Some important questions

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1. An electron and a proton are moving under the influence of mutual forces. In calculating the change in the kinetic energy of the system 
during motion, one ignores the magnetic force of one on another. This is because, 

(a) the two magnetic forces are equal and opposite, so they produce
no net effect.

(b) the magnetic forces do no work on each particle.

(c) the magnetic forces do equal and opposite (but non-zero) work on each particle. 

(d) the magnetic forces are necessarily negligible.


» A proton is kept at rest. A positively charged particle is released from rest at a distance d in its field. Consider two experiments; one in which the charged particle is also a proton and in another, a positron. In the same time t, the work done on the two moving charged particles is

(a) same as the same force law is involved in the two experiments.
(b) less for the case of a positron, as the positron moves away
more rapidly and the force on it weakens.
(c) more for the case of a positron, as the positron moves away a
larger distance.
(d) same as the work done by charged particle on the stationary proton.

» A man squatting on the ground gets straight up and stand. The force of reaction of ground on the man during the process is 

(a) constant and equal to mg in magnitude.
(b) constant and greater than mg in magnitude.
(c) variable but always greater than mg.
(d) at first greater than mg, and later becomes equal to mg.

» A ball is travelling with uniform trans lat or y motion. This means that

(a) it is at rest.
(b) the path can be a straight line or circular and the ball travels
with uniform speed.
(c) all parts of the ball have the same velocity (magnitude and direction) and the velocity is constant.
(d) the centre of the ball moves with constant velocity and the ball spins about its centre uniformly.

» A body is falling freely under the action of gravity alone in vacuum. Which of the following quantities remain constant during the fall? 

(a) Kinetic energy.
(b) Potential energy.
(c) Total mechanical energy.
(d) Total linear momentum.

» A mass of 5 kg is moving along a circular path of radius 1 m. If the mass moves with 300 revolutions per minute, its kinetic energy  would be :

(a) 250π ²
(b) 100π ²
(c) 5π ²
(d) 0

» A bullet of mass m fired at 30° to the horizontal leaves the barrel of the gun with a velocity v. The bullet hits a soft target at a height h above the ground while it is moving downward and emerges out with half the kinetic energy it had before hitting the target. Which of the following statements are correct in respect of bullet 
after it emerges out of the target?

(a) The velocity of the bullet will be reduced to half its initial value. 
(b) The velocity of the bullet will be more than half of its earlier velocity. 
(c) The bullet will continue to move along the same parabolic path. 
(d) The bullet will move in a different parabolic path.
(e) The bullet will fall vertically downward after hitting the target.
(f) The internal energy of the particles of the target will increase.

» A rough inclined plane is placed on a cart moving with a constant velocity u on horizontal ground. A block of mass M rests on the incline. Is any work done by force of friction between the block and incline? Is there then a dissipation of energy?

» Conservation of momentum in a collision between particles can be understood from 

(a) conservation of energy.
(b) Newton’s first law only.
(c) Newton’s second law only.
(d) both Newton’s second and third law.

» A hockey player is moving northward and suddenly turns westward with the same speed to avoid an opponent. The force that acts on 
the player is

(a) frictional force along westward.
(b) muscle force along southward.
(c) frictional force along south-west.
(d) muscle force along south-west.

»  A girl riding a bicycle along a straight road with a speed of 5 m s–1 throws a stone of mass 0.5 kg which has a speed of 15 m s–1 with respect to the ground along her direction of motion. The mass of the girl and bicycle is 50 kg. Does the speed of the bicycle change after the stone is thrown? What is the change in speed, if so?



» A person driving a car suddenly applies the brakes on seeing a child on the road ahead. If he is not wearing seat belt, he falls forward and hits his head against the steering wheel. Why?

» A mass of 2kg is suspended with thread AB . Thread CD of the same type is attached to the other end of 2 kg mass. Lower thread is pulled gradually, harder and harder in the downward direction so as to apply force on AB. Which of the threads will break and why?

» Two masses of 5 kg and 3 kg are suspended
with help of massless inextensible strings. Calculate T1 and T2 when whole system is going upwards with acceleration = 2 m s2 
(use g = 9.8 m s–2 ). 
Some important questions Some important questions. 

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