LESSON 02 · MOTION
Why does a sliding object stop?
The short answer
A moving object keeps moving unless a force changes its motion. On Earth, friction between surfaces and air resistance push against motion, so sliding things slow down and stop.
Forces are pushes and pulls
A force is a push or a pull. Forces can start something moving, speed it up, slow it down, or change its direction. When you kick a ball, your foot gives it a push. When the ball rolls to a stop, other forces are at work.
Moving things tend to keep moving
An object that is moving tends to keep moving in a straight line at the same speed. This tendency is called inertia, and it is described by Newton's first law of motion. Far out in space, away from air and surfaces, a thrown object would keep drifting for a very long time.
Friction pushes back
Friction is a force between two surfaces that rub or slide against each other, and it acts against the motion. Rough surfaces, such as carpet or a towel, usually create more friction than smooth ones, such as a polished table or ice. Friction also turns some of the energy of motion into heat, which is why rubbing your hands together warms them.
Air resistance slows things too
Moving through air means pushing air out of the way. This air resistance, also called drag, grows as things move faster. A flat sheet of paper falls slowly because of air resistance, but the same sheet crumpled into a ball falls much faster.
Try it at home
Test three surfaces
You will need
- A small toy car or a ball
- A board or a large hardcover book to use as a ramp
- A few books to raise one end
- A tape measure or ruler
- A towel and a smooth floor or table
What to do
- Rest one end of the ramp on a stack of books.
- Let the car roll down the ramp onto the smooth floor. Do not push it; just let go.
- Measure how far it travels past the end of the ramp.
- Lay the towel at the end of the ramp and repeat. Keep the same ramp height and starting point.
- Try each surface three times and compare the average distances.
SafetyKeep the ramp low and clear the area so nobody trips.
What's happening?
The car starts each run with the same energy because the ramp height stays the same. The towel creates more friction than the smooth floor, so it slows the car sooner. Changing only one thing at a time is what makes this a fair test.
Key words
- Force
- A push or a pull that can change how something moves.
- Inertia
- The tendency of an object to keep doing what it is doing: staying still or moving.
- Friction
- A force between surfaces that acts against motion.
- Air resistance
- The push of air against a moving object, also called drag.
- Fair test
- An experiment in which only one thing is changed at a time.
Check your understanding
Why does a hockey puck slide farther on ice than on grass?
Ice is smooth and creates very little friction, so less force acts against the puck's motion.
What is inertia?
The tendency of an object to keep doing what it is doing, whether that is staying still or moving.
Why should the ramp height stay the same in the experiment?
So the car starts each run with the same energy. Then any difference comes from the surface, which keeps the test fair.