How Forces Affect Motion — Class 9 Science NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 9 Science chapter "How Forces Affect Motion" — 8 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 9 Science chapter "How Forces Affect Motion" — 8 important questions with detailed answers for CBSE board…
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NCERT Solutions for Class 9 Science — How Forces Affect Motion. Step-by-step answers to all 8 textbook questions from this chapter, written for CBSE board preparation and free to use.
How Forces Affect Motion — All 8 Questions Solved
Q1. State Newton's First Law of Motion.
An object at rest remains at rest, and an object in motion continues moving in a straight line at constant velocity unless acted upon by an external force. This is the law of inertia.
Q2. What is inertia? Give an example.
Inertia is the tendency of an object to resist changes in its state of motion or rest. A car suddenly stopping causes passengers to lurch forward (they tend to continue moving). Heavier objects have greater inertia.
Q3. State Newton's Second Law of Motion and write its formula.
The acceleration of an object is directly proportional to the applied force and inversely proportional to its mass.
Formula: F = ma
Where F = force (N), m = mass (kg), a = acceleration (m/s²)
Q4. A 2 kg mass experiences a force of 10 N. Find its acceleration.
Step 1: m = 2 kg, F = 10 N
Step 2: F = ma
Step 3: a = F/m = 10/2 = 5 m/s²
Q5. State Newton's Third Law of Motion.
For every action, there is an equal and opposite reaction. When object A exerts a force on object B, object B exerts an equal and opposite force on object A. Forces always occur in pairs.
Q6. Define momentum and write its formula.
Momentum is the product of mass and velocity of an object.
Formula: p = mv
Where p = momentum (kg m/s), m = mass (kg), v = velocity (m/s)
It is a vector quantity with direction.
Q7. Find the momentum of a 5 kg object moving at 8 m/s.
Step 1: m = 5 kg, v = 8 m/s
Step 2: p = mv = 5 × 8 = 40 kg m/s
Q8. State the Law of Conservation of Momentum.
In the absence of external forces, the total momentum of a system remains constant. For two colliding objects: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂
Where u = initial velocities, v = final velocities after collision.
How Forces Affect Motion — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. Which of the following is NOT an effect that a force can produce on an object?
- A. Change the speed of the object.
- B. Change the direction of motion of the object.
- C. Change the mass of the object.
- D. Change the shape or size of the object.
Answer: C. Change the mass of the object. — Forces can change an object's speed, direction, or shape, but they do not change its mass. Mass is an intrinsic property of an object.
MCQ 2. According to Newton's First Law of Motion, an object at rest will remain at rest, and an object in motion will continue in motion with the same speed and in the same direction, unless acted upon by a/an:
- A. Balanced force
- B. Unbalanced force
- C. Frictional force
- D. Gravitational force
Answer: B. Unbalanced force — Newton's First Law (Law of Inertia) states that an object's state of motion changes only when an external unbalanced force acts upon it.
MCQ 3. What is the SI unit of force?
- A. Joule
- B. Watt
- C. Newton
- D. Pascal
Answer: C. Newton — The SI unit of force is the Newton (N), named after Sir Isaac Newton for his contributions to classical mechanics.
MCQ 4. Inertia is the natural tendency of an object to:
- A. Accelerate when a force is applied.
- B. Resist changes in its state of motion.
- C. Exert a force on another object.
- D. Move in a circular path.
Answer: B. Resist changes in its state of motion. — Inertia is the property of an object to resist any change in its state of rest or uniform motion. The greater the mass, the greater the inertia.
MCQ 5. A force of 10 N acts on an object of mass 2 kg. What will be the acceleration produced?
- A. 0.2 m/s²
- B. 5 m/s²
- C. 10 m/s²
- D. 20 m/s²
Answer: B. 5 m/s² — According to Newton's Second Law, Force (F) = mass (m) × acceleration (a). So, a = F/m = 10 N / 2 kg = 5 m/s².
MCQ 6. Which of the following scenarios best demonstrates Newton's Third Law of Motion?
- A. A car speeding up when the accelerator is pressed.
- B. A book remaining on a table unless pushed.
- C. A rocket propelling upwards by expelling gases downwards.
- D. A ball rolling down a slope due to gravity.
Answer: C. A rocket propelling upwards by expelling gases downwards. — Newton's Third Law states that for every action, there is an equal and opposite reaction. The rocket pushing gases downwards (action) results in the gases pushing the rocket upwards (reaction).
MCQ 7. A body is moving with a constant velocity. What is the net force acting on the body?
- A. Non-zero and in the direction of motion.
- B. Non-zero and opposite to the direction of motion.
- C. Zero.
- D. Cannot be determined.
Answer: C. Zero. — According to Newton's First Law, if an object is moving with a constant velocity (zero acceleration), the net force acting on it must be zero.
MCQ 8. Momentum of an object is defined as the product of its:
- A. Force and acceleration
- B. Mass and velocity
- C. Mass and acceleration
- D. Force and displacement
Answer: B. Mass and velocity — Momentum (p) is a measure of the quantity of motion an object possesses, calculated as the product of its mass (m) and velocity (v): p = mv.
MCQ 9. A batsman hits a cricket ball, which then flies into the air. Which force is primarily responsible for bringing the ball back down to the ground?
- A. Muscular force
- B. Frictional force
- C. Gravitational force
- D. Magnetic force
Answer: C. Gravitational force — Gravitational force (gravity) is the attractive force between any two objects with mass, and it is responsible for pulling the ball back towards the Earth.
MCQ 10. Why does a passenger tend to fall backwards when a bus suddenly starts from rest?
- A. Due to the bus pulling the passenger backwards.
- B. Due to the inertia of rest of the passenger.
- C. Due to the inertia of motion of the passenger.
- D. Due to the frictional force between the passenger and the seat.
Answer: B. Due to the inertia of rest of the passenger. — The passenger's body tends to maintain its state of rest due to inertia, so when the bus starts moving forward, the lower part of the body moves with the bus, while the upper part lags behind, making the passenger fall backward.
MCQ 11. If the mass of an object is doubled, and the force applied to it is also doubled, how does its acceleration change?
- A. It quadruples.
- B. It doubles.
- C. It halves.
- D. It remains the same.
Answer: D. It remains the same. — According to F=ma, acceleration (a) = F/m. If F becomes 2F and m becomes 2m, then a_new = (2F)/(2m) = F/m. So, the acceleration remains the same.
MCQ 12. Which of the following is an example of an unbalanced force?
- A. A book lying still on a table.
- B. A car moving at a constant speed on a straight road.
- C. A ball accelerating as it rolls down a hill.
- D. Two teams pulling a rope in a tug-of-war, with the rope not moving.
Answer: C. A ball accelerating as it rolls down a hill. — An unbalanced force causes a change in an object's state of motion, such as acceleration (change in speed or direction). In the other cases, the net force is zero.
MCQ 13. A 5 kg object experiences a change in velocity from 2 m/s to 10 m/s in 4 seconds. What is the net force acting on the object?
- A. 10 N
- B. 50 N
- C. 2 N
- D. 1 N
Answer: A. 10 N — First, calculate acceleration (a) = (final velocity - initial velocity) / time = (10 - 2) m/s / 4 s = 8 m/s / 4 s = 2 m/s². Then, calculate force (F) = mass (m) × acceleration (a) = 5 kg × 2 m/s² = 10 N.
MCQ 14. The Law of Conservation of Momentum states that:
- A. Momentum is always converted into energy.
- B. The total momentum of an isolated system remains constant.
- C. Momentum can only be created, not destroyed.
- D. Momentum is conserved only in elastic collisions.
Answer: B. The total momentum of an isolated system remains constant. — The Law of Conservation of Momentum states that for an isolated system, the total momentum before a collision or interaction is equal to the total momentum after the collision or interaction.
MCQ 15. When a bullet is fired from a gun, the gun recoils. This phenomenon is best explained by:
- A. Newton's First Law of Motion
- B. Newton's Second Law of Motion
- C. Newton's Third Law of Motion
- D. Law of Conservation of Energy
Answer: C. Newton's Third Law of Motion — The firing of a bullet and the recoil of the gun is an example of action and reaction. The gun exerts a force on the bullet (action), and the bullet exerts an equal and opposite force on the gun (reaction), causing it to recoil, which is Newton's Third Law.
MCQ 16. Which of the following statements is true about friction?
- A. Friction always aids motion.
- B. Friction always opposes motion.
- C. Friction only acts on smooth surfaces.
- D. Friction is independent of the nature of the surfaces in contact.
Answer: B. Friction always opposes motion. — Friction is a force that opposes the relative motion or tendency of motion between two surfaces in contact.
MCQ 17. A force of 20 N produces an acceleration of 4 m/s² in an object. What is the mass of the object?
- A. 80 kg
- B. 5 kg
- C. 0.2 kg
- D. 24 kg
Answer: B. 5 kg — Using Newton's Second Law, F = ma, so m = F/a = 20 N / 4 m/s² = 5 kg.
MCQ 18. If an object is moving at a constant speed in a circular path, its velocity is:
- A. Constant.
- B. Changing.
- C. Zero.
- D. Cannot be determined.
Answer: B. Changing. — Velocity is a vector quantity, meaning it has both magnitude (speed) and direction. Even if the speed is constant, the direction of motion is continuously changing in a circular path, so the velocity is changing.
MCQ 19. Which of the following is not a contact force?
- A. Frictional force
- B. Muscular force
- C. Gravitational force
- D. Normal force
Answer: C. Gravitational force — Gravitational force is a non-contact force, acting even when objects are not physically touching. Frictional, muscular, and normal forces require direct contact.
MCQ 20. Two objects, A and B, are subjected to the same net force. If object A has twice the mass of object B, how does the acceleration of A compare to that of B?
- A. Acceleration of A is twice that of B.
- B. Acceleration of A is half that of B.
- C. Acceleration of A is the same as that of B.
- D. Acceleration of A is four times that of B.
Answer: B. Acceleration of A is half that of B. — From F=ma, a = F/m. If F is constant, acceleration is inversely proportional to mass. If mass A is 2m_B, then a_A = F/(2m_B) = (1/2) * (F/m_B) = (1/2) * a_B. So, acceleration of A is half that of B.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.