A soccer ball sits on the grass until Lena kicks it. The kick sends it flying. Something acted on the ball, and it came from somewhere: her foot.
Physics has one word for that kind of acting. Watch who does what to whom.
Lena's foot pushes the soccer ball. A dog's leash pulls the dog's collar. A table pushes up on the book resting on it.
Every case has the same shape: one object pushes or pulls another object.
That is what a force is: a push or a pull exerted on one object by another object.
So naming a force means naming both objects. 'The table pushes up on the book' names the table and the book.
A hammer strikes a nail. Which statement names the force on the nail in full?
Mira hangs her backpack on a wall hook. The hook holds the backpack up. Which two objects does that holding force name?
A force is not something an object has or carries. The flying ball does not hold Lena's kick inside it. A force lasts only as long as the two objects interact. Once her foot leaves the ball, that push is over.
The two objects do not need to touch. A magnet pulls a steel paper clip across a gap: two objects again, the magnet and the clip.
Tobias says: 'When a bat hits a baseball, the force on the ball names two objects, the bat and the ball.' Is Tobias correct?
A rope tows a sled across the snow. Name the force on the sled in full.
Naming a force means naming both objects: the object exerting the push or pull, and the object it acts on.
The rope pulls the sled. The two objects are the rope and the sled.
A gust of wind flattens a flag against its pole. Which two objects does that pushing force name?
Air counts as an object: it can push on things.
The force is the air's push on the flag.
The air and the flag: the air pushes the flag.
A pickup truck tows a trailer along a level road. Which statement names the towing force in full?
Four students describe forces in different scenes. Which description is complete?
A magnet pulls a steel paper clip toward it across a 1 cm gap, with nothing touching. Which statement is correct?