Friction & Common Forces — Drills · Grade 12 Science
Chapter: Forces, Rotation & EquilibriumWhat these drills covers
By the end of Friction & Common Forces you should be able to:
- I can distinguish static from kinetic friction and calculate a friction force using f = μN.
- I can identify the normal force, tension, and weight acting on an object.
- I can resolve weight into components on an inclined plane and predict the resulting motion.
6 questions, numeric, multiple choice and written answer, each with a worked explanation.
Vocabulary used: static friction, kinetic friction, coefficient of friction (μ), normal force (N), tension.
Problem 1
Answer and explanation
40 N
f = μₖN = 0.5 × 80 = 40 N.
Problem 2
Answer and explanation
19.6 N
N = mg = 10 × 9.8 = 98 N. f = μₖN = 0.20 × 98 = 19.6 N.
Problem 3
- The maximum static friction is at least as large as kinetic friction
- Friction depends only on how fast the object moves
- Friction always equals the object's weight
- Kinetic friction is greater than static friction
Answer and explanation
The maximum static friction is at least as large as kinetic friction
The maximum static friction (μₛN) is usually greater than kinetic friction (μₖN), which is why it takes more force to start sliding than to keep sliding.
Problem 4
- twice its weight
- its weight, mg
- its mass
- zero
Answer and explanation
its weight, mg
On level ground with only gravity acting, the normal force balances the weight, so N = mg.
Problem 5
Answer and explanation
To speed up, the tires push backward on the road, and by friction (and Newton's third law) the road pushes the car forward. To stop, the brakes slow the tires and friction between tires and road pushes backward on the car, decelerating it. On ice, where friction is tiny, a car can neither accelerate nor brake effectively.
Problem 6
Answer and explanation
4.9 m/s²
On a frictionless ramp, a = g sinθ = 9.8 × sin 30° = 9.8 × 0.5 = 4.9 m/s² (mass cancels out).