For two thousand years people believed that a moving object needs a push to keep moving. Everyday life seems to agree: stop pedaling and your bike slows down. Galileo and then Newton saw that the bike slows because of friction and air, not because motion runs out. With zero net force, an object keeps the velocity it has.
Key idea
Newton's first law: if the net force on an object is zero, its velocity does not change. At rest, it stays at rest. Moving, it keeps moving at the same speed in the same direction. This resistance to changes in motion is called inertia, and more mass means more inertia.
Why it works
A force is needed to change motion, not to keep it. Slide a puck on rough carpet and it stops almost at once; on smooth ice it glides much farther. Make the surface smoother and smoother, and the puck slows less and less. In the limit of no friction at all, nothing would slow it, so it would glide forever.
Worked example: A probe in deep space
A space probe far from any planet coasts at 17 km/s with its engines off. Ignoring the faint pull of the distant Sun, how fast is it going a year later?
- Far out in space there is almost no friction, air or nearby gravity, so the net force is about zero.
- Zero net force means the velocity does not change.
Answer: Still about 17 km/s, in the same direction
Worked example: A sudden stop
A bus brakes hard. Why do the standing passengers lurch forward?
- The brakes push the bus backward, but nothing pushes the passengers backward at first.
- By inertia, their bodies keep moving forward at the old speed while the bus slows under them.
Answer: Their bodies keep moving forward (inertia)
Common mistake
Believing a moving object needs a force to keep it moving and stops when that force "runs out." No force runs out. A rolling ball stops because friction and air push against it. Check: name the object that causes the slowing. There always is one.
Voyager 1 was launched in 1977. With its engines off, it is still coasting out of the solar system at about 17 km/s.
See it for yourself in the Science Lab: The First Law: Inertia.