A man pushes a shopping cart at a constant speed. What happens if the man increases the force on the cart?

The cart would speed up.

Explanation:

According to Newton's 1st law, object subjected to no force, or net force 0, would have a constant speed. In our case the cart is initially at constant speed, meaning the man exerts a force that is equal to friction force. If he increases the force on the cart, the net force would no longer be 0. The cart would gain an acceleration and increases its speed.