OAR OAR Mechanical Comprehension: Forces 3 — Questions and Answers
Question 1: 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)
- 20 N
- 28.3 N (Correct answer)
- 40 N
- 14.1 N
Correct 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.
Question 2: Which of Newton's laws states that for every action there is an equal and opposite reaction?
- First Law
- Second Law
- Third Law (Correct answer)
- Law of Universal Gravitation
Correct answer: Third Law
Newton's Third Law states that action and reaction forces are equal in magnitude and opposite in direction.
Question 3: A car of mass 1,200 kg decelerates from 20 m/s to rest in 4 seconds. What braking force was applied?
- 6,000 N (Correct answer)
- 4,800 N
- 300 N
- 240 N
Correct answer: 6,000 N
a = (0 − 20)/4 = −5 m/s²; F = ma = 1200 × 5 = 6,000 N.
Question 4: Which type of friction acts on an object that is already in motion across a surface?
- Static friction
- Rolling friction only
- Kinetic friction (Correct answer)
- Normal friction
Correct answer: Kinetic friction
Kinetic (sliding) friction acts on objects in motion and is typically less than static friction.
Question 5: 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?
- 51.96 N
- 30 N (Correct answer)
- 60 N
- 34.64 N
Correct answer: 30 N
Horizontal component = F cos 60° = 60 × 0.5 = 30 N.
Question 6: What happens to the normal force on an object resting on a horizontal surface when a downward vertical force is added?
- Normal force decreases
- Normal force remains the same
- Normal force increases (Correct answer)
- Normal force becomes zero
Correct 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.
Question 7: 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?
- 6 m/s²
- 2 m/s² (Correct answer)
- 4 m/s²
- 1 m/s²
Correct answer: 2 m/s²
Net force = 20 − 10 = 10 N east; a = F/m = 10/5 = 2 m/s².
OAR OAR Mechanical Comprehension: Forces 3
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)