Different forces, all measured in newtons (N).
Quick check
A book rests on a flat table. Which force balances gravity pulling it down?
Friction (kinetic)
Friction is proportional to how hard the surfaces press together (the normal force N), not to the contact area. μ is the coefficient of friction.
Field forces vs contact forces
- Field forces act at a distance — gravity, electric, magnetic.
- Contact forces need touching — normal, friction, tension, applied push/pull.
- Weight is mg; on the Moon the same mass weighs about 1/6 as much.
- Tension pulls *along* a rope and is the same throughout an ideal (massless) rope.
Recap
- All forces are measured in newtons (N).
- The normal force adjusts itself to whatever is needed (up to breaking).
- Friction opposes relative motion (or attempted motion).
Common forces
- Gravity — mg, pulling everything toward Earth.
- Normal — surface pushing back.
- Friction — resists relative motion.
- Tension — pulling along a rope or cable.
- Applied — anything you push or pull with.