← Mechanics

Forces, friction & slopes (every paper)

Free-body diagram → F(net) = ma along the motion.

Method

  • Draw all forces on the object.
  • Resolve weight on a slope: mg sin θ down-slope, mg cos θ into it.
  • F(net) = ma including friction f opposing motion.
Exam-style 20 kg crate, 60 N push, 15 N friction Find the acceleration. F(net) = 60 − 15 = 45 N a = F/m = 45/20 = 2.25 m s⁻² ✓ 2.3 m s⁻² (2 s.f.) in the direction of the push. ℹ

Static versus kinetic friction

Before an object starts moving, STATIC friction can take any value up to a maximum, exactly opposing the applied force so the object stays still — this is why a light push on a heavy crate produces no motion at all. Once the object is sliding, KINETIC friction applies, and it is usually treated as roughly constant regardless of speed. A question about whether an object moves at all is a static-friction question; a question about acceleration once moving is a kinetic-friction question.

Practise just this