Learn to tell static friction from kinetic friction, use f = mu × N to find the friction force acting on a sliding object, and combine it with an applied force to find the net force and the acceleration it produces.
ObjectiveP.2.2Read4 minExam weightForces and Newton’s Laws ≈ 20% of Physics
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You can tell static friction apart from kinetic friction and explain why more force is needed to start something sliding than to keep it sliding.
You can calculate a friction force with f = mu × N once you know the coefficient of friction and the normal force.
You can combine an applied force with a friction force to find the net force on an object, then find its acceleration.
What the lesson concludes
Friction always opposes motion, or attempted motion, between two surfaces in contact.
Static friction holds an object still up to a maximum; kinetic friction, usually smaller, takes over once the object slides.
The friction force is f = mu × N, where N is the normal force. Only the coefficient of friction and the normal force matter, not the size of the contact area.
On a flat, level surface with no other vertical forces, the normal force equals the object's weight, N = m × g.
Net force is the applied force minus the friction force opposing it, and that same net force is the F that goes into a = F / m.