4. An open cart on a level surface is rolling without frictional loss through a

Question

4. An open cart on a level surface is rolling without frictional loss through a vertical downpour of rain, as shown above. As the cart rolls, an appreciable amount of rainwater accumulates in the cart. The speed of the cart will
(A) increase because of conservation of momentum (B) increase because of conservation of mechanical energy
(C) decrease because of conservation of momentum (D) decrease because of conservation of mechanical energy
(E) remain the same because the raindrops are falling perpendicular to the direction of the cart's motion


9. A child pushes horizontally on a box of mass m which moves with constant speed v across a hori¬zontal floor. The coefficient of friction between the box and the floor is µ. At what rate does the child do work on the box?
(A) µmgv (B) mgv (C) µmg/v (D) µmg/v (E) µmv2



13. Which of the following is true about the net force on an uncharged conducting sphere in a uniform electric field?
(A) It is zero. (B) It is in the direction of the field.
(C) It is in the direction opposite to the field.
(D) It produces a torque on the sphere about the direction of the field.
(E) It causes the sphere to oscillate about an equilibrium position.

Explanation / Answer

(C) decrease because of conservation of momentum cause there is no friction force, so the energy is the same. the vertical rain have no horizontal velocity, so its horizontal momentum is zero. Thats mean the total velocity of the cart + water after is smaller than that of cart initially. -------------- A) friction force is mg*u. the rate equal to force * velocity = mg*u*v ------------- A) its zero. since the total charge cant be change, so if initially the sphere is uncharged, then the net charge after that is also zero

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