Why a box does not budge
The applied force must beat friction.
A heavy box
You push a heavy box on the floor. At first it does not move. You push harder and it begins to move.
Why did it not move at first?
What this lesson covers
The idea
Friction between surfaces acts opposite to the applied force, so an object at rest starts moving only when the applied force exceeds the force of friction, giving a net force in the direction of motion.
A heavy box
You push a heavy box on the floor. At first it does not move. You push harder and it begins to move.
Why did it not move at first?
- Friction opposed your push
- There was no force on the box
- Friction pushed along with you
Push until it moves
Push gently, then harder. The largest friction here is 8 N.
Beating friction
Friction between the box and the floor pushes against your push. For a small push, the two forces balanced and the box stayed at rest. Only when your push became larger than friction was there a net force in the direction of motion.
Friction between surfaces acts opposite to the applied force, so an object at rest starts moving only when the applied force exceeds the force of friction, giving a net force in the direction of motion.
Notes
Friction acts opposite to the applied force. An object at rest moves only when the applied force exceeds friction.
Check yourself
Why did the box not move when you pushed it gently?
When does an object at rest start to move?
The largest friction on a block is 8 N. You push it with 12 N. What is the net force?
Answer: 4 N
12 N − 8 N = 4 N, in the direction of the push.
A box is pushed to the right. In which direction does the friction on it act?
- Friction acted opposite to the push, and the push was not larger than friction — correct. Yes!
- No force was acting on the box. Your push acted on the box, and friction acted against it.
- Friction acted in the same direction as the push. Friction acts opposite to the applied force.
- When the applied force equals the force of friction. Equal forces in opposite directions are balanced, so the object stays at rest.
- When the applied force is larger than the force of friction — correct. Yes!
- When the force of friction becomes zero. Friction is still there. The object moves because the applied force exceeds it.
- To the right. Friction acts opposite to the applied force.
- Downwards. Friction acts opposite to the applied force, along the surface. The weight is the force that acts downwards.
- To the left — correct. Yes!