potential.

Q.No.12.9: - Do electrons tend to go to region of high potential or of low potential.

Ans: - By convention we use positive potential as a high and negative potential as low potential. So according to this convention we can say that electrons which have negative charge tend to go to the region of high potential (positive) from low potential (negative).

Q.No.12.8: - Is it true that Gauss’s Law states that the total number of lines of forces crossing any closed surface in the outward direction is proportional to the net positive charge enclosed within surface.

Q.No.12.8: - Is it true that Gauss’s Law states that the total number of lines of forces crossing any closed surface in the outward direction is proportional to the net positive charge enclosed within surface.
Ans: - Yes this is true. According to Gauss’s law the total no of electric field lines coming out of a closed surface is proportional to the total no of charges present in that closed surface.

Mathematically,
The above statement is true.

Q.No.12.7: - Is E necessarily zero inside a charged rubber balloon if balloon is spherical? Assume that charge is distributed uniformly over the surface.

Q.No.12.7: - Is E necessarily zero inside a charged rubber balloon if balloon is spherical? Assume that charge is distributed uniformly over the surface.
Ans: - According to Gauss’s Law if the charge enclosed in the closed surface is zero then flux will be zero out of that surface. In this case the charge is distributed uniformly over the surface and no charge is present

Hence electric field intensity will be zero inside a spherical balloon which has uniformly distributed charge on its surface.

Monday 18 November 2013

Q.No.12.6: - If a point charge q of mass m is released in a non-uniform electric field, will it make a rectilinear motion?

Q.No.12.6: - If a point charge q of mass m is released in a non-uniform electric field, will it make a rectilinear motion?

Ans: - A rectilinear motion means a motion along a straight line. If a point charge q of mass m is placed at any point in the non-uniform electric field. Suppose this field is caused by a positive point charge then the charge will experience a repulsive force (as the force will exert on the charge along the line joining the two charges) and the charge will follow a straight path which makes a rectilinear motion.

Q.No.12.5: - Electric lines of forces never cross. Why?

Q.No.12.5: - Electric lines of forces never cross. Why?

Ans: - Electric lines of force never cross each other. This is because E has only one direction at any given point. If the lines cross, E could have more than one direction which is physically not correct.

Q.No.12.4: - Describe the force or forces on a positive point charge when placed between parallel plates. (a) with similar and equal charges. (b) with opposite and equal charges.

Q.No.12.4: - Describe the force or forces on a positive point charge when placed between parallel plates.
(a)    with similar and equal charges.
(b)    with opposite and equal charges.
Ans: - (a). If the plates have similar and equal charge in magnitude and we place the positive point charge between such plates then both the plates want to repel this positive point charge with same Coulomb’s force. The net force on the charge will be zero.
          (b). If the plates have similar and opposite charge in magnitude and we place the positive point charge between such plates then positive plate will repel the positive charge and exert a force F1 while negative plate will attract it and exert an attractive force F2.

          Net force        =           F        =      F1   +     F2  

Monday 14 October 2013

Q.No.12.3: - How can you identify that which plate of a capacitor is positively charged?


Q.No.12.3: - How can you identify that which plate of a capacitor is positively charged?
Ans: - For this purpose we can use Gold Leaf Electroscope (GLE). We bring the disc of a positively charged electroscope close to the plate of the capacitor. If the divergence of the gold leaf increases, then the plate is positively charged and if the divergence in the leaf decreases, then the plate is negatively charged.
                                                  Or
Rub together a plastic rod and a piece of fur, both initially uncharged, the rod acquires a negative charge (since it takes electrons from the fur) and the fur acquires a positively charge of the same magnitude ( since it has lost as many electrons as the rod has gained). Now bring the plastic rod to one of the capacitors plate is there is a repulsion between the plastic rod and the capaticor’s plate then it means that the plate is negatively charged and the other has positive charge and vice versa.

Q.No.12.2: - Suppose that you follow an electric field line due to a positive point charge. Do electric field and the potential increase or decrease?


Q.No.12.2: - Suppose that you follow an electric field line due to a positive point charge. Do electric field and the potential increase or decrease?
Ans: - If we follow an electric field line due to a positive point charge then we will move away from the positive point charge because the electric field produced by a positive charge is away from charge. Electric field and electric potential both inversely dependent on the distance from the charge to the selected point (say P). So as we follow the positive charge then we will move away from the charge. The formula of electric field  and electric potential is
                    E    =    qq′/4πЄ0r2      and       V     =     q/4πЄ0r    
So as we follow then electric field and electric potential will decrease.

Q.No.12.1: - The potential is constant throughout a given region of space. Is the electrical field zero or nonzero in this region? Explain.


Q.No.12.1: - The potential is constant throughout a given region of space. Is the electrical field zero or nonzero in this region? Explain.
Ans: - The electric field has a relation with change in electrical potential which is
                                                E   =   -ΔV/Δr
In this case ΔV is the change in electrical potential and if the change in potential is zero then E will be zero and ΔV will be zero only when there is no change in electrical potential. This is given in question that electrical potential is constant throughout the given region of space hence, E = 0 when ΔV=0
Mathematically,
                                  E   =   -ΔV/Δr   =    -0/Δr    =   0

Friday 12 April 2013

Test F.Sc Physics 1st year Chapter No.3 and 4. Long Questions


Q.No.3: - Attempt any two                               2    5    10
(i) Calculate the (a) time of flight (b) height of projectile (c) range of projectile
(ii) Derive the relation of absolute of P.E at the surface of Earth.
(iii) State and explain law of conservation of energy.
(iv) Find the final velocities of balls after an elastic collision.

Test F.Sc Physics Chapter no.3 and 4 short questions


Q.No.2: -Write the answer of the following questions.   2   x     10
(i) Prove that the units of impulse and momentum is same.
(ii) State isolated system and also describe that why is it important for law of conservation of momentum?
(iii) If two balls collide and after collision the magnitude of their velocities will remain same but their direction changes. Explain which type of collision both the balls will suffer.
(iv) Let two balls collide in such a way that their collision is perfectly elastic and both the balls are of same mass and the velocity of second ball is zero. Find the velocities of both the balls after collision?
(v) If you move with a bag in your hand and you move in the forward direction then what will be work done by your force and gravitational force? Explain it.

Test F.Sc Physics 1st year Chapter No.3 and 4. M.C.Qs



(i) The dimensions of impulse is that of
(a) Power            (b) velocity             (c) momentum            (d) acceleration
(ii) The time rate of change of momentum is called
(a) Velocity          (b) distance             (c) momentum           (d) force
(iii) The time rate of change speed is called
(a) Acceleration     (b) jerk                  (d) velocity                 (d) non of these
(iv) When we projected the ball with some velocity vi at an angle with the x-axis then the value of velocity at the highest point of its flight will be equal to its
(a) x-component    (b) y-component    (c) (d) non of these
(v) At which angle the range of projectile will be equal

(a) ϴ=450 , 300      (a) ϴ=300 , 600      (a) ϴ=600, 900            (d) non of these
(v) We can calculate the momentum by using



(vi) The work done will be zero if ϴ is
(a) 2700, 900            (b) 00, 1800            (b) 900, 00             (b) 00, 3600
(vii) The work done by gravitational force is
(a) Independent of path (d) dependent of path(c) both (a) and (b).
(d) Non of these
(viii) The dimension of power is

(ix) is the dimension of
(a) Power                 (b) velocity                (c) work                  (d) K.E
(x) The absolute P.E at infinite distance will be
(a) ∞                         (b) 0                            (c) maximum            (d) non of these

Test Chapter No.13 current electricity 2nd year F.Sc Long questions


                                                                Long Questions
Q.No.3: -  Explain the following question.  Attempt all  ( Mark 3+3=6 each question)
Q.No.1:- (i) Find the power dissipation in a resistor also give its units.
(ii) How many electron pass through an electric bulb in one minute if the 300mA current is passing through it?
Q.No.2: -(i) State and explain electromotive force and internal resistance of a source of emf. Also calculate the emf for a source of emf. Why potential difference is less than the emf. 
(ii) A charge of 90 C passes through a wire in 1 hour and 15 minutes. What is the current in the wire?

Short Questions for fsc 2nd year 13 chapter current electricity


Q.No.2: -Write the brief answers of the following questions.  Attempt Any 4  (mark 8)
(i) Why does the resistance of a conductor rise with temperature?
(ii) Do bends in a wire affects its electrical resistance?
(iii) Is the filament resistance lower or higher in a 500W, 220V light bulb in a 100W bulb?
(iv) How the heat produced when current flows in a conductor, on which factors the heat of the resistance depends?
(v) Define ohm law. Why the ohmic device doesn’t exist?
(vi) Draw the graph between current and voltage for semiconductor diode and also describe that why the graph turn towards current axis and not toward the voltage axis?
(vii) Define current also write its unit.

M.C.Q's Chapter No 13 Current Electricity 2nd year


Q.No.1: - Tick the correct answer.                        Mark 10
(i) Which type of energy is stored in a cell?
(a) Heat energy       (b) Potential energy     (c) Kinetic Energy        (d) Chemical energy
(ii) The pattern of magnetic field formed around a straight conductor is
(a) Circular              (b) elliptical                 (c) bar magnetic            (d) non of these
(iii) We can find resistance from the Ohm law by using the relation
(a) R=VI                  (b) R=V/I                     (c) V=IR                       (d) No of these
(iv) When the resistors are connected in series then the equivalent resistance will be equal to the
(a) sum of resistances   (b) sum of the reciprocal of all the resistances (c) average of the resistances (d) zero
(v) the units of conductivity is
(a) (ohm.m)-1           (b) mho.m-1                 (c) m.ohm-1                    (d) both (a) and (b)
(vi) The resistivity is directly proportional to the
(a) Square of the area of the conductor (b) inverse of the resistance of the conductor
(c) Inverse of length of conductor (d) current of the conductor
(vii) On the resistor the silver band shows the tolerance of
(a) 5%                    (b) 7%                          (c) 10%                          (d) 20%
(viii) The power dissipation in a resistor is equal to
(a) P=VI                (b) P=V2I                     (c) P=R2I                       (d) P=V2/I
(ix) The resistance of a resistor with color code first “red” second “violet” third “orange” forth “silver”
(a) 47000 Ω           (b) 4000 Ω                    (c) 2700 Ω                     (d) 3700 Ω
(x) The color code of “Yellow” on a resistor is
(a) 1                       (b)4                               (c)8                                  (d) 3 

Tuesday 2 April 2013

Q.No.3.9: - Define impulse and show that how it is related to linear momentum?

Q.No.3.9: - Define impulse and show that how it is related to linear momentum?
Ans: - Impulse: -  If a very large force exert for a very short interval of time then product of such force and time is called impulse.
     Mathematically

Relation between impulse and linear momentum: -  
            According to Newton’s second law of motion, the force is defined as the rate of change of momentum.
          Thus if force F acting on a body for time ∆t, changes its momentum from mvf to mvi. Then the force is written as;

Hence, impulse is equal to the change of linear momentum.

Q.No.3.8: - Find the change in momentum for an object subjected, to a given force for a given time and state lawof motion in terms of momentum.

Q.No.3.8: - Find the change in momentum for an object subjected, to a given force for a given time and state lawof motion in terms of momentum.
Ans: - Consider a body of mass “m” moving with an initial velocity vi. Let us suppose that an external force F acts upon it for time “t” after which velocity becomes vf. If this force produces an acceleration a, then it is expressed as,

This is Newton’s 2nd law in terms of momentum.

Q.No.3.13: - At what point or points in its path does a projectile have its minimum speed, its maximum speed?

Q.No.3.13: - At what point or points in its path does a projectile have its minimum speed, its maximum speed?
Ans: - (i) The speed of projectile is minimum at the maximum height because the vertical component is zero at the maximum height.
 (ii) The speed of a projectile is maximum at the point of projection and also just before it strikes the ground (level of projection) because the vertical component of velocity vy is maximum at these points.

Q.No.3.12:- Explain what is meant by projectile motion. Derive expressions for (a) the time of flight (b) the range of projectile. Show that the range of projectile is maximum when projectile is thrown at an angle of 450 with the horizontal.

Q.No.3.12:- Explain what is meant by projectile motion. Derive expressions for (a) the time of flight (b) the range of projectile. Show that the range of projectile is maximum when projectile is thrown at an angle of 450 with the horizontal.
Ans: - in the book

Q.No.3.11: - Explain the difference between elastic and inelastic collisions. Explain how would a bouncing ball behave in each case? Give plausible reasons for the fact that K.E is not conserved in most cases?

Q.No.3.11: - Explain the difference between elastic and inelastic collisions. Explain how would a bouncing ball behave in each case? Give plausible reasons for the fact that K.E is not conserved in most cases?
Ans: - Elastic Collision:-
                                Such a collision in which K.E and linear momentum remain same before and after collision is called elastic collision.
Inelastic Collision: -
                        Such a collision in which K.E and linear momentum does not remain same before and after collision is called inelastic collision.
         If the bounding ball collide with the floor in such a way that it rebounds to the initial height then its collision will be elastic because both K.E and linear momentum will remain same but if it does not rebound to the same height then this collision will be inelastic collision because much of the momentum will be change before and after collision.
               In most of the cases the K.E is not conserved because if we think for a moment then we arrive at a result that elastic collision is an ideal one. K.E does not conserved because of some frictional effects and much of the K.E is lost in the form of sound, heat and work done against the colliding object etc…

NoQ..3.10: - State the law of conservation of linear momentum, pointing out the importance of isolated system. Explain, why under certain conditions, the law is useful even though the system is not completely isolated?

NoQ..3.10: - State the law of conservation of linear momentum, pointing out the importance of isolated system. Explain, why under certain conditions, the law is useful even though the system is not completely isolated?
Ans: - Statement:-
                       It states that the total linear momentum of an isolated system always remains constant.
            Isolated system is important in many respects we study the properties of gases in an isolated system. Firing a bullet from a gun, the shooting of missile etc… The isolated system is an ideal system and such ideal systems do not exist. The momentum tells us about the motion of the bodies. So if we want to know the motion of bodies then we use this law and ignore the external force on the system.

Q.No.3.7: - Motion with constant velocity is a special case of motion with constant acceleration. Is this statement true? Discuss.

Q.No.3.7: - Motion with constant velocity is a special case of motion with constant acceleration. Is this statement true? Discuss.
Ans: -  Yes this statement is true because when the body moves with constant velocity then its acceleration will be zero and zero is itself a constant value so we can say that acceleration is also constant during constant velocity.

Q.No.3.6: - Explain the circumstances in which the velocity v and acceleration a of a car are (i) Parallel (ii) Anti-parallel (iii) Perpendicular (iv) v is zero but a is not zero. (v) a is zero but v is not zero.

Q.No.3.6: - Explain the circumstances in which the velocity v and acceleration a of a car are
(i) Parallel    (ii) Anti-parallel      (iii) Perpendicular     (iv) v is zero but a is not zero.    (v) a is zero but v is not zero.
Ans: - (i) When the car is moving with increasing velocity then the direction of acceleration will be parallel to the velocity.
(ii) When the car is moving with decreasing velocity then the direction of acceleration will be anti-parallel to the velocity. ( the direction of velocity depends upon the change in displacement so when the body move slowly then it will move in the forward direction but change in displacement will be in the forward direction. On the other hand the direction of acceleration depends upon the direction of change in velocity and in this case the velocity of the car is decreasing so the direction of acceleration will be opposite to the direction of velocity)
(iii) When the car will move in a circle then the direction of velocity and acceleration will be perpendicular to each other. ( in this case the direction of velocity will be along the tangent to the circle and the direction of acceleration depends upon the change in velocity so if we take the vector sum then the resultant will always point toward the center of the circle so we can say that velocity and acceleration will be perpendicular to each other in this case )
(iv) When the brakes are applied on the moving car, it slows down and comes to rest due to negative acceleration in the opposite direction. Thus, velocity v is zero but a is not zero.
(v) When the car is moving with constant velocity then there will be no change in velocity so acceleration will be zero but velocity will not be zero.

Q.No.3.5: - A man standing on the top of a tower throws a ball straight up with initial velocity Vi and at the same time throws a second ball straight downward with the same speed. Which ball will have larger speed when it strikes the ground ignore air friction?

Q.No.3.5: - A man standing on the top of a tower throws a ball straight up with initial velocity Vi and at the same time throws a second ball straight downward with the same speed. Which ball will have larger speed when it strikes the ground ignore air friction?
Ans: - Both balls will have same speed on striking the ground. When a body is projected vertically upward with certain initial velocity, then it will hit the ground with the same velocity. Thus, when the ball is thrown straight up with the initial velocity vi when it returns back. Hence, this ball will strike the ground with the same speed. When a second ball is thrown vertically downward with initial velocity vi it will also strike the ground with the same speed. It results that, in both the cases, the balls will hit the ground with the same speed. But the difference is that both the balls will hit the ground at different times due to different heights.

Q.No.3.4: - Specify the correct statement:- (a) An object can have a constant velocity even its speed is (a) An object can have a constant velocity even its speed is (a) An object can have a constant velocity even its speed is Changing (b) An object can have a constant speed even its velocity is Changing. (c) An object can have a zero velocity even its acceleration is not zero.(d) An object subjected to a constant acceleration can reverse its velocity.

Q.No.3.4: - Specify the correct statement:-
                 (a)      An object can have a constant velocity even its speed is  Changing.
(b)           An object can have a constant speed even its velocity is
Changing
(c)            An object can have a zero velocity even its acceleration is not zero.
(d)           An object subjected to a constant acceleration can reverse its velocity.
Ans: - The correct answer is (b).

Q.No.3.3: - Can the velocity of an object reverse direction when acceleration is constant? If so, give an example.

Q.No.3.3: - Can the velocity of an object reverse direction when acceleration is constant? If so, give an example.
Ans: - Yes, it is possible that the velocity of an object reverse direction when acceleration is constant. For example when we throw a body upward then the direction of acceleration will be downward while acceleration remains same.

Q.No.3.2: - An object is thrown vertically upward. Discuss the sign of acceleration due to gravity relative to velocity, while the object is in air.

Q.No.3.2: - An object is thrown vertically upward. Discuss the sign of acceleration due to gravity relative to velocity, while the object is in air.
Ans: - When the object is thrown vertically upward then the body will displace in the upward direction so the direction of velocity is in the upward direction but we noticed that after covering so vertical upward distance the body will slow which shows that the velocity decrease. At this time the direction of acceleration will be downward because the direction of change in velocity is downward.
          During the downward flight  we noticed that the body we move rapidly from the extreme position which means that the velocity will increase and at that time the direction of acceleration will be toward the downward direction. So we can say that in both the flights the sign of gravitational acceleration “g” is downward.

Q.No.3.1: - What is the difference between uniform and variable velocity. From the explanation of variable velocity, define acceleration. Give SI units of velocity and acceleration.

Q.No.3.1: - What is the difference between uniform and variable velocity. From the explanation of variable velocity, define acceleration. Give SI units of velocity and acceleration.
Ans: -
Uniform Velocity: -
                      If a body covers equal displacements in equal intervals of time, however small may be, the body is said to have uniform velocity. Its motion is said to be uniform. But be remember that the direction of body all also remain same. If one of them (magnitude or direction) changes then the body will not move with uniform velocity.
Variable Velocity: -
                       If a body covers unequal displacements in equal intervals of time it is said to be moving with variable velocity. If the body is moving in such a way that the magnitude of its velocity is same but its direction is changing then the body is said to be moving with variable velocity.
Acceleration: -  
                   Time rate of change of velocity of a body is called acceleration. This means that the velocity is changing, so from the definition of variable velocity we can say that variable velocity is also called acceleration.
SI Units: - The SI units of velocity are m/s.
                   The SI units of acceleration are ms-2.

Monday 14 January 2013

P.No.5.1:- A tiny laser beam is directed from the Earth to the Moon. If the beam is to have a diameter of 2.50m at the moon, how small must divergence angle be for the beam? The distance of Moon from the Earth is 380000000m

P.No.5.1:- A tiny laser beam is directed from the Earth to the Moon. If the beam is to have a diameter of 2.50m at the moon, how small must divergence angle be for the beam? The distance of Moon from the Earth is 380000000m 

 Solution:-
Given Data:-
          Length of arc  =  S  = 2.50m              (Diameter of beam)