Unit 2, lesson 5. About 15 minutes.

What is a force?

What can start a ball rolling, stop it, and even change its direction?

Play

Getting the experiment ready…

Explain

A push moves things away, and a pull brings them close.

These are called forces.

A force is a push or a pull. You can't see a force, but you can see what it does.

Forces can make things start moving, stop, speed up, slow down, change direction or change shape.

We measure forces in newtons. One newton is about the weight of a small apple in your hand.

When two forces push against each other, the bigger one wins. What is left over is called the net force.

A force is an interaction that can change an object's motion or shape. It has a size and a direction, which makes it a vector. The SI unit is the newton (N).

Only the net force changes an object's motion. In the simulation, two forces act along one line, so we subtract them. If they are equal, the forces are balanced and the net force is zero.

Worked example: 60 N acts right and 20 N acts left. The net force is 60 − 20 = 40 N to the right.

Want the full story?

The simulation uses smooth wheels and ignores friction, so a balanced box that is already moving keeps moving. In real life, friction would also act. Forces in different directions add as vectors, so you would need geometry as well as subtraction.

The Formula

net force = force one way − force the other way

F_{\text{net}} = F_1 - F_2

$F_{\text{net}}$: the total force left over, measured in newtons (N)
$F_1$: the force pushing right, measured in newtons (N)
$F_2$: the force pushing left, measured in newtons (N)

Try the numbers

Force right: 40 NForce left: 0 N

Net force (positive means right): 40 N

Try it at home

You need a toy car or an empty box, a table or smooth floor.

You need: a toy car or an empty box, a table or smooth floor.

Push the car with one finger. Now pull it back.

Give the toy a gentle push and watch it go.
Push it from both sides at once. Try to keep it still.
Now push harder on one side.

What's happening: when your pushes match, the forces balance and the toy stays put. When one is bigger, there is a net force and the toy moves that way.

Physics is life

Your body

Your muscles pull on your bones to move you.

Your muscles can only pull. To throw a ball, they pull on your bones to make your arm push.

Daily life

You pull a door to open it.

Opening a door, kicking a ball and squeezing toothpaste are all forces.

Nature

The wind pushes the trees.

Wind pushes on trees, and flowing water pushes on rocks.

Space

Earth pulls you to the ground.

Gravity is a pull. It keeps the Moon near Earth and keeps you on the ground.

Quick check

Question 1 of 3

Which unit do we use to measure force?

Words to know

Force: A push or a pull on something.

Force: A push or a pull on something.
Newton (N): The unit of force. One newton is about the weight of a small apple.
Net force: The total force left over after adding up all the pushes and pulls.
Balanced forces: Forces that cancel out, so the net force is zero.

Next up

Newton's First Law: things keep doing what they're doing