Gravity exerts a balanced force on the object. An unbalanced force causes acceleration. Friction is applied to the object. The forces acting on the object are in equilibrium.
I gave my students this multiple choice question: A 20 N object is placed on a surface and starts to slide. What is the most likely reason the object begins to move?

Only one answer is correct among these 4 choices:
- Gravity exerts a balanced force on the object.
- An unbalanced force causes acceleration.
- Friction is applied to the object.
- The forces acting on the object are in equilibrium.

Could you tell me which one is correct?

An unbalanced force causes acceleration.