Chapter 6 of the new NCERT Class 9 Science textbook Exploration (2026-27) — How Forces Affect Motion. Below are 22 questions from this chapter with answers and step-by-step explanations, including 8 diagram-based questions with their figures. Try each one before revealing the answer — and if a concept doesn't click, Vidya ma'am teaches this exact chapter live in the EduLevel app.
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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.
A sailor jumps out from a small boat towards the shore as shown in the figure. Does the boat move as a result? If it does, specify the direction and explain why.
Explanation: While jumping, the sailor pushes the boat backward with his feet; this is the action force. By Newton's third law, the boat exerts an equal and opposite reaction force on the sailor, which carries him forward towards the shore. The action force acting on the boat makes the boat move in the opposite direction, i.e. backward, away from the shore. The same result follows from conservation of momentum: the total momentum was initially zero, so when the sailor gains forward momentum, the boat gains an equal momentum backward.
A block resting on a table is acted upon by two forces of 10 N and 6 N. For each of the three cases illustrated in the accompanying figure, calculate the magnitude of the net force on the block and state its direction.
Explanation: Forces acting along the same straight line add up when they point the same way and subtract when they point opposite ways. In case (a) both forces act in the same direction, so the net force is 10 + 6 = 16 N in the direction in which the forces are applied. In case (b) the 10 N force acts towards the right and the 6 N force towards the left, so the net force is 10 - 6 = 4 N towards the right, along the 10 N force. In case (c) the directions are interchanged, so the net force is again 10 - 6 = 4 N, but now towards the left, along the 10 N force.
A wooden block is pulled by a spring balance on a horizontal surface, as shown in the figure. The force is gradually increased until the block just begins to move. (i) What does the reading on the spring balance represent at this exact moment? (ii) If the block were pulled at a constant velocity, what would be the net force acting on it? (iii) Explain how the spring balance reading relates to the magnitude of the force of friction.
Explanation: (i) The block just begins to move when the applied pull barely overcomes the maximum static friction, so at that exact moment the spring balance reading equals the limiting (maximum static) friction force. (ii) At constant velocity the acceleration is zero, so by Newton's first and second laws (F = ma with a = 0) the net force on the block is zero. (iii) Whether the block is on the verge of moving or sliding uniformly, the pulling force and the friction force balance each other; hence the spring balance reading gives the magnitude of friction - the limiting static friction at the instant of slipping and the sliding friction during uniform motion.
In an arm-wrestling match between players R and S, their arms begin to tilt forward. At this moment, are the forces applied by the players balanced? If they are not balanced, identify which player is applying the greater force.
Explanation: As long as the clasped hands stay still, the two players' forces are equal and opposite, so they are balanced. The instant the arms begin to tilt, that state of rest changes, and a change of motion can only happen under an unbalanced net force, so at this instant the forces are not balanced. In arm-wrestling both elbows stay planted on the table, so the joined hands cannot swing sideways towards R or towards S; they can only tip towards you or away from you, which is why the question describes the tilt as being to the front, out of the page. A player is losing when the back of his own hand is being driven down towards the table, and in the figure it is R's hand that faces you - you are looking at the back of R's hand, with R's fingers curled over S's fist. So a tilt to the front means R's hand is the one being pushed down: R is giving way, and therefore S exerted the larger force.
Consider a weightlifter holding a barbell stationary as shown in the corresponding figure. Identify two forces acting on the barbell. State whether these forces are balanced or unbalanced when the barbell is held steady.
Explanation: Two forces act on the stationary barbell: the force of gravity, i.e. the weight of the barbell, pulling it vertically downward, and the upward muscular force applied by the weightlifter's hands supporting it. Since the barbell is held steady, its state of motion is not changing, so the net force on it must be zero. Therefore the two forces are equal in magnitude, opposite in direction, and balanced.
While sitting on a chair with wheels, you push a large table away from you. What happens to you and the chair? In which direction do you move?
Explanation: When you push the table, you exert a force on it directed away from you; this is the action force. By Newton's third law, the table simultaneously exerts an equal and opposite reaction force on you, directed away from the table. Because the chair is on wheels and friction is very small, this reaction force is unbalanced and sets you and the chair in motion. You therefore roll backward, away from the table, in the direction opposite to your push, as the arrows in the figure show.
Two blocks, P and Q, are on a smooth horizontal surface as shown in the figure. Two forces of 4 N and 5 N act on block P in opposite directions. Block Q is observed to be moving with a constant velocity. Which of the following statements is correct?
Explanation: Block P has unbalanced forces (5N > 4N), so it has a net force. Block Q moves at a constant velocity, which means its acceleration is zero, and therefore the net force on it is zero according to Newton's first law.
When a net force is applied to an object, we observe that the object accelerates. Which statement correctly describes this acceleration?
Explanation: Newton's Second Law states that acceleration is directly proportional to the net force (F=ma) and in the same direction as the force.
A hand cart loaded with vegetables collides with an identical but empty hand cart. Which statement accurately describes the forces during the collision?
Explanation: According to Newton's Third Law, for every action, there is an equal and opposite reaction. The forces the carts exert on each other are an action-reaction pair and must be equal in magnitude.
The position-time graphs for four objects (A, B, C, and D) moving along a straight line are provided. On which of these objects is a net force acting?
Explanation: A net force causes acceleration. The slope of a position-time graph represents velocity. Graph C is a curve, indicating that its slope (velocity) is changing, which means the object is accelerating.
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