Why things stop
Friction brings moving objects to rest.
The bicycle coasts
You stop pedalling a bicycle on a straight road. It rolls on for a while and then stops.
Why does it stop?
What this lesson covers
The idea
A moving object left to itself slows down and stops because friction acts against its motion; a force must be applied continuously only to counter the force of friction.
The bicycle coasts
You stop pedalling a bicycle on a straight road. It rolls on for a while and then stops.
Why does it stop?
- A force acts against its motion
- Nothing acts on it, it just gets tired
- The road pushes it forward
Push or no push
Play with the push off. Then switch the push on.
Friction acts against motion
When the pushing stopped, the block slowed down and stopped. Friction acts against the motion. To keep the block moving at the same speed, the push had to be there all the time, but only to counter friction.
A moving object left to itself slows down and stops because friction acts against its motion; a force must be applied continuously only to counter the force of friction.
Notes
A moving object left alone slows and stops because friction acts against its motion. We must apply a force continuously only to counter friction.
Check yourself
A ball rolls along the ground and stops. Why?
Why must we apply a force continuously to keep an object moving at the same speed on a rough floor?
In the picture the push equalled the force of friction and the speed stayed the same. What was the net force?
In which direction does the force of friction on a moving object act?
- The force of friction acts against its motion — correct. Yes!
- No force acts on it, so it stops. A force is acting: friction acts against the motion and brings the ball to rest.
- Friction pushes it forward. Friction acts against the direction of motion.
- To stop it from becoming lighter. The force is needed only to counter friction, which acts against the motion.
- To counter the force of friction — correct. Yes!
- To increase the force of friction. The force is applied to counter friction, not to increase it.
- Equal to the push. Friction acts opposite to the push and cancels it, so the net force is zero.
- Equal to the force of friction. The push cancels friction, so the net force is zero.
- Zero — correct. Yes!
- Opposite to the direction of motion — correct. Yes!
- In the direction of motion. If it did, a moving object would speed up. Friction acts against the motion.
- Upward. The upward force on an object resting on a surface is the normal force.