No force, no change
You stop pedalling and the cycle slows down. Does a moving thing need a push to keep going?
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- A cart moves at 6 m/s. No force acts on it.
- Friction now pushes back on it: 2 N.
- The speed drops to 3 m/s.
- Friction is a force. It slows the cart.
With no friction, a moving thing needs no push to keep going. The cycle slows because friction acts on the cycle.
The rule
Constant velocity means the speed and direction do not change.
An object at rest stays at rest unless a net force acts. A moving object keeps a constant velocity unless a net force acts.
The net force is all the forces on an object added up. Forces in the same direction add. Forces in opposite directions subtract.
Equal and opposite forces on one object are balanced. The net force is then zero.
Friction is a force. It acts against the motion. Less friction means a longer slide. With no friction and no other force, a moving thing keeps moving.
This is Newton's first law.
| Net force | What the object does |
|---|---|
| Zero, object at rest | Stays at rest |
| Zero, object moving | Keeps the same velocity |
| Not zero | Its velocity changes |