QCE Physics IA1 free practice questions with answers

Question 1 (5 marks) [Forces and Newton's laws]

A 24 kg crate is pulled along a horizontal floor by a constant horizontal force of 140 N. The coefficient of kinetic friction between the crate and the floor is 0.30.

(a) What is the magnitude of the friction force on the crate?

(a) Answer: 70.6 N

• On a horizontal surface the normal force equals the weight: N = mg = 24 × 9.8 = 235 N.
• f = μN = 0.30 × 235 = 70.6 N

(b) What is the acceleration of the crate?

(b) Answer: 2.89 m s−²

• Net force = applied force − friction = 140 − 70.6 = 69.4 N.
• a = F_net ÷ m = 69.4 ÷ 24 = 2.89 m s−²

Newton's second law: find the NET force (add forces in the direction of motion, subtract opposing ones), then a = F_net ÷ m.