Step-by-step answers by SciNCERT Decoder by Kuldeep S. Take g = 9.8 m/s².
These How Forces Affect Motion NCERT Solutions give clear answers to every in-text question, activity, example and the Revise, Reflect, Refine questions of Class 9 Science Chapter 6. Each answer is short and shows the steps, so you can write it in the exam. To learn the theory first, read our How Forces Affect Motion Class 9 Notes. You can also open your NCERT Science textbook or find more chapters on our Class 9 study material page.
How Forces Affect Motion NCERT Solutions: Quick Answer Key
Use this table to check your numerical answers fast. The full steps are given below.
| Question | Answer |
|---|---|
| Example 6.4 (weightlifter) | 294 N upward |
| Example 6.5 (block) | (i) 0 m, (ii) 0.4 m |
| Example 6.6 (sports car) | 3000 N east, 0, 3000 N west |
| Example 6.8 (gun and bullet) | Gun 0.4 m/s², bullet 20 m/s² |
| Revise Q4 (snake boat) | 18,000 N forward |
| Revise Q6 (position–time graphs) | Object C |
| Revise Q11 (velocity–time graph) | 25 N |
| Revise Q12 (bullet) | 500 N |
| Revise Q13 (football) | 0.015 s |
| Revise Q14 (rough patch) | 10 m |
| Revise Q15 (tractor) | a = a₁a₂ ÷ (a₁ + a₂) |
Why does a canoe move forward when the canoeist pushes the water backwards with the paddle? Why does it move faster when the push is harder?
The same canoeist uses the same paddle force in an empty canoe and in a canoe with a passenger. Which moves faster?
The activities in these How Forces Affect Motion NCERT Solutions show what you should see and why.
| Activity | What you observe | Conclusion |
|---|---|---|
| 6.1 Coin stack on different surfaces | The stack slides farthest on polished marble and the shortest on the wooden table | Friction is smaller on smoother surfaces, so the velocity decreases more slowly |
| 6.2 Spring balance and a block | The reading is smallest on the surface where the coins went farthest | The reading gives an approximate measure of friction |
| 6.3 Cart and cup (double the cup’s mass) | The cart takes less time to cross the table | For the same mass, acceleration increases as force increases (a₁ ÷ a₂ = T₂² ÷ T₁²) |
| 6.4 Cart and cup (double the cart’s mass) | The cart takes more time | For the same force, acceleration decreases as mass increases |
| 6.5 Wheeled chair and table | Pushing the table away rolls you back. Pulling it towards you rolls you forward | The table pushes you with a force equal and opposite to yours |
| 6.6 Two spring balances hooked together | Both show the same reading every time | The two forces are equal in magnitude |
| 6.7 Balloon on a thread | Air rushes out backwards and the balloon moves forward along the thread | The air pushes the balloon opposite to its own motion (third law, like a rocket) |
List two forces acting on a barbell lifted by a weightlifter. Are they balanced when she holds it steady?
In an arm-wrestling match, are the forces of the two players balanced at the instant the arms tilt? If not, who exerts the larger force?
An object moves with constant velocity. Is a net force acting on it?
No net force acts on an object. Which are possible: (i) it stays at rest, (ii) it keeps moving with constant velocity, (iii) it moves with constant acceleration?
Real objects always have forces on them. Show with an example that the net force can still be zero.
A toy car of 100 g moves with a constant velocity of 0.5 m/s. What is the net force on it?
Two children of different masses sit on identical swings. For the same initial acceleration, which child needs the larger force?
How does bubble wrap or hay protect glass items during transport?
Why does a firefighter sometimes struggle to hold a hose pipe?
A spacecraft is in a region of space where gravity is negligible. How can it change its velocity?
Here are the solutions of the examples in the How Forces Affect Motion NCERT Solutions, written in simple steps.
Forces of 10 N and 6 N act on a block in three ways. Find the net force in each case.
(b) 10 N to the right, 6 N to the left: 10 − 6 = 4 N, to the right.
(c) 6 N to the right, 10 N to the left: 10 − 6 = 4 N, to the left.
A person pushes a moving box forward with a force equal to the friction on it. Will the box keep moving or stop?
Draw the position–time and velocity–time graphs of an object with no net force on it.
Object at rest: the position–time graph is a horizontal line (position does not change). The velocity–time graph is a horizontal line on the time axis (velocity = 0).
Object moving with constant velocity: the position–time graph is a straight, sloping line. The velocity–time graph is a horizontal line above the time axis.
A weightlifter holds a barbell with 10 kg fixed on each side of a 10 kg bar. What force does she apply to keep it steady?
A 25 kg block rests on a floor. The maximum friction is 50 N. Find the displacement in 2 s if it is pushed with (i) 50 N, (ii) 55 N.
(ii) Net force = 55 − 50 = 5 N. a = 5 ÷ 25 = 0.2 m/s². s = ut + ½at² = 0 + ½ × 0.2 × 2² = 0.4 m forward.
A 1500 kg sports car moves east with the velocity–time graph shown. Find the force in 0–5 s, 5–10 s and 10–15 s.
5–10 s: the graph is horizontal, so the velocity is constant. a = 0, so F = 0.
10–15 s: a = (0 − 10) ÷ 5 = −2 m/s². F = 1500 × (−2) = −3000 N, that is 3000 N towards the west (opposite to the motion).
The Earth and a fruit pull each other with equal forces. Why does only the fruit seem to move?
A 0.1 kg bullet is fired from a 5 kg gun with a force of 2 N. Find the initial accelerations of the bullet and the gun.
Gun: a = 2 ÷ 5 = 0.4 m/s².
Bullet: a = 2 ÷ 0.1 = 20 m/s².
The forces are equal, but the accelerations differ because the masses differ.
These are the end-of-chapter questions of the How Forces Affect Motion NCERT Solutions, with full steps.
A table is moved across the floor at constant velocity by a horizontal force F. What is the friction from the floor?
Choose the right word for a ball on a frictionless surface.
(ii) With a net force along the motion, the speed increases.
(iii) With a net force opposite to the motion, the speed decreases.
Block P has forces of 5 N and 4 N in opposite directions. Block Q moves with constant velocity. Which statement is correct?
In a snake boat, 95 oarsmen row forward and 5 row the wrong way. Each applies 200 N. Find the net force.
When a net force acts on an object, it accelerates…
Position–time graphs of four objects A, B, C and D are given. On which does a net force act?
A sailor jumps from a small boat to the shore. Does the boat move?
Why is a landing mat or sand bed used in high jump?
A loaded hand cart hits an identical empty hand cart. During the collision…
Plot the force–mass graph from the acceleration–mass graph given in the book.
Find the force on a 10 kg object from its velocity–time graph.
A 50 g bullet moving at 100 m/s stops after going 50 cm into a wooden block. Find the stopping force.
v² = u² + 2as gives 0 = 10,000 + 2 × a × 0.5, so a = −10,000 m/s².
F = ma = 0.05 × (−10,000) = −500 N. The stopping force is 500 N, opposite to the motion.
A footballer kicks a 0.4 kg ball to 108 km/h with a force of 800 N. Find the contact time.
A 2 kg object moving at 10 m/s meets a rough patch with friction 7 N, and an extra 3 N force also opposes the motion. How far does it go before it stops?
A tractor with force F gives acceleration a₁ to a harrow (m₁) and a₂ to a trolley (m₂). Find the acceleration if the harrow is placed on the trolley.
a = F ÷ (m₁ + m₂) = 1 ÷ (1 ÷ a₁ + 1 ÷ a₂) = a₁a₂ ÷ (a₁ + a₂).
A bar magnet and a compass needle exert equal forces on each other. Why does only the needle move?
Summary
You have now seen all the How Forces Affect Motion NCERT Solutions for Class 9 Science Chapter 6. Remember three ideas. Only a net force changes motion. F = ma links force, mass and acceleration. Action and reaction act on different objects. To revise the theory with diagrams, examples and MCQs, open our How Forces Affect Motion Class 9 Notes, and then continue with Work, Energy and Simple Machines.