Exam-style CBSE practice questions on Newton's Second Law (How Forces Affect Motion). Try each one first, then reveal the correct answer and a step-by-step explanation. Free, from EduLevel — the AI teacher for CBSE.
A force acting on a body of mass 5 kg produces an acceleration of 2 m/s². What is the magnitude of the force?
Explanation: By Newton's second law, F = ma. Here m = 5 kg and a = 2 m/s², so F = 5 × 2 = 10 N.
One newton (1 N) is the same as which of the following?
Explanation: From F = ma, one newton is the force that gives a mass of 1 kg an acceleration of 1 m/s². So 1 N = 1 kg × 1 m/s² = 1 kg·m/s². The unit kg·m/s (without the second squared) is the unit of momentum, not force.
A football of mass 2 kg is moving with a velocity of 5 m/s. What is its momentum?
Explanation: Momentum = mass × velocity = 2 × 5 = 10 kg·m/s. Remember that the SI unit of momentum is kg·m/s, while the newton (N) is the unit of force, so 10 N is a unit mistake.
The mass of a school bag is 10 kg. Taking g = 10 m/s², what is its weight?
Explanation: Weight is the force with which the Earth pulls a body, so W = m × g. Here W = 10 × 10 = 100 N. Since weight is a force, it is measured in newtons (N), not in kg.
A car of mass 1000 kg starts from rest and reaches a velocity of 20 m/s in 10 s. What is the force applied by its engine?
Explanation: Acceleration = change in velocity ÷ time = (20 − 0) ÷ 10 = 2 m/s². By Newton's second law, F = ma = 1000 × 2 = 2000 N.
A force of 60 N acts on a body of mass 12 kg. What acceleration does it produce?
Explanation: From F = ma, acceleration a = F ÷ m = 60 ÷ 12 = 5 m/s². Multiplying 60 × 12 or dividing the other way round (12 ÷ 60) are common errors; acceleration is always force divided by mass.
A cricket ball of mass 0.15 kg moving at 20 m/s is caught by a fielder and brought to rest. What is the magnitude of the change in momentum of the ball?
Explanation: Initial momentum = m × v = 0.15 × 20 = 3 kg·m/s. Final momentum = 0.15 × 0 = 0 kg·m/s because the ball comes to rest. So the change in momentum = 3 − 0 = 3 kg·m/s.
The momentum of a body increases from 30 kg·m/s to 60 kg·m/s in 5 s. What is the force acting on it?
Explanation: By Newton's second law, force = change in momentum ÷ time taken. Change in momentum = 60 − 30 = 30 kg·m/s. So F = 30 ÷ 5 = 6 N.
The mass of an object is 6 kg on the Earth. What will its mass be on the Moon, where gravity is about one-sixth of that on the Earth?
Explanation: Mass is the amount of matter in a body and does not change from place to place, so it stays 6 kg on the Moon. Only weight, which equals m × g, becomes about one-sixth on the Moon because g is smaller there. Dividing the mass by 6 confuses mass with weight.
A force of 18 N gives a mass m1 an acceleration of 3 m/s² and a mass m2 an acceleration of 6 m/s². What acceleration will the same force produce if m1 and m2 are tied together?
Explanation: From m = F ÷ a, m1 = 18 ÷ 3 = 6 kg and m2 = 18 ÷ 6 = 3 kg. When tied together, the total mass = 6 + 3 = 9 kg. So acceleration = F ÷ total mass = 18 ÷ 9 = 2 m/s². Averaging the two accelerations to get 4.5 m/s² is wrong because it is the masses that add up, not the accelerations.
A vehicle of mass 800 kg moving at 20 m/s is brought to rest in 4 s by applying brakes. What is the magnitude of the braking force?
Explanation: The velocity changes by 20 − 0 = 20 m/s in 4 s, so the retardation = 20 ÷ 4 = 5 m/s². Braking force F = ma = 800 × 5 = 4000 N. Multiplying mass directly by velocity (800 × 20 = 16000) gives momentum in kg·m/s, not force.
A bullet of mass 20 g is fired from a gun with a velocity of 400 m/s. What is the momentum of the bullet?
Explanation: First convert the mass into SI units: 20 g = 20 ÷ 1000 = 0.02 kg. Momentum p = m × v = 0.02 × 400 = 8 kg·m/s. Forgetting to convert grams into kilograms gives the wrong answer 8000 kg·m/s.
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