Question

A cart is moving along a long, frictionless horizontal air track. Two students are pushing on it:

Bob is doing positive work, and Alice is doing negative work If the cart is moving to the left and slowing down, which statement below is true?

A) Bob pushes to the right; the net work done on the cart is positive,

B) Bob pushes to the left; the net work done on the cart is positive

C) Bob pushes to the right: the net work done on the cart is negative

D) Bob pushes to the left; the net work done on the cart is negative

E)Bob pushes to the right; the net work done on the cart is zero.

Answer 1

The cart moves to left, the direction of velocity is to the left. It is given that the Bob is doing a positive work. So, the force applied by Bob is in the same direction as the displacement of the cart. The displacement of the cart is to the left, therefore the force applied by Bob will also be to the left.

By the work energy theorem we know that the net work done is equal to the change in kinetic energy of the system.

Since the object slows down the kinetic energy reduces, the final kinetic energy is less than the initial kinetic energy of the system. Therefore the net work done on the cart is negative.

Option (D) is correct.