List-I shows some situations with specified observers. Surface-1 refers to block and surface-2 refers to lower contacting surface. In all cases the velocity of the block is in the ground frame. List-I

Question

List-I shows some situations with specified observers. Surface-1 refers to block and surface-2 refers to lower contacting surface. In all cases the velocity of the block is in the ground frame.

List-I	List-II
(P) Block projected on rough surface, observer on ground.	(1) Work done by friction on surface-2 is negative.
(Q) Block projected on rough surface, observer moving with constant velocity v₀.	(2) Till block stops in given reference frame, friction on surface-1 is in +ve x direction.
(R) Block projected with velocity v₀ on a rough belt moving with constant velocity 2v₀; observer on belt.	(3) Till block stops in given reference frame, friction on surface-1 is in -ve x direction.
(S) Block on rough belt moving with initial velocity v₀ and constant acceleration a₀. Initially block is at rest; observer is on ground.	(4) Net work done by kinetic friction is $- \frac{1}{2} m v_{0}^{2}$
	(5) When block stops in given reference frame, friction is static in nature.

TestHub · Accepted Answer

P: stationary ground; friction on the block acts left.

$W_{f, 2} = 0, W_{f, net} = - \frac{1}{2} m v_{0}^{2}$

P goes to 3, 4.

Q: ground moves left; friction on it acts right. Block friction acts left.

$W_{f, 2} < 0, W_{f, net} = - \frac{1}{2} m v_{0}^{2}$

Q goes to 1, 3, 4.

R: the faster belt pulls the block right.

$W_{f, net} = - \frac{1}{2} m v_{0}^{2}$

R goes to 2, 4.

S: friction pulls the block right and opposes the moving belt. After slipping, static friction accelerates the block with the belt.

S goes to 1, 2, 5. Answer: A.

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