OAR Mechanical Comprehension: Forces Flashcards

A 4 kg object is suspended by two ropes that each make a 45° angle with the ceiling. What is the tension in each rope? (g = 10 m/s²)

Answer: 28.3 N

Each rope's vertical component must support half the weight: T sin 45° = 20 N → T = 20/0.707 ≈ 28.3 N.

Which of Newton's laws states that for every action there is an equal and opposite reaction?

Answer: Third Law

Newton's Third Law states that action and reaction forces are equal in magnitude and opposite in direction.

A car of mass 1,200 kg decelerates from 20 m/s to rest in 4 seconds. What braking force was applied?

Answer: 6,000 N

a = (0 − 20)/4 = −5 m/s²; F = ma = 1200 × 5 = 6,000 N.

Which type of friction acts on an object that is already in motion across a surface?

Answer: Kinetic friction

Kinetic (sliding) friction acts on objects in motion and is typically less than static friction.

A 60 N force is applied at an angle of 60° above the horizontal to push a box. What is the horizontal component of this force?

Answer: 30 N

Horizontal component = F cos 60° = 60 × 0.5 = 30 N.

What happens to the normal force on an object resting on a horizontal surface when a downward vertical force is added?

Answer: Normal force increases

The surface must push up harder to keep the object stationary, so the normal force increases by the added downward force.

A 5 kg block on a frictionless surface is pushed by two horizontal forces: 20 N east and 10 N west. What is the magnitude of the block's acceleration?

Answer: 2 m/s²

Net force = 20 − 10 = 10 N east; a = F/m = 10/5 = 2 m/s².