NCERT Physics Class 11 Exemplar Ch 6 Work Energy and Power Part 4

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Q 19. A man, of mass m, standing at the bottom of the staircase, of height L climbs it and stands at its top.

(a) Work done by all forces on man is equal to the rise in potential energy mgL.

(b) Work done by all forces on man is zero.

(c) Work done by the gravitational force on man is mgL.

(d) The reaction force from a step does not do work because the point of application of the force does not move while the force exists.

Ans: (b), (d)

Q 20. A bullet of mass m fired at 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.

Ans: (b), (d), (f)

Q 21. Two blocks and having equal mass are free to move on a horizontal frictionless surface. is attached to a massless spring as shown in Fig. Initially is at rest and is moving toward with speed v and collides head-on with.

Two blocks m1 andm2 equal mass are free frictionless

Two Blocks m1 andm2 Equal Mass Are Free Frictionless

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(a) While spring is fully compressed all the KE of is stored as PE of spring.

(b) While spring is fully compressed the system momentum is not conserved, though final momentum is equal to initial momentum.

(c) If spring is massless, the final state of the is state of rest.

(d) If the surface on which blocks are moving has friction, then collision cannot be elastic.

Ans: (c)