Different classes of forces
Contact force- A force that must have contact to the object to push or pull, like a game of pool, when the cue stick hits the ball.pool_table.jpg
The most common contact force is the force of friction; you cannot avoid it.

Action-at-a-distance force- A force that can pull or push an object without direct contact. The force of gravity and the force of magnetism are two common examples for Action-at-a-distance forces.
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