Class 9 Science Exploration Chapter 6 Solutions (NCERT 2026–27) – How Forces Affect Motion

These Class 9 Science Exploration Chapter 6 solutions cover How Forces Affect Motion from the new NCF-2023 textbook (2026–27).

Class: 9 Subject: Science Book: Exploration Chapter: 6 Exercise: Revise, Reflect, Refine (16 Qs) Session: 2026–27
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Class 9 Science Exploration Chapter 6 Solutions – Overview

Chapter 6 of Exploration, How Forces Affect Motion, explains what causes a change in motion. It introduces force and net (unbalanced) force, Newton’s three laws of motion, the ideas of inertia and momentum, the key relation F = ma, the concept of impulse, the role of friction, and action–reaction pairs. These Class 9 Science Exploration Chapter 6 solutions answer every textbook question step by step.

Key Concepts & Definitions

Force: a push or pull that can change the speed, direction or shape of an object. Net (unbalanced) force: the single resultant of all forces; only a net force changes motion.

Inertia: the tendency of an object to resist a change in its state of rest or motion; it increases with mass.

Momentum (p): p = mass × velocity (kg m s-1); a vector.

Friction: a force that opposes relative motion between surfaces in contact.

Action–reaction: forces always occur in equal and opposite pairs acting on two different bodies (Newton’s third law).

Newton’s Laws & Key Formulas

First law (inertia): a body continues at rest or in uniform motion unless acted on by a net force.

Second law: net force = rate of change of momentum ⇒ F = ma.

Third law: to every action there is an equal and opposite reaction.

Impulse: F × t = change in momentum = mv − mu.

Useful: p = mv; and the equations of motion v = u + at, v2 = u2 + 2as.

“Think It Over” — Answers

Why does a canoe move forward when the canoeist pushes the water backward?

ANSWERBy Newton’s third law: the paddle pushes the water backward (action), and the water pushes the canoe forward with an equal and opposite force (reaction), so the canoe moves ahead.

Why does the canoe move faster when the canoeist pushes harder?

ANSWERA harder push means a greater force. By Newton’s second law (F = ma), a greater force produces a greater acceleration, so the canoe gains speed more quickly.

Class 9 Science Exploration Chapter 6 Solutions — Revise, Reflect, Refine

1. Using a horizontal force F, a table is moved across the floor at a constant velocity. How much is the frictional force exerted by the floor on the table?

ANSWER At constant velocity the acceleration is zero, so the net force is zero (Newton’s first law). Therefore friction balances the applied force exactly: frictional force = F (acting opposite to the motion).

2. For a ball moving on a smooth frictionless surface, choose the option that makes each statement physically correct. (i) If no net force is applied, the velocity of the ball will remain the same / increase / decrease. (ii) If a net force is applied in the direction of its motion, the magnitude of the velocity will remain the same / increase / decrease. (iii) If a net force is applied opposite to its motion, the magnitude of the velocity will remain the same / increase / decrease.

ANSWER (i) remain the same (Newton’s first law — no net force, no change in velocity). (ii) increase (force along motion speeds it up). (iii) decrease (force opposite to motion slows it down).

3. Two forces of 4 N and 5 N act in opposite directions on block P, while block Q moves with a constant velocity (Fig. 6.36). Which statement is correct? (i) P experiences a net force and Q does not. (ii) P does not experience a net force and Q does. (iii) Both P and Q experience a net force. (iv) Neither P nor Q experiences a net force.

ANSWER Correct option: (i). On P the forces are unequal and opposite, so net force = 5 − 4 = 1 N (not zero). Q moves at constant velocity, so its net force is zero.

4. While practising for the snake boat race, 100 oarsmen row a boat: 95 row backwards to propel the boat forward, but 5 row in the opposite direction by mistake. If each applies a horizontal force of 200 N, what is the net force on the boat? (Ignore drag.)

ANSWER Forward force = 95 × 200 = 19000 N. Backward force = 5 × 200 = 1000 N. Net force = 19000 − 1000 = 18000 N, directed forward.

5. When a net force acts on an object, the object accelerates: (i) opposite to the force, with acceleration proportional to the force. (ii) opposite to the force, with acceleration proportional to the mass. (iii) in the direction of the force, with acceleration inversely proportional to the force. (iv) in the direction of the force, with acceleration proportional to the force.

ANSWERCorrect option: (iv) — by Newton’s second law (F = ma), the acceleration is in the direction of the net force and is proportional to it (for a fixed mass).

6. The position–time graphs for four objects A, B, C and D moving along a straight line are given in Fig. 6.37. A net force acts on: (i) Object A (ii) Object B (iii) Object C (iv) Object D

ANSWER Correct option: (iii) Object C. A net force means acceleration, which shows up as a curved position–time graph (changing slope). Object C’s graph is curved, so a net force acts on it. Objects A and D are straight lines (uniform velocity) and B is a horizontal line (at rest) — all have zero net force.

7. A sailor jumps out from a small boat to the shore (Fig. 6.38). As the sailor jumps forward, will the boat move? If yes, in which direction and why?

ANSWER Yes, the boat moves backward (away from the shore). By Newton’s third law, as the sailor pushes the boat backward to jump forward, the boat pushes the sailor forward, so the boat recoils in the opposite direction to the sailor’s jump.

8. During a high jump event, a landing mat or sand bed is placed for the athlete to fall upon (Fig. 6.39). Explain the reason behind it.

ANSWERThe soft mat increases the time over which the athlete is brought to rest. Since impulse F × t = change in momentum is fixed, a longer time means a smaller stopping force on the body, reducing the chance of injury.

9. A hand cart loaded with vegetables collides with an identical but empty hand cart. During the collision: (i) the loaded cart exerts a force of larger magnitude on the empty cart. (ii) the empty cart exerts a force of larger magnitude on the loaded cart. (iii) neither cart exerts a force on the other. (iv) both carts exert an equal magnitude of force on each other.

ANSWERCorrect option: (iv) — by Newton’s third law the two carts exert equal and opposite forces on each other, whatever their loads. (The empty cart accelerates more because it has less mass.)

10. The acceleration–mass graph for the acceleration produced by a force on objects of different masses is plotted in Fig. 6.40. Plot the force–mass graph for this case.

ANSWER From Fig. 6.40 the same force gives a = 10 m s-2 at 1 kg, 5 m s-2 at 2 kg, 2.5 m s-2 at 4 kg, and so on — in every case F = ma = 10 N. Since the force is the same (10 N) for all masses, the force–mass graph is a horizontal straight line at F = 10 N (parallel to the mass axis).

11. The velocity–time graph of an object of mass 10 kg moving along a straight line is shown in Fig. 6.41. Calculate the force acting on the object using the graph.

ANSWER From the graph the velocity rises from 10 m s-1 (at t = 0) to 30 m s-1 (at t = 8 s). Acceleration a = slope = (30 − 10)/8 = 20/8 = 2.5 m s-2. Force F = ma = 10 × 2.5 = 25 N.

12. A bullet of mass 50 g moving at 100 m s-1 enters a heavy stationary wooden block and stops after penetrating 50 cm. Estimate the stopping force on the bullet (assume constant acceleration).

ANSWER m = 0.05 kg, u = 100 m s-1, v = 0, s = 0.5 m. v2 = u2 + 2as ⇒ 0 = 1002 + 2a(0.5) ⇒ a = −10000 m s-2. F = ma = 0.05 × (−10000) = −500 N. The stopping force is 500 N (opposing the motion).

13. A footballer kicks a penalty with a speed of 108 km h-1. The estimated force imparted was 800 N and the ball’s mass was 0.4 kg. Calculate the time of contact between the foot and the ball.

ANSWER v = 108 km h-1 = 108 × 5/18 = 30 m s-1; the ball starts from rest (u = 0). Impulse: F × t = m(v − u) ⇒ t = m v / F = (0.4 × 30)/800 = 12/800 = 0.015 s.

14. An object of mass 2 kg moving at a constant 10 m s-1 enters a rough patch where friction is 7 N. At the same time, an extra 3 N force opposing the motion is applied. How much distance does the object travel before coming to rest?

ANSWER Total opposing force = 7 + 3 = 10 N, so a = F/m = −10/2 = −5 m s-2. v2 = u2 + 2as ⇒ 0 = 102 + 2(−5)s ⇒ s = 100/10 = 10 m.

15. A tractor pulls a harrow of mass m1 with net force F giving acceleration a1, and pulls a trolley of mass m2 with the same force F giving acceleration a2. If it now pulls the trolley with the harrow on it (same force F), find the resulting acceleration in terms of a1 and a2. (Ignore friction.)

ANSWER From F = ma: m1 = F/a1 and m2 = F/a2. Combined: a = F/(m1 + m2) = F/(F/a1 + F/a2) = 1/(1/a1 + 1/a2). So a = a1a2 / (a1 + a2).

16. When a bar magnet’s pole is brought close to a magnetic compass, the magnet and the needle exert equal and opposite magnetic forces (Newton’s third law). Yet the compass needle moves while the bar magnet does not (Fig. 6.42). Explain why.

ANSWER The forces are equal in magnitude, but the two bodies have very different masses. The compass needle is very light and freely pivoted, so the same force gives it a large acceleration (a = F/m) and it swings round. The bar magnet is much heavier (and usually held), so the same force produces a negligible acceleration and it appears to stay still.

Common Mistakes to Avoid

Watch out for these

  • Thinking a moving object always needs a force — at constant velocity the net force is zero.
  • Confusing mass and weight: mass (kg) is the amount of matter; weight (N) is the gravitational force on it.
  • Forgetting that action and reaction act on different bodies, so they never cancel each other.
  • Not converting units (g to kg, cm to m, km h-1 to m s-1) before using F = ma.
  • Dropping the negative sign for a stopping/retarding force, or reporting acceleration instead of force.
  • Reading a straight position–time graph as “accelerating” — only a curved graph means a net force.

Extra Practice Questions

Very Short Answer Type Questions

Q1. State the SI unit of force.

ANSWERThe newton (N); 1 N = 1 kg m s-2.

Q2. Which physical quantity is the product of mass and velocity?

ANSWERMomentum (p = mv).

Q3. Name the law that explains the recoil of a gun.

ANSWERNewton’s third law of motion.

Short Answer Type Questions

Q1. Why is it dangerous to jump out of a moving bus?

ANSWERBecause of inertia of motion: your body is moving with the bus’s speed. On landing, your feet stop but the upper body tends to keep moving forward, which can make you fall. Running a few steps in the direction of motion helps.

Q2. A force of 20 N acts on a 4 kg object. Find its acceleration.

ANSWERa = F/m = 20/4 = 5 m s-2.

Long Answer Type Question

Q1. State Newton’s second law of motion and use it to derive F = ma.

ANSWER Newton’s second law: the rate of change of momentum of a body is directly proportional to the net force applied, and the change takes place in the direction of the force. If a mass m changes velocity from u to v in time t, change in momentum = m(v − u). Rate of change = m(v − u)/t = ma (since (v − u)/t = a). So F ∝ ma, and choosing the unit of force (newton) to make the constant 1 gives F = ma.

MCQs & Assertion–Reason

1. The SI unit of force is the:

(a) joule    (b) newton    (c) watt    (d) pascal

2. Newton’s first law is also called the law of:

(a) momentum    (b) inertia    (c) gravitation    (d) action

3. A net force of 10 N acts on a 2 kg body. Its acceleration is:

(a) 5 m s-2    (b) 20 m s-2    (c) 0.2 m s-2    (d) 12 m s-2

4. Momentum is the product of:

(a) mass and acceleration    (b) force and time    (c) mass and velocity    (d) force and distance

5. A body moves at constant velocity. The net force on it is:

(a) zero    (b) equal to its weight    (c) increasing    (d) maximum

6. The recoil of a gun is explained by Newton’s:

(a) first law    (b) second law    (c) third law    (d) law of gravitation

7. Inertia of a body depends on its:

(a) speed    (b) mass    (c) shape    (d) colour

8. A curved position–time graph indicates:

(a) rest    (b) uniform velocity    (c) acceleration (a net force)    (d) zero force

9. A cushioned landing reduces injury because it:

(a) increases the force    (b) increases the stopping time    (c) increases momentum    (d) reduces mass

10. For the same force, a body of larger mass has:

(a) larger acceleration    (b) smaller acceleration    (c) zero acceleration    (d) the same acceleration

Answer key: 1-(b), 2-(b), 3-(a), 4-(c), 5-(a), 6-(c), 7-(b), 8-(c), 9-(b), 10-(b).

For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.

A-R 1. Assertion: A body at constant velocity has zero net force on it.

Reason: At constant velocity the acceleration is zero, so by F = ma the net force is zero.

A-R 2. Assertion: Action and reaction forces do not cancel each other.

Reason: They act on two different bodies.

A-R 3. Assertion: A heavier body has greater inertia.

Reason: Inertia increases with the mass of a body.

A-R 4. Assertion: For the same force, a lighter body accelerates more.

Reason: Acceleration is directly proportional to mass.

A-R 5. Assertion: A cricketer pulls their hands back while catching a fast ball.

Reason: Increasing the time of catch reduces the force on the hands.

Answer key: 1-(A), 2-(A), 3-(A), 4-(C), 5-(A).

Quick Revision Summary

  • Only a net (unbalanced) force changes an object’s motion.
  • First law = inertia; second law: F = ma; third law: equal and opposite action–reaction.
  • Momentum p = mv; impulse F × t = change in momentum.
  • For a fixed force, acceleration is inversely related to mass (a = F/m).
  • Friction opposes relative motion; at constant velocity friction equals the applied force.
  • A curved position–time graph means acceleration (a net force).

Real-life Applications

Newton’s laws are everywhere: seat belts and air bags increase stopping time to lower the force in a crash, rockets and jet engines move by pushing gases backward (third law), a cricketer draws the hands back while catching, athletes land on soft mats, and rowing, swimming and walking all work by pushing against water or ground.

How to score full marks in this chapter

Convert all units before using F = ma, and keep the direction/sign of the force. For collisions and recoil, quote the correct Newton’s law. For graph questions, remember that a curved position–time graph means a net force, and read slopes carefully for velocity–time graphs.

Frequently Asked Questions

What is Class 9 Science Exploration Chapter 6 about?

How forces affect motion — net force, Newton’s three laws, inertia, momentum, F = ma, impulse, friction and action–reaction pairs.

What are Newton’s three laws of motion?

First: an object keeps its state of rest or uniform motion unless a net force acts. Second: F = ma. Third: every action has an equal and opposite reaction.

What is the difference between mass and inertia?

Mass is the quantity of matter in a body; inertia is its tendency to resist a change in motion. Greater mass means greater inertia.

Are these Class 9 Science Exploration Chapter 6 solutions free?

Yes. All solutions are free and follow the official NCERT Exploration textbook for 2026–27.

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