Question

A person gives a shopping cart an initial push along a horizontal floor to get it moving, and then lets go. The cart travels forward along the floor, gradually slowing as it moves. Consider the horizontal force(s) on the cart while it is moving forward and slowing. Which of the following statements is correct?
Question 36 options:

Both a forward and a backward force are acting on the cart, but the forward force is larger.

Only a forward force is acting which diminishes with time.

Both a forward and a backward force are acting on the cart, but the backward force is larger.

Only a backward force is acting, no forward force.

Answer 1

Answer:

Only a backward force is acting, no forward force.

Explanation:

• Once released from the initial push, in absence of friction, the shopping cart would continue moving forward at a constant speed forever.
• As it would move at a constant speed, no net force would be acting on it.
• So, if it is gradually slowing, there must  be a net force producing an acceleration in a direction opposite to the movement.
• This force is the kinetic friction force, and is the only force acting on the cart in the horizontal direction.
• As any friction force, opposes to the relative movement between the cart and the horizontal floor, which means that is directed backward.
• This is consistent with the direction of the acceleration of the cart.