You've met forces in Newton's Third Law and Friction. Before using Newton's laws on any problem, physicists draw a free-body diagram: one object, alone, with every force on it drawn as an arrow.
Key idea
Draw the object as a box or a dot. For every force on it, draw an arrow from its center in the force's direction and name the object that exerts it. Contact forces come from things touching it; gravity comes from Earth.
Why it works
Newton's laws relate the forces on one object to that object's motion. A diagram that leaves out a force, or adds one that isn't real, gives the wrong net force. Naming who exerts each force is the test: "the floor pushes up" is a real force; "the motion pushes forward" has no one doing the pushing, so it isn't a force.
- Earth's gravity: always, straight down, $F_g = mg$.
- Each surface it touches: a normal force (perpendicular to the surface) and maybe friction (along the surface).
- Each rope or string attached: a tension, pulling along the rope.
- Anything else touching it: a push, a spring, the air.
Worked examples
Worked example: A box pushed across a floor
You push a box across a rough floor at a steady speed. What forces act on it?
- Earth: gravity, down. The floor: a normal force, up, and kinetic friction, opposite to the sliding.
- Your hand: the push, along its direction.
- Four forces. At steady speed the push balances friction and the normal force balances gravity.
Answer: Gravity, normal force, friction and your push
Worked example: A ball in flight
A ball flies up and to the right after you throw it. Ignoring air, what forces act on it?
- Nothing touches it, so there are no contact forces.
- Only Earth's gravity, straight down. Your throw ended when the ball left your hand.
Answer: Only gravity
Common mistake
Drawing a "force of motion" in the direction the object moves, or including a force the object exerts on something else (like the box pushing on the floor). Moving objects keep moving on their own, so no force is needed to "carry" the motion; and forces the box exerts belong on the floor's diagram, not the box's. Check: for every arrow, finish the sentence "___ pushes or pulls on the box".
Try it in the Science Lab: Forces at Work builds diagrams for many everyday scenes.