Newton's first law

The idea

An object at rest remains at rest, and an object in motion continues to move with constant velocity, unless a net force acts upon it; so, without friction, force is needed only to start, change or stop motion.

An ice puck

A puck slides on very smooth ice. It travels a long way before it slows down.

If there were no friction at all, what would the puck do?

• Keep moving with the same velocity
• Stop after a short distance
• Turn round and come back

Take friction away

Try the block at rest, then moving without friction, then with friction.

Force changes motion

Without a net force, the block at rest stayed at rest and the moving block kept moving at constant velocity. Friction is a force, so with friction the moving block stopped. A force is needed only to start, change or stop the motion.

An object at rest remains at rest, and an object in motion continues to move with constant velocity, unless a net force acts upon it; so, without friction, force is needed only to start, change or stop motion.

Notes

No net force: an object at rest stays at rest, and one in motion keeps constant velocity. Without friction, force is needed only to start, change or stop motion.

Check yourself

The net force on an object is zero. What does it do?

Without friction, when is a force needed?

What does constant velocity mean?

A person pushes a moving box with a force equal to the force of friction on it. What does the box do?

• It must be at rest. A moving object with zero net force keeps moving with constant velocity.
• It slows down gradually. Slowing down needs a net force, such as the force of friction.
• It stays at rest, or keeps moving with constant velocity — correct. Yes!
• To start, change or stop the motion — correct. Yes!
• To keep a moving object moving at constant velocity. Once the object is moving, no further force is required to keep it moving with constant velocity.
• Never, even to stop a moving object. To change the velocity, or to stop a moving object, a force has to be applied.
• Only the speed stays the same. Constant velocity also means the direction does not change.
• No change in the magnitude or the direction of the velocity — correct. Yes!
• The object is at rest. At rest means zero velocity. A constant velocity can also be non-zero.
• It comes to rest after some time. The two forces are equal and opposite, so the net force is zero and the box keeps its velocity.
• It speeds up. There is no net force, so its velocity does not change.
• It continues to move with constant velocity — correct. Yes!