Nothing changes for free
Why Do Things Keep Moving?
An object keeps doing exactly what it was doing until something makes it stop.
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Step 1 of 5
Everyone's instinct is wrong here.
Nothing in the list is the motion running out For 2,000 years people thought moving things naturally slow down. Actually they only slow because something — air, friction, a wall — is stopping them.
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Step 2 of 5
Remove the opposition and motion never ends.
A puck on smooth ice slides a long way. A probe in deep space, with nothing to rub on, keeps its speed for billions of years.
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Step 3 of 5
More mass means more stubbornness.
The same push, and far less happens Inertia grows with mass. Nudging an empty trolley is easy; nudging a loaded one takes far more effort to get moving and to stop.
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Step 4 of 5
You feel it in every vehicle.
When a bus brakes you lurch forward — you were moving and you kept moving. When it accelerates you press back into the seat.
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Step 5 of 5
Turning counts as changing too.
Inertia resists any change, including direction. In a fast corner you slide sideways because your body wants to keep going in a straight line.
This is exactly what a seatbelt does. In a crash the car stops; without a belt, you do not.
The short version
Objects keep doing what they are doing — inertia is why change of speed or direction always needs a force.
Try it yourself
Put a card on a glass with a coin on top. Flick the card sideways. The coin barely moves and drops straight in.