Push a door, pull a wagon, lift a backpack: each is a force. A force always involves two objects, one that pushes or pulls and one that gets pushed or pulled. Forces are measured in newtons (N), an SI unit; a small apple weighs about 1 N, and 1 kN is 1,000 N.
Key idea
A force is a push or a pull by one object on another. It has a size (in newtons) and a direction, so we draw it as an arrow.
Why it works
Ask "what is doing the pushing?" for every force. A book on a table feels two: Earth pulls it down (gravity), and the table pushes it up (the normal force). Some forces need touching, like a push, friction or the pull of a rope. Others act across a gap: gravity, magnetic forces and electric forces.
Worked example: Name the forces
A lamp hangs from the ceiling by a cord. What forces act on the lamp, and what exerts each one?
- Gravity: Earth pulls the lamp down.
- Tension: the cord pulls the lamp up.
- Nothing else touches the lamp, so those are the only two.
Answer: Gravity (by Earth) and tension (by the cord)
Worked example: After the throw
A baseball has just left the pitcher's hand. Which forces act on it now?
- Earth still pulls it down: gravity.
- The air pushes against its motion: air resistance.
- The hand is no longer touching it, so the hand exerts no force.
Answer: Gravity and air resistance only
Common mistake
Talking about "the force of the ball," as if a moving object carries a force inside it. A force is always a push between two objects, and it lasts only while they interact. Once the ball leaves the hand, the hand's force is gone. Check: for every force you name, say what object exerts it. If you can't, it isn't a force.
Try building free-body diagrams in the Science Lab's Newton's Laws course.