A bat meets a fastball. For a few thousandths of a second, bat and ball press on each other with forces of thousands of newtons. Then the ball is gone the other way.
Interactions like that one are everywhere, and they earn their own name.
A hammer strikes a nail. For a tiny fraction of a second, hammer and nail press on each other with large forces.
Now the same hammer rests on the nail while Haruki leans on it, pressing just as hard, for three whole seconds.
The strike packed its large forces into a brief moment.
The lean spread forces of the same size over a long time.
When two objects exert large forces on each other for a brief time, we call the interaction a collision.
Is the strike a collision? Yes: large forces, brief time.
Is the lean a collision? No: the forces are large, but not brief.
The hammer goes back to striking, with a quick tap. Brief again, so a collision again.
Two bumper cars bump: a collision. A dropped ball of clay lands on the floor: a collision.
The sorting line: a collision is over in a blink, so a force that goes on and on marks the interaction as a steady force, not a collision.
Esther's shopping cart bumps into a display stand, rattling it in an instant. Is the interaction between the cart and the stand a collision?
Gunnar leans against a loaded bookcase, pressing on it steadily for five seconds. Is the interaction between Gunnar and the bookcase a collision?
A tennis racket strikes a ball. Is the interaction a collision?
During the strike, racket and ball exert large forces on each other.
The contact lasts a few thousandths of a second: brief.
Large forces for a brief time is the collision pattern.
Yes: the racket-and-ball interaction is a collision.
A tow bar drags a car along a highway at a steady speed for an hour. Is the interaction between the tow bar and the car a collision?
The tow bar and the car exert forces on each other the whole trip.
The forces last an hour: nothing brief about them.
A collision needs the forces packed into a brief time.
No: the towing interaction is a steady force, not a collision.
Which of these interactions is a collision?
A crane cable lowers a beam, supporting it for a full minute. This interaction is not a collision. What is the reason?
Which pair of features makes an interaction between two objects a collision?
Two objects exert large forces on each other for a brief time. Type the name for this kind of interaction.