When two bodies interact, they exert a force on each other-
If object A exerts a force on object B, then object B exerts the exact opposite force on object A.
- This means, if you push something, e.g. a shopping trolley, the trolley will push back against you, just as hard. As soon as you stop pushing, so does the trolley.
- The force of you pushing the trolley is called the action force.
- The force of the trolley pushing back against you is called the reaction force.
If the forces are always equal, how does anything ever go anywhere?
Think of a pair of ice skaters:
- Skater A = mass 55kg.
- Skater B = mass 65kg.
When skater A pushes on skater B (the action force), she feels an equal and opposite force from Skater B's hand (the reaction force). Both skaters feel the same sized force, in opposite directions, and so accelerate away from each other. Skater A will be accelerated more than Skater B, because she has a smaller mass.
Zero Resultant Force and Motion🚙
In I/GCSE Physics, you will have to know how no resultant force means no change in velocity
- If there is zero resultant force acting on a body (the forces are balanced), then the body will remain at rest, or else if it's already moving, it'll just carry on at the same velocity.
- A resultant force is the overall force acting on a body.
When a train or car or bus or anything else is moving at a constant velocity, then the forces on it must be balanced - there is no resultant force.
- To keep going at a steady speed, there must be zero resultant force.
Drafted by Catrina (Physics)