Case Based Questions: Force and Laws of Motion | Science Class 9 PDF Download

Q1. Read the following passage and answer the questions that follow:

i. What type of force is preventing the box from moving when a small force is applied? (1 mark)
ii. Explain why the box starts moving when the children push harder. (2 mark)
iii. How does this scenario relate to Newton’s First Law of Motion? (1 mark)

Ans:
i. The force preventing the motion of the box is friction, which acts in the opposite direction of the applied force.
ii. The box starts moving because when the applied force becomes greater than the force of friction, there is an unbalanced force acting on the box, which changes its state of rest to motion.
iii. This scenario illustrates Newton’s First Law of Motion, which states that an object remains in its state of rest or uniform motion unless acted upon by an unbalanced external force. Initially, the forces were balanced, but once the applied force exceeded friction, motion began.

Q2. Read the following passage and answer the questions that follow:

i. What is inertia? (1 mark)
ii. Explain why a passenger moves forward when a bus suddenly stops. (2 mark)
iii. How does wearing a seatbelt in a car help in such situations? (1 mark)

Ans:
i. Inertia is the tendency of an object to resist any change in its state of motion or rest.
ii. When a bus suddenly stops, the lower part of the passenger’s body comes to rest with the bus, but the upper part of the body continues to move forward due to inertia of motion, making the passenger lurch forward.
iii. A seatbelt applies a force that prevents the passenger from moving forward suddenly, reducing the risk of injury by counteracting the effects of inertia.

Q3. Read the following passage and answer the questions that follow:

i. What physical quantity is defined as the product of mass and velocity? (1 mark)
ii. How does pulling back hands while catching a ball reduce the impact force? (2 mark)
iii. How does this principle apply to airbags in vehicles? (1 mark)

Ans:
i. Momentum is defined as the product of mass and velocity.
ii. Pulling back the hands increases the time over which the momentum of the ball is reduced to zero, thereby reducing the rate of change of momentum and hence the force of impact.
iii. Airbags in cars function similarly by increasing the time of impact in a collision, reducing the force exerted on passengers and minimizing injuries.

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Q4. Read the following passage and answer the questions that follow:

i. State Newton’s Third Law of Motion. (1 mark)
ii. Why does a gun recoil when a bullet is fired? (2 mark)
iii. Explain how this principle is applied in rocket propulsion. (1 mark)

Ans:
i. Newton’s Third Law of Motion states: For every action, there is an equal and opposite reaction.
ii. When a bullet is fired, the gun exerts a forward force on the bullet, and the bullet exerts an equal and opposite force on the gun, causing it to recoil.
iii. Rocket propulsion works on the same principle. The rocket expels hot gases downward, and as a reaction, the rocket moves upward with an equal force in the opposite direction.

Q5. Read the following passage and answer the questions that follow:

i. What is the equation used to calculate force using Newton’s Second Law? (1 mark)
ii. Calculate the force needed to stop the car. (2 mark)
iii. How does this concept explain the importance of applying brakes gradually instead of suddenly? (1 mark)

Ans:
i. The force is given by F = ma, where m is mass and a is acceleration.
ii. Given:
Mass, m = 1000 kg
Initial velocity, u = 30 m/s
Final velocity, v = 0 m/s
Time, t = 5 s
Acceleration, a = (v - u) / t = (0 - 30) / 5 = -6 m/s²
Force, F = ma = (1000 × -6) = -6000 N (negative sign indicates deceleration).

iii. If brakes are applied gradually, the stopping time increases, reducing the acceleration and thus minimizing the force on passengers, preventing injuries.

Q6. Read the following passage and answer the questions that follow:

i. What happens to the velocity of a marble rolling up an inclined plane? (1 mark)
ii. How did Galileo’s experiment lead to the First Law of Motion? (2 mark)
iii. In real life, why does a rolling ball eventually stop even on a smooth surface? (1 mark)

Ans:
i. The velocity of a marble decreases when rolling up an inclined plane due to the force of gravity acting against its motion.
ii. Galileo’s experiment showed that in the absence of external forces (like friction), an object in motion continues moving at a constant velocity. This principle was later formulated as Newton’s First Law of Motion, stating that an object remains in motion unless acted upon by an external force.
iii. In real life, a rolling ball eventually stops due to friction and air resistance, which act as external forces slowing it down.

Q7. Read the following passage and answer the questions that follow:

i. What is inertia? (1 mark)
ii. Why does the stone require more force than the rubber ball to move? (2 mark)
iii. How does the concept of inertia explain why it is harder to push a truck than a bicycle? (1 mark)

Ans:
i. Inertia is the property of an object to resist any change in its state of motion or rest.
ii. The stone requires more force than the rubber ball because inertia is directly proportional to mass. Since the stone has more mass, it has greater inertia, making it harder to move.
iii. A truck has much more mass than a bicycle, meaning it has greater inertia and requires a larger force to overcome its resistance to motion.

Q8. Read the following passage and answer the questions that follow:

i. Define momentum. (1 mark)
ii. How does Newton’s Second Law explain the changes in the ball’s velocity in this situation? (2 mark)
iii. Why does a heavier football require more force to accelerate than a lighter one? (1 mark)

Ans:
i. Momentum (p) is the product of an object’s mass and velocity, given by the formula p = mv.
ii. According to Newton’s Second Law, the force applied to the ball causes a change in its momentum. When a player kicks the ball, they exert a force, leading to a change in its velocity. The goalkeeper also applies a force in the opposite direction to stop the ball, reducing its momentum to zero.
iii. A heavier football has a greater mass, so according to F = ma, a larger force is required to give it the same acceleration as a lighter ball.

Q9. Read the following passage and answer the questions that follow:

i. State Newton’s Third Law of Motion. (1 mark)
ii. Why does the gun move backward with less velocity than the bullet? (2 mark)
iii. How is this principle applied in rocket propulsion? (1 mark)

Ans:
i. Newton’s Third Law of Motion states: For every action, there is an equal and opposite reaction.
ii. The gun moves backward with less velocity because its mass is much greater than that of the bullet. Since momentum (p) = mv, the gun’s recoil velocity is small to balance the large velocity of the bullet, keeping the total momentum conserved.
iii. Rockets work on the same principle: they expel exhaust gases downward at high velocity, and in reaction, the rocket moves upward with an equal and opposite force.

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Q10. Read the following passage and answer the questions that follow:

i. What equation relates force, mass, and acceleration? (1 mark)

ii. Calculate the acceleration of the truck. (1 mark)

iii. How does increasing the mass of a vehicle affect the force needed to accelerate it? (1 mark)

Ans:
i. The equation is F = ma, where F is force, m is mass, and a is acceleration.
ii. Using the equation s = ut + (1/2) at², where:
Initial velocity u = 0 (since the truck starts from rest)
Distance s = 400 m
Time t = 20 s
Solving for acceleration:
400 = 0 + (1/2) a (20)²
400 = 200a
a = 2 m/s²
iii. Increasing the mass of a vehicle increases inertia, meaning a larger force is required to accelerate it, as per Newton’s Second Law (F = ma).

Q11. Read the following passage and answer the questions that follow:

i. What physical quantity changes when the goalkeeper catches the football? (1 mark)
ii. Calculate the force exerted by the goalkeeper on the ball if its mass is 0.5 kg. (2 marks)
iii. Explain why the goalkeeper pulls his hands backward while catching the ball. (1 mark)

Ans:
i. Momentum of the ball changes.
ii. Given:
Mass of ball, m = 0.5 kg
Initial velocity, u = 15 m/s
Final velocity, v = 0 m/s
Time taken, t = 0.2 s
Using Newton’s Second Law:
F = m × a
Acceleration,
a = (v - u) / t = (0 - 15) / 0.2 = -75 m/s²
Force,
F = 0.5 × (-75) = -37.5 N
iii. The goalkeeper pulls his hands backward to increase the time of impact. This reduces the rate of change of momentum, thereby decreasing the force exerted on his hands and preventing injury.

Q12. Read the following passage and answer the questions that follow:

i. Which law of motion explains this phenomenon? (1 mark)
ii. A passenger of mass 60 kg is sitting in the bus. If the bus stops suddenly with an acceleration of 5 m/s², calculate the force exerted on the passenger. (2 marks)
iii. Why do passengers in a moving bus need to hold onto support when standing? (1 mark)

Ans:
i. Newton’s First Law of Motion (Law of Inertia).
ii. Given:
Mass, m = 60 kg
Acceleration, a = 5 m/s²
Using Newton’s Second Law:
F = m x a = 60 x 5 = 300N
iii. Standing passengers hold onto support because of inertia. If the bus suddenly starts or stops, their body resists the change in motion, causing them to fall. Holding onto a support provides stability and prevents falling.

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Q13. Read the following passage and answer the questions that follow:

i. Which type of force initially prevents the box from moving? (1 mark)
ii. If the box has a mass of 20 kg and the force required to move it is 100 N, calculate the coefficient of static friction between the box and the floor. (Take acceleration due to gravity, g = 10 m/s²)  (2 marks)
iii. Why is it easier to push an object once it starts moving than when it is at rest? (1 mark)

Ans:
i. Static friction force.
ii. Given:
Normal force, N = mg = 20 x 10 = 200N
Force required to move, F = 100N
Coefficient of static friction,
µ_s = F/N = 100/200 = 0.5
iii. Once an object starts moving, kinetic friction (which is usually less than static friction) acts instead of static friction. This makes it easier to push the object.

Q14: Read the following passage and answer the questions that follow:

i. Which law of motion explains the recoil of the gun? (1 mark)
ii. A bullet of mass 10 g is fired with a velocity of 400 m/s from a gun of mass 4 kg. Find the recoil velocity of the gun. (2 marks)
iii. Why does the bullet move forward with greater velocity than the gun’s recoil velocity? (1 mark)

Ans:
i. Newton’s Third Law of Motion (Action-Reaction Law).
ii. Given:
Mass of bullet, m₁ = 0.01 kg
Velocity of bullet v₁ = 400 m/s
Mass of gun, m₂ = 4 kg
Recoil velocity of gun, v₂
By conservation of momentum:
m₁v₁ = m₂v₂
0.01 x 400 = 4 x v₂
v₂ = 4/4 = 1 m/s (backward)
iii. The bullet has a smaller mass than the gun. Since momentum is conserved, a small mass moving forward at a high velocity results in a large mass (gun) moving backward at a much lower velocity.

Q15: Read the following passage and answer the questions that follow:

i. Which property of an object resists the change in motion in the above situation? (1 mark)
ii. If a person has a mass of 70 kg and the bus decelerates at a rate of 4 m/s², calculate the force exerted on the person due to inertia.
(2 marks)
iii. Why do safety belts help in preventing injuries in such situations? (1 mark)

Ans:
i. Inertia (Newton’s First Law).
ii. Given:
Mass, m = 70 kg
Deceleration, a = 4 m/s
Force, F = m x a = 70 x 4 = 280 N
iii. Safety belts prevent sudden forward motion by applying an opposite force, reducing the risk of impact injuries.