Newton's first law of motion

Kim is skateboarding right into the curb! Let's pause the film! What will happen to Kim, when we restart the film? And why? To answer that, we need to know something about which physical laws govern the motion of stuff.

Let's start here, on flat ground. What happens when Kim speeds up the skateboard, like this? The board rolls straight ahead, but the speed decreases all the time, and eventually it stops. But what is it making the skateboard stop? The speed seems to...

just... run out? The skateboard actually continues to go, straight ahead, at constant speed, until a force affects it. And the force that affects the skateboard here, is friction, against the ground. Let's try this: we place Kim in space instead!

Here there's no friction, and no air resistance. Kim fires the rocket engine for a moment... And Kim continues to move! Straight ahead. At constant speed.

As long as no friction, and no other force, affects the jetpack, it'll continue with the same speed, in the same direction, forever. Or, until it runs into something. This isn't only true for jetpacks and skateboards, but for all things in the entire universe. For something to change speed or direction, it must be affected by a force. And that force can be friction.

Or a kick. Or a rocket engine. Or gravity. Nothing changes speed nor direction unless it's exposed to a force. Here Kim's riding straight ahead, at constant speed.

Friction slows the skateboard down. The friction arrow points backwards. But Kim is constantly kicking to get new speed -- adding more force -- directed forward. Kim's forward directed force is exactly as big as the backward force of friction. If we add these two forces together, they cancel each other out.

The result is... no force at all! When we add forces together like this, we call the result: resultant force. When the resultant force is zero, we say that the forces balance. And right here, the forces balance, the resultant force is zero... ...

so, here, the skateboard keeps both its speed and its direction. It takes a force... an unbalanced force, that is, a resultant force that is not zero, to change the speed or direction of an object. This, is Newton's first law of motion. We can state it like this: An object at rest will remain at rest... ...

and an object in motion stays in motion, with the same speed and in the same direction... ... unless acted upon by an unbalanced force. So, now that we know Newton's first law of motion, we can go back to Kim, by the curb! At first, both Kim and the skateboard moved. Then the skateboard is suddenly affected by a force, and it changes its speed -- it stops!

But, there's no force affecting Kim. So Kim keeps going... ... with the same speed as before. Now, it's mostly gravity affecting Kim... Kim is affected by an unbalanced force, in direction toward the ground...

and therefore changes direction... Ouch! And here's the friction force, slowing down the movement, 'til a complete stand still, or well, lying-down-still. Now there are no unbalanced forces. The resultant force is zero.

Kim's body is at rest, and stays at rest. So... For something to change its speed or direction, it must be exposed to a force. It's as if matter doesn't want to change its speed or direction. This unwillingness to change has a name: inertia.

Inertia is a property of matter. This is why Newton's first law of motion is also called: the law of inertia.