This MCQ module is based on: How Forces Affect Motion — NCERT Exercises
TOPIC 22 OF 50
How Forces Affect Motion — NCERT Exercises
🎓 Class 9
Science
CBSE
Theory
Ch 6 — How Forces Affect Motion
⏱ ~13 min
🌐 Language: [gtranslate]
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This assessment will be based on: How Forces Affect Motion — NCERT Exercises
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Chapter Summary
Chapter 6 ties together the everyday experience of pushing, pulling, and watching things move with three universal laws stated by Newton. Below are the takeaway ideas you must hold securely for the board exam.
ForceA push or pull that can change the state of rest, state of motion, speed, direction, or shape of a body. SI unit: newton.
Types of forceContact (muscular, friction, normal, tension); non-contact (gravitational, electrostatic, magnetic).
Balanced vs unbalancedBalanced ⇒ net force = 0 ⇒ no change in motion (only deformation possible). Unbalanced ⇒ change in motion.
Newton's First LawA body retains its state of rest or uniform motion unless an unbalanced external force acts on it. Mass measures inertia.
Newton's Second Law\(F = ma\); also \(F = \Delta p / t\). Direction of acceleration matches direction of net force.
Newton's Third LawEvery action has an equal and opposite reaction. The two forces act on different bodies.
Momentum\(p = mv\), kg·m/s, vector. Heavier or faster ⇒ more momentum.
Conservation of momentumFor an isolated system: total momentum before = total momentum after. Basis of recoil and rocket propulsion.
Keyword Grid
ForcePush or pull producing change.
Net ForceVector sum of all forces on a body.
Balanced ForcesCancel out — no motion change.
Unbalanced ForcesNet force ≠ 0 — motion changes.
InertiaResistance to change of motion.
MassQuantitative measure of inertia.
Acceleration\(a = (v-u)/t\), unit m/s².
Momentum\(p = mv\), unit kg·m/s.
Impulse\(F t = \Delta p\).
Action–ReactionEqual-opposite force pair.
RecoilBackward motion of gun on firing.
ConservationTotal \(p\) constant in isolated system.
NCERT Exercises — Step-by-Step Solutions
Click "Show Solution" to view the worked answer. Try each problem yourself before peeking.
Q 1 Define force. State its SI unit.
A force is a push or pull on a body that may change its state of rest, its state of uniform motion, its direction, or its shape.SI unit: newton (N), where \(1 \text{ N} = 1 \text{ kg} \cdot \text{m/s}^2\).
Q 2 Distinguish between balanced and unbalanced forces with one example each.
Balanced: equal forces in opposite directions sum to zero, so motion does not change. Example — a book on a table (weight balanced by normal force).Unbalanced: the forces do not cancel; net force ≠ 0 and motion changes. Example — pushing a stationary trolley harder than friction so it starts moving.
Q 3 State Newton's First Law of Motion. Why is it called the law of inertia?
A body remains at rest, or moves uniformly in a straight line, unless acted upon by an unbalanced external force.It is called the law of inertia because it expresses the natural tendency of bodies to resist any change in their state of motion — that resistance is inertia.
Q 4 When a stationary bus suddenly starts, passengers fall backward. Explain.
The lower body of the passenger, in contact with the bus floor, accelerates with the bus. The upper body, having no direct force pushing it forward, tends to remain at rest due to inertia of rest.Result: the upper body lags behind the lower, and the passenger is thrown backward.
Q 5 A force of 5 N gives a body an acceleration of 10 m/s². Find its mass.
Given: \(F = 5\) N, \(a = 10\) m/s².\(m = F/a = 5/10 = 0.5\) kg.Answer: Mass = 0.5 kg.
Q 6 A 50 g bullet leaves a 5 kg gun at 350 m/s. Find the recoil velocity of the gun.
Initial momentum of (gun + bullet) = 0.After firing: \(0.05 \times 350 + 5 \times v = 0\)\(v = -17.5/5 = -3.5\) m/s.Answer: The gun recoils with a velocity of 3.5 m/s opposite to the bullet.
Q 7 A car of mass 1200 kg accelerates from rest to 20 m/s in 8 s. Find the force exerted by the engine.
\(a = (20 - 0)/8 = 2.5\) m/s².\(F = ma = 1200 \times 2.5 = 3000\) N.
Q 8 Why is it dangerous to jump out of a moving bus?
A passenger sitting in a moving bus shares its forward velocity. On jumping out, the feet hit the ground and stop suddenly, but the upper body continues moving forward due to inertia of motion. This sudden imbalance can throw the person down and cause injury. The recommended way to alight is to run a few steps in the direction of motion so that the body decelerates gradually.
Q 9 State Newton's Third Law of Motion and give two examples.
Statement: To every action there is an equal and opposite reaction; the two forces act on different bodies.Examples: (i) A swimmer pushes water backward (action) and is pushed forward by the water (reaction). (ii) A rocket expels gases downward (action) and is propelled upward by the gases (reaction).
Q 10 Define momentum. State its SI unit. A 2 kg ball moves at 4 m/s — find its momentum.
Momentum is the product of the mass and velocity of a body: \(p = mv\). It is a vector along the velocity. SI unit: kg·m/s.For the ball: \(p = 2 \times 4 = 8\) kg·m/s.
Q 11 A truck of mass 3000 kg moving at 4 m/s strikes a stationary car of mass 1000 kg. They lock together. Find their common velocity after collision.
Conservation of momentum: \(m_1 u_1 + m_2 u_2 = (m_1+m_2)v\).\(3000(4) + 1000(0) = 4000 v\)\(v = 12000/4000 = 3\) m/s.
Q 12 Why does a cricketer pull his hands backward while catching a fast ball?
The change in momentum \(\Delta p\) of the ball is fixed by its mass and incoming velocity. By \(F = \Delta p/t\), increasing the time \(t\) for which the ball decelerates reduces the average force on the hands.Pulling the hands back stretches \(t\), so \(F\) becomes smaller, and the cricketer avoids injury.
Q 13 A force of 20 N acts on a 4 kg trolley for 3 s. Find (a) acceleration, (b) final velocity if it started from rest, (c) change in momentum.
(a) \(a = F/m = 20/4 = 5\) m/s².(b) \(v = u + at = 0 + 5 \times 3 = 15\) m/s.(c) \(\Delta p = m(v-u) = 4 \times 15 = 60\) kg·m/s. (Or directly: \(\Delta p = F t = 20 \times 3 = 60\) kg·m/s — same answer.)
Q 14 Two bodies of masses 4 kg and 6 kg moving at 5 m/s and 3 m/s in the same direction collide. After collision, the 4 kg body moves at 2 m/s. Find the velocity of the 6 kg body.
Conservation of momentum: \(m_1 u_1 + m_2 u_2 = m_1 v_1 + m_2 v_2\).\(4(5) + 6(3) = 4(2) + 6 v_2\)\(20 + 18 = 8 + 6 v_2\) ⇒ \(6 v_2 = 30\) ⇒ \(v_2 = 5\) m/s.Answer: The 6 kg body moves at 5 m/s in the original direction.
Q 15 A motorbike of mass 150 kg is moving at 90 km/h. The rider applies brakes and stops the bike in 5 s. Find (a) deceleration and (b) the braking force.
Convert: \(u = 90 \times 5/18 = 25\) m/s, \(v = 0\), \(t = 5\) s.(a) \(a = (0 - 25)/5 = -5\) m/s². Magnitude of deceleration = 5 m/s².(b) \(F = ma = 150 \times 5 = 750\) N (opposite to motion).
Exam-prep tip: Always write the formula first, then substitute units, then compute the number. Carry direction signs (±) when working with vectors such as velocity or momentum, and double-check that both sides of any conservation equation use the same direction convention.
Frequently Asked Questions — NCERT Exercises & Intext Questions
How do I solve NCERT Class 9 Science Chapter 6 (How Forces Affect Motion) exercise questions for the CBSE board exam?
Solve NCERT Chapter 6 — How Forces Affect Motion — exercise questions by first reading the question carefully, writing down the given data, recalling the relevant concepts like force, Newton's laws, inertia, and applying them step by step. This Part 4 covers every intext and end-of-chapter exercise from the NCERT textbook. Write balanced equations, label diagrams clearly and show each step — CBSE Class 9 examiners award step marks even if the final answer has a small slip. Practising these solutions strengthens conceptual clarity and builds speed for both the school exam and the upcoming Class 10 board exam.
Are the NCERT intext questions from How Forces Affect Motion important for the Class 9 Science exam?
Yes, NCERT intext questions for Chapter 6 How Forces Affect Motion are highly important for the CBSE Class 9 Science exam. Many questions in school and competitive papers are directly lifted or only slightly modified from these intext questions, and they test the foundational concepts — force, Newton's laws, inertia — that chapter-end questions and the Class 10 board build on. Attempt every intext question first, then move on to the exercises. This practice ensures complete NCERT coverage, which is the CBSE syllabus's primary source.
What types of questions from How Forces Affect Motion are asked in the Class 9 Science exam?
The Class 9 Science paper (CBSE pattern) asks a mix of question types from How Forces Affect Motion: 1-mark MCQ and assertion-reason, 2-mark short answers, 3-mark explanations, 5-mark long answers with diagrams or derivations, and 4-mark competency-based / case-study questions. These test understanding of force, Newton's laws, inertia, momentum. Practising every NCERT exercise and intext question prepares you to answer all of these formats with confidence.
How many marks does Chapter 6 — How Forces Affect Motion — typically carry in the Class 9 Science paper?
Chapter 6 — How Forces Affect Motion — is part of the CBSE Class 9 Science syllabus and typically contributes 5–9 marks in the annual paper, depending on the year's weightage. Questions are drawn from definitions, reasoning, numerical/descriptive problems and diagrams on topics like force, Newton's laws, inertia. Solving the NCERT exercises in this part is essential because CBSE directly references the NCERT Exploration textbook for question design.
Where can I find step-by-step NCERT solutions for Chapter 6 How Forces Affect Motion Class 9 Science?
You can find complete, step-by-step NCERT solutions for Chapter 6 How Forces Affect Motion Class 9 Science on MyAiSchool. Every intext and end-of-chapter exercise question is solved with full working, labelled diagrams and CBSE-aligned mark distribution. Solutions highlight key points about force, Newton's laws, inertia that examiners look for. This makes revision quick and exam-focused for Class 9 CBSE students.
What is the best way to revise How Forces Affect Motion for the Class 9 Science exam?
The best way to revise How Forces Affect Motion for the CBSE Class 9 Science exam is a three-pass approach. First pass: skim the chapter and note down key terms like force, Newton's laws, inertia in a one-page mind map. Second pass: solve every NCERT intext and exercise question without looking at the solution, then self-check. Third pass: attempt sample papers and competency-based questions under timed conditions. This structured revision secures full marks for this chapter.
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