Class 9 Science Important Questions and Answers: How Forces Affect Motion
Premium NCERT, Board Exam, Exemplar and Olympiad-level question answers for Class 9 Science. Learn force, inertia, momentum, Newton’s three laws of motion, law of conservation of momentum, impulse, action-reaction pairs and important numericals.
Short Introduction
Force is a push or pull that can change the state of rest, state of motion, speed, direction or shape of an object. The chapter How Forces Affect Motion explains how objects move under the effect of forces.
Newton’s laws of motion form the foundation of mechanics. These laws explain inertia, acceleration, force, momentum, action-reaction pairs and the law of conservation of momentum.
Exam focus: Newton’s laws, inertia, momentum, force formula, impulse, action-reaction examples, conservation of momentum and numerical problems are very important for CBSE exams.
Chapter Overview
1. Force
A force is a push or pull that can change motion, direction or shape of an object.
2. Inertia
Inertia is the tendency of an object to resist any change in its state of rest or motion.
3. First Law of Motion
An object remains at rest or in uniform motion unless acted upon by an external unbalanced force.
4. Second Law of Motion
The rate of change of momentum is directly proportional to the applied force.
5. Third Law of Motion
For every action, there is an equal and opposite reaction.
6. Conservation of Momentum
Total momentum of a system remains constant if no external force acts on it.
Important Keywords
Important Definitions and Formulae
Force
F = ma
Momentum
p = mv
Change in Momentum
Change in p = mv – mu
Force from Momentum
F = (mv – mu) ÷ t
Impulse
Impulse = Force × Time
Impulse-Momentum Relation
Impulse = Change in momentum
Momentum Conservation
m1u1 + m2u2 = m1v1 + m2v2
SI Unit of Force
newton or kg m/s2
SI Unit of Momentum
kg m/s
Symbols: F = force, m = mass, a = acceleration, p = momentum, u = initial velocity, v = final velocity, t = time.
Newton’s Laws of Motion at a Glance
Law of Inertia
An object remains in its state of rest or uniform motion in a straight line unless acted upon by an external unbalanced force.
Example: Passengers fall forward when a moving bus suddenly stops.
Law of Force and Acceleration
The rate of change of momentum of an object is directly proportional to the applied unbalanced force and takes place in the direction of force.
Formula: F = ma
Law of Action and Reaction
To every action, there is an equal and opposite reaction.
Example: A gun recoils backward when a bullet is fired forward.
Important Very Short Answer Questions
Q1. Define force.
Answer: Force is a push or pull that can change the state of rest, motion, speed, direction or shape of an object.
Q2. What is the SI unit of force?
Answer: The SI unit of force is newton, written as N.
Q3. Define one newton force.
Answer: One newton is the force required to produce an acceleration of 1 m/s2 in a body of mass 1 kg.
Q4. What is inertia?
Answer: Inertia is the tendency of a body to resist any change in its state of rest or uniform motion.
Q5. Which property of a body measures its inertia?
Answer: Mass measures the inertia of a body.
Q6. State Newton’s first law of motion.
Answer: A body remains at rest or in uniform motion in a straight line unless acted upon by an external unbalanced force.
Q7. State Newton’s second law of motion.
Answer: The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction of force.
Q8. State Newton’s third law of motion.
Answer: For every action, there is an equal and opposite reaction.
Q9. Define momentum.
Answer: Momentum is the product of mass and velocity of a body. It is represented by p = mv.
Q10. What is the SI unit of momentum?
Answer: The SI unit of momentum is kg m/s.
Q11. Define impulse.
Answer: Impulse is the product of force and time for which the force acts. It is equal to change in momentum.
Q12. What is the law of conservation of momentum?
Answer: If no external force acts on a system, the total momentum of the system remains constant.
Q13. What are balanced forces?
Answer: Balanced forces are equal and opposite forces acting on a body that do not change its state of motion.
Q14. What are unbalanced forces?
Answer: Unbalanced forces are forces that change the state of rest or motion of a body.
Q15. Why does a gun recoil when a bullet is fired?
Answer: A gun recoils due to Newton’s third law of motion and conservation of momentum.
Short Answer Questions
Q1. Differentiate between balanced and unbalanced forces.
Answer:
| Balanced Forces | Unbalanced Forces |
|---|---|
| They are equal in magnitude and opposite in direction. | They are not equal in magnitude or do not cancel each other. |
| They do not change the state of motion of a body. | They can change the state of rest or motion of a body. |
| Net force is zero. | Net force is not zero. |
| Example: A book resting on a table. | Example: A football moving after being kicked. |
Q2. Differentiate between mass and weight.
Answer:
| Mass | Weight |
|---|---|
| Mass is the amount of matter present in a body. | Weight is the force with which Earth attracts a body. |
| It remains constant everywhere. | It changes from place to place. |
| Its SI unit is kilogram. | Its SI unit is newton. |
| It is a scalar quantity. | It is a vector quantity. |
Q3. Explain inertia of rest with an example.
Answer: Inertia of rest is the tendency of a body to remain at rest. Example: When a bus suddenly starts, passengers fall backward because their lower body moves with the bus but the upper body tends to remain at rest.
Q4. Explain inertia of motion with an example.
Answer: Inertia of motion is the tendency of a moving body to continue moving. Example: When a moving bus suddenly stops, passengers fall forward because their bodies tend to remain in motion.
Q5. Explain inertia of direction with an example.
Answer: Inertia of direction is the tendency of a body to resist change in direction. Example: When a car turns suddenly, passengers are pushed sideways due to inertia of direction.
Q6. Why is it difficult to stop a moving truck than a moving bicycle moving with the same speed?
Answer: A truck has more mass than a bicycle. Since momentum = mass × velocity, the truck has greater momentum. Therefore, more force is required to stop it.
Q7. Why does a cricket player move his hands backward while catching a fast ball?
Answer: The player moves his hands backward to increase the time taken to stop the ball. Since force = change in momentum ÷ time, increasing time reduces the force on the hands.
Q8. Why are seat belts used in cars?
Answer: Seat belts protect passengers from falling forward when a moving car stops suddenly. They reduce the effect of inertia of motion and prevent injury.
Q9. Why do athletes run some distance before taking a long jump?
Answer: Athletes run before jumping to gain momentum. Greater momentum helps them cover a longer distance during the jump.
Q10. Why does a boat move backward when a person jumps out of it?
Answer: When a person jumps forward, he applies a backward force on the boat. According to Newton’s third law, the boat moves backward due to the reaction force.
Long Answer Questions
Q1. Explain Newton’s first law of motion with examples.
Answer: Newton’s first law states that an object remains in its state of rest or uniform motion in a straight line unless acted upon by an external unbalanced force.
Explanation:
- If a body is at rest, it will remain at rest unless an external force acts on it.
- If a body is moving with uniform velocity, it will continue moving unless an external force changes its motion.
- This law is also called the law of inertia.
Examples:
- Passengers fall backward when a bus suddenly starts.
- Passengers fall forward when a moving bus suddenly stops.
- Dust comes out of a carpet when it is beaten with a stick.
Q2. Explain Newton’s second law of motion and derive F = ma.
Answer: Newton’s second law states that the rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction of force.
Let mass of body = m
Initial velocity = u
Final velocity = v
Time taken = t
Initial momentum = mu
Final momentum = mv
Change in momentum = mv – mu = m(v – u)
Rate of change of momentum = m(v – u) ÷ t
Since acceleration a = (v – u) ÷ t,
Force F = ma
Therefore, F = ma.
Q3. Explain Newton’s third law of motion with examples.
Answer: Newton’s third law states that for every action, there is an equal and opposite reaction.
Important points:
- Action and reaction forces are equal in magnitude.
- They act in opposite directions.
- They act on two different bodies.
- They occur simultaneously.
Examples:
- When we walk, our feet push the ground backward and the ground pushes us forward.
- A gun recoils backward when a bullet is fired forward.
- A swimmer pushes water backward and water pushes the swimmer forward.
- A rocket moves upward because gases are pushed downward.
Q4. Explain the law of conservation of momentum.
Answer: The law of conservation of momentum states that when no external force acts on a system, the total momentum of the system remains constant.
Mathematical form:
Initial momentum = Final momentum
m1u1 + m2u2 = m1v1 + m2v2
Example:
- When a gun fires a bullet, the bullet moves forward and the gun recoils backward.
- Before firing, total momentum is zero.
- After firing, forward momentum of bullet is equal and opposite to backward momentum of gun.
- Hence, total momentum remains conserved.
Q5. Explain action and reaction forces. Why do they not cancel each other?
Answer:
- Action and reaction forces are equal and opposite forces described by Newton’s third law.
- They act on two different bodies.
- Forces cancel only when they act on the same body in opposite directions.
- Since action and reaction act on different bodies, they do not cancel each other.
Example: When a person pushes a wall, the person applies force on the wall and the wall applies an equal and opposite force on the person.
Important Numericals with Solutions
Numerical 1: A force of 20 N acts on a body of mass 5 kg. Find acceleration.
Solution:
Force, F = 20 N
Mass, m = 5 kg
F = ma
a = F ÷ m = 20 ÷ 5 = 4 m/s2
Numerical 2: A body of mass 10 kg moves with acceleration 3 m/s2. Find the force acting on it.
Solution:
Mass, m = 10 kg
Acceleration, a = 3 m/s2
F = ma
F = 10 × 3 = 30 N
Numerical 3: Find the momentum of a body of mass 8 kg moving with velocity 5 m/s.
Solution:
Mass, m = 8 kg
Velocity, v = 5 m/s
Momentum, p = mv
p = 8 × 5 = 40 kg m/s
Numerical 4: A body of mass 2 kg changes its velocity from 3 m/s to 7 m/s. Find the change in momentum.
Solution:
Mass, m = 2 kg
Initial velocity, u = 3 m/s
Final velocity, v = 7 m/s
Change in momentum = mv – mu = m(v – u)
= 2(7 – 3) = 2 × 4 = 8 kg m/s
Numerical 5: A force changes the velocity of a 4 kg body from 2 m/s to 10 m/s in 4 s. Find the force.
Solution:
m = 4 kg, u = 2 m/s, v = 10 m/s, t = 4 s
Acceleration, a = (v – u) ÷ t
a = (10 – 2) ÷ 4 = 8 ÷ 4 = 2 m/s2
F = ma = 4 × 2 = 8 N
Numerical 6: A force of 50 N acts on a body for 3 s. Find impulse.
Solution:
Force, F = 50 N
Time, t = 3 s
Impulse = F × t
Impulse = 50 × 3 = 150 N s
Numerical 7: A ball of mass 0.5 kg moving at 20 m/s is brought to rest in 0.1 s. Find the force applied.
Solution:
m = 0.5 kg, u = 20 m/s, v = 0 m/s, t = 0.1 s
F = m(v – u) ÷ t
F = 0.5(0 – 20) ÷ 0.1
F = -10 ÷ 0.1 = -100 N
Answer: Force = 100 N opposite to the direction of motion.
Numerical 8: A bullet of mass 0.02 kg is fired with velocity 300 m/s. Find its momentum.
Solution:
m = 0.02 kg
v = 300 m/s
p = mv
p = 0.02 × 300 = 6 kg m/s
Numerical 9: A gun of mass 5 kg fires a bullet of mass 0.05 kg with velocity 200 m/s. Find recoil velocity of the gun.
Solution:
Initial total momentum = 0
Mass of bullet, m = 0.05 kg
Velocity of bullet, v = 200 m/s
Mass of gun, M = 5 kg
Let recoil velocity of gun = V
Momentum of bullet + Momentum of gun = 0
0.05 × 200 + 5V = 0
10 + 5V = 0
V = -2 m/s
Answer: Recoil velocity of gun = 2 m/s backward.
Numerical 10: A 2 kg object moving at 6 m/s collides with a stationary 4 kg object. After collision, both move together. Find their common velocity.
Solution:
m1 = 2 kg, u1 = 6 m/s
m2 = 4 kg, u2 = 0 m/s
Let common velocity after collision = v
Initial momentum = Final momentum
m1u1 + m2u2 = (m1 + m2)v
2 × 6 + 4 × 0 = (2 + 4)v
12 = 6v
v = 2 m/s
Numerical 11: A 60 kg person jumps from a boat of mass 120 kg with velocity 4 m/s. Find the recoil velocity of the boat.
Solution:
Initial total momentum = 0
Mass of person = 60 kg
Velocity of person = 4 m/s
Mass of boat = 120 kg
Let recoil velocity of boat = V
60 × 4 + 120V = 0
240 + 120V = 0
V = -2 m/s
Answer: Boat moves backward with velocity 2 m/s.
Numerical 12: A force of 100 N acts on a body and changes its momentum by 500 kg m/s. Find the time for which force acts.
Solution:
Force, F = 100 N
Change in momentum = 500 kg m/s
Force = Change in momentum ÷ Time
Time = Change in momentum ÷ Force
t = 500 ÷ 100 = 5 s
Case-Study Based Questions
Case Study 1: Sudden Braking of a Bus
A bus is moving at a high speed. Suddenly, the driver applies brakes. The passengers sitting inside the bus are pushed forward due to the sudden stopping of the bus.
Q1. Which law of motion explains this situation?
Answer: Newton’s first law of motion explains this situation.
Q2. Which type of inertia is involved?
Answer: Inertia of motion is involved.
Q3. Why do passengers fall forward?
Answer: Their lower body stops with the bus, but the upper body tends to continue moving forward due to inertia.
Q4. Which safety device helps reduce injury in such cases?
Answer: Seat belts help reduce injury.
Case Study 2: Catching a Cricket Ball
A cricket player catches a fast-moving ball by moving his hands backward. This increases the time taken to stop the ball and reduces the force experienced by the hands.
Q1. Which law of motion is used here?
Answer: Newton’s second law of motion is used here.
Q2. What happens when stopping time is increased?
Answer: The force experienced by the hands decreases.
Q3. Which quantity changes when the ball is stopped?
Answer: Momentum of the ball changes.
Q4. Write the relation between force and change in momentum.
Answer: Force = Change in momentum ÷ Time.
Case Study 3: Recoil of a Gun
When a bullet is fired from a gun, the bullet moves forward with high velocity and the gun recoils backward. Before firing, both bullet and gun are at rest.
Q1. Which law explains recoil of a gun?
Answer: Newton’s third law of motion and conservation of momentum explain recoil of a gun.
Q2. What is the total momentum before firing?
Answer: Total momentum before firing is zero.
Q3. Why does the gun move backward?
Answer: The bullet moves forward and the gun gets an equal and opposite momentum backward.
Q4. Why is recoil velocity of the gun smaller than the velocity of the bullet?
Answer: The gun has much greater mass than the bullet, so its recoil velocity is smaller.
Assertion-Reason Questions
Choose the correct option:
A. Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
B. Both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
C. Assertion is true but Reason is false.
D. Assertion is false but Reason is true.
Q1. Assertion: Newton’s first law is also called the law of inertia.
Reason: It explains the tendency of a body to resist change in its state of rest or motion.
Answer: A. Both are true and Reason correctly explains Assertion.
Q2. Assertion: A heavy truck has more inertia than a small car.
Reason: Inertia depends on mass.
Answer: A. Both are true and Reason correctly explains Assertion.
Q3. Assertion: Action and reaction forces cancel each other.
Reason: Action and reaction forces act on the same body.
Answer: Both Assertion and Reason are false. Action and reaction forces act on different bodies, so they do not cancel each other.
Q4. Assertion: Momentum depends on both mass and velocity.
Reason: Momentum is given by p = mv.
Answer: A. Both are true and Reason correctly explains Assertion.
Q5. Assertion: A cricket player moves his hands backward while catching a ball.
Reason: Increasing the time of stopping reduces the force on the hands.
Answer: A. Both are true and Reason correctly explains Assertion.
Q6. Assertion: Conservation of momentum is valid when no external force acts on the system.
Reason: External force can change the total momentum of a system.
Answer: A. Both are true and Reason correctly explains Assertion.
Exam Tips
- Write Newton’s laws in exact CBSE-style language.
- Always mention that Newton’s first law is the law of inertia.
- Remember that mass is a measure of inertia.
- In second law questions, use the phrase “rate of change of momentum”.
- For numericals, write given values before applying formula.
- Use SI units: force in newton, mass in kg, velocity in m/s and momentum in kg m/s.
- In third law examples, clearly identify action and reaction pairs.
- Remember that action and reaction forces act on different bodies.
- For conservation of momentum, write initial momentum = final momentum.
- Use negative sign to show opposite direction in recoil and collision numericals.
Quick Revision Box
Force
A push or pull that can change rest, motion, direction or shape.
Inertia
Tendency of a body to resist change in its state of rest or motion.
Mass
Measure of inertia of a body.
First Law
Object remains at rest or in uniform motion unless external force acts.
Second Law
Force is proportional to rate of change of momentum.
Third Law
Every action has an equal and opposite reaction.
Momentum
Product of mass and velocity: p = mv.
Impulse
Product of force and time; equal to change in momentum.
Conservation of Momentum
Total momentum remains constant if no external force acts.
Balanced Force
Net force is zero and motion does not change.
Unbalanced Force
Net force is not zero and motion can change.
Action-Reaction
Equal and opposite forces acting on different bodies.
FAQ Section
What is the main focus of How Forces Affect Motion?
The main focus is force, inertia, Newton’s laws of motion, momentum, impulse and law of conservation of momentum.
What is Newton’s first law of motion?
Newton’s first law states that a body remains at rest or in uniform motion in a straight line unless acted upon by an external unbalanced force.
Why is Newton’s first law called the law of inertia?
It is called the law of inertia because it describes the tendency of a body to resist change in its state of rest or motion.
What is Newton’s second law of motion?
Newton’s second law states that the rate of change of momentum is directly proportional to the applied force and takes place in the direction of force.
What is the formula of force?
The formula of force is F = ma, where m is mass and a is acceleration.
What is Newton’s third law of motion?
Newton’s third law states that for every action, there is an equal and opposite reaction.
What is momentum?
Momentum is the product of mass and velocity of a body. It is given by p = mv.
What is the SI unit of momentum?
The SI unit of momentum is kg m/s.
What is impulse?
Impulse is the product of force and time. It is equal to change in momentum.
What is the law of conservation of momentum?
The law states that if no external force acts on a system, the total momentum of the system remains constant.
Why does a gun recoil after firing?
A gun recoils because the bullet moves forward and the gun gets equal and opposite momentum backward due to conservation of momentum.
Why does a cricket player move his hands backward while catching a ball?
Moving hands backward increases the stopping time and reduces the force experienced by the hands.
Final Conclusion
How Forces Affect Motion is one of the most important Class 9 Physics chapters because it explains how force changes motion. Newton’s three laws of motion help us understand daily-life examples such as walking, catching a ball, recoil of a gun, sudden braking of vehicles and rocket motion.
To score well in CBSE exams, students should learn definitions, write Newton’s laws accurately, practise numericals based on F = ma and p = mv, and understand the law of conservation of momentum with examples.
Leave a Reply