What Galileo found (and what we found) is that once an object is set into motion, no force is necessary to keep it moving; an object's inertia makes it tend to keep moving. In the real world, we don't really get to witness situations in which objects keep moving forever because virtually all objects moving on the earth are being acted upon by friction, and it is friction that often causes objects to slow down and eventually stop.
If we could remove friction and all other external forces acting on an object, it would keep moving forever once it is set into motion. Think about objects moving in outer space, for instance. This tendency to resist change in motion is called INERTIA.
Alternatively, if we set the skater down at rest on a horizontal surface, the skater would stay at rest if there were no external forces acting on the skater. This tendency to resist change in motion is also called INERTIA.
In general, if zero total outside force acts on an object, its inertia makes it tend to do one of the following two things:
If we could remove friction and all other external forces acting on an object, it would keep moving forever once it is set into motion. Think about objects moving in outer space, for instance. This tendency to resist change in motion is called INERTIA.
Alternatively, if we set the skater down at rest on a horizontal surface, the skater would stay at rest if there were no external forces acting on the skater. This tendency to resist change in motion is also called INERTIA.
In general, if zero total outside force acts on an object, its inertia makes it tend to do one of the following two things:
- if the object is at rest, its inertia makes it tend to stay at rest, or
- if the object is already moving, its inertia makes it tend to want to keep moving with a constant speed in a straight line.