Answer:
the unbalanced force
Explanation:
An unbalance forces are defined as the forces which causes a change in the speed or velocity of a moving object. A force causes a change in the motion of a body and an unbalanced force causes the body to change its velocity and speed or a change in its direction.
An unbalanced force will start movement in a static object or it can cause to speed up the moving object or it can cause to stop the moving object.
Thus an unbalanced force causes the cart to speed up.
