What you are looking at
A puck rests on a surface. Give it a push and watch what happens. On a rough surface friction is a net force
that drags it to a stop; on frictionless ice or in empty space there is
no net force, so the
puck glides on at constant speed indefinitely (it wraps around the screen).
The law of inertia
Newton's first law:
an object at rest stays at rest, and an object in motion stays in motion at
constant velocity, unless acted on by a net external force. Motion needs no cause to continue —
only a change in motion does. The push gives the puck velocity; once your hand leaves, the only horizontal
force is friction.
net F = 0 ⟹ velocity is constant
Friction is the hidden force
In everyday life things slow down and stop, which fooled thinkers for centuries into believing motion needs a
continuous push. It doesn't — friction does. Here the deceleration is a = μg, the friction force is μmg.
Notice that the
deceleration doesn't depend on mass: a heavier puck has more friction force
but also more inertia, and the two cancel. Set μ = 0 and the puck never stops.
Things to try
Push on carpet, then on ice — same push, wildly different distance. Switch to "Space" (μ = 0) and the puck
coasts forever: that is inertia with nothing to hide it. Change the mass and confirm the stopping distance is
the same for a given surface and push speed.