"Newton's Third Law: Action-Reaction Force Pairs"

Forces come in pairs

A force is a push or a pull. Some forces act by contact, like friction, tension, and the normal force (the push of a surface, such as a desk, on an object resting on it). Others act at a distance, like gravity and magnetism. Here is the surprising part: a force never exists alone. A force always comes from an interaction between two objects.

When you push on a wall, the wall pushes back on your hand. You can feel it: press hard on a wall and your hand gets squished a little.

The two forces are called an action-reaction force pair. Which one is the "action" doesn't matter. They happen at the same time.

To name a pair, use this pattern: "A pushes on B" and "B pushes on A."

• Action: Emma's hand pushes on the wall.

• Reaction: the wall pushes on Emma's hand.

Different objects, different effects

Here is the key idea: the two forces of a pair act on different objects. The kick acts on the ball. The reaction acts on the foot.

That is why they never cancel each other. To cancel, forces must act on the same object.

Equal forces do not mean equal motion. Jayden stands on a skateboard and pushes a wall. The wall pushes back on Jayden with the same force. Jayden has little mass and wheels under him, so he rolls away. The wall is attached to the whole building and the ground, so it does not seem to move at all.

Balanced and unbalanced forces

Now look at forces that act on the same object. Add them to find the net force ($F_{net}$).

• Forces in the same direction: add.

• Forces in opposite directions: subtract.

If the net force is $0\ \text{N}$, the forces are balanced and the object's motion does not change. If the net force is not zero, the forces are unbalanced and the motion changes. The net force points in the direction of the larger force.

Pair or balanced? Telling them apart

Both ideas use "equal and opposite" forces, so they are easy to mix up. Use one test: do the two forces act on the same object or on different objects?

A book rests on a desk. Gravity pulls down on the book with $10\ \text{N}$. The desk's normal force pushes up on the book with $10\ \text{N}$. These two are equal and opposite, but both act on the book. They are balanced forces, not a pair.

The true pairs are:

• Earth pulls the book down, and the book pulls Earth up.

• The desk pushes up on the book, and the book pushes down on the desk.

A spring scale reads force in newtons (N). Mass is measured in kilograms (kg), but weight is the force of gravity on an object, measured in N. On Earth, each kilogram weighs about $10\ \text{N}$, so a $2\ \text{kg}$ backpack weighs about $20\ \text{N}$. When the backpack hangs at rest, the scale reads about $20\ \text{N}$ up, balancing the $20\ \text{N}$ of gravity down.

If Malik pulls the backpack upward so it speeds up, the scale's upward force is larger than gravity. The forces are unbalanced. The net force points up.

Back to the start: the ball pushes on the player's foot with the same force as the kick. The forces act on different objects, so they don't cancel. The ball speeds up because the kick is an unbalanced force on the ball.

Summary

• A force is an interaction between two objects, so forces come in action-reaction pairs.

• Newton's Third Law: the two forces are equal in size and opposite in direction.

• The forces of a pair act on different objects, so they never cancel.

• Net force: add forces in the same direction, subtract forces in opposite directions.

• Balanced forces ($F_{net} = 0\ \text{N}$) act on the same object and do not change its motion. Unbalanced forces change it.

• To find a reaction, swap the objects: "A on B" becomes "B on A."