The Law of Interaction says that whenever one object exerts a force on another object, the second object exerts an equal and opposite force back on the first. All forces come in pairs like this: you can't have one force without the other.
Because the forces act on different objects, we almost need to make two pictures.
1. A car coming to a sudden halt:
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|
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| The force of the seatbelt on the passenger | The force of the passenger on the seatbelt |
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2. An astronaut during a shuttle launch:
The upward force of the couch on the astronaut is the same size as the
downward force of the astronaut on the couch.
3.
A collision between a car and a truck:
The force of the truck on the car is the same size as the force of
the car on the truck (no matter what their masses, or how fast either was going).
"How can that be? It would seem the guy in the truck comes out a
lot better!"
Explanation