A net force that causes motion is called...

• A. Friction
• B. Balanced Forces
• C. Unbalanced Forces
• D. Normal Force

Answer: (C)

When several forces act on an object, you combine them to get the net force. A nonzero net force changes the object's motion; it causes acceleration according to F_net = m a. The net force that causes motion is called an unbalanced force. For example, pushing a cart with more force than the opposing friction produces a net forward force and the cart speeds up; if the forward push exactly balances friction, the net force is zero and the cart moves at a constant velocity or stays at rest. Friction is one of the forces that can oppose motion, not the net force by itself. Balanced forces produce no change in motion, and the normal force is a contact force that supports weight, not the force driving motion.