AP® Physics C: Mechanics · Unit 2 · Topic 2.2
Forces as Interactions
A force is never a lone push or pull — it is always one half of an interaction between two objects. Every force you can name has a specific other object exerting it, and that object feels an equal, opposite force back.
Forget the idea of force as some quantity an object just carries around. A force always traces back to a pair: something doing the pushing or pulling, and something receiving it. Gravity comes from the Earth's mass reaching across space to pull on you; you pull back on the Earth just as hard, though its huge mass makes that effect unnoticeable. Contact forces work the same way at a smaller scale. When you lean on a wall, atoms in your hand compress atoms in the wall; those atoms resist being squeezed together and push back. That push is the normal force. Tension is the same idea stretched instead of squeezed: pull the ends of a rope apart, and its atoms resist being pulled apart, yanking back on whatever holds each end.
This pairing structure explains the hockey puck problem. Once the stick stops touching the puck, there is no object left supplying a force along the direction of motion, so there is nothing to call a 'force of motion.' Friction is zero on ideal ice, gravity and the normal force cancel vertically, and the puck simply keeps whatever velocity it already had. Motion needs no cause; only a change in motion does. Belt-lifting fails for a related reason: both forces in that interaction — your hand pulling up on the belt, the belt pulling down on your hand — act on your own body. They are internal, so they cancel inside the system and leave no net external force to lift you off the ground.
This idea sets up Newton's third law formally and feeds directly into free-body diagrams, where every force drawn on an object must have an identifiable source object pushing or pulling on it from outside.
Key terms
- Interaction
- A pair of equal and opposite forces that two objects exert on each other.
- Normal force
- The contact push that arises when surfaces' atoms resist being compressed into each other.
- Tension
- The contact pull transmitted through a stretched rope or rod as its atoms resist being pulled apart.
- Internal force
- A force exerted by one part of a system on another part of the same system, which cancels when analyzing the system as a whole.
- Net external force
- The vector sum of all forces on an object coming from outside the object, which determines its acceleration.
Watch it explained
- How it worksHow does every single force trace back to two objects either touching or reaching across space?
- How it worksHow do the normal force and tension both come from atoms being squeezed or stretched?
- Common mistakeWhy isn't there a force of motion pushing a hockey puck along after the stick leaves it?
- Going furtherWhat happens to the contact force between two stacked blocks when you push harder on the outer one?
Check yourself
End of topic.