CLASS 9 FORCE AND LAWS OF MOTION

 

Worksheet 1

Class 9 - Science

Chapter: Force and Laws of Motion

Maximum Marks: 30

Time: 1 Hour

Section A (1 mark each)

1. Define inertia.
2. Give one example of Newton’s Third Law of Motion in daily life.
3. A body is moving with uniform velocity. What is the net force acting on it?
4. Which law of motion explains the working of a rocket?
5. Why do passengers move forward when a moving bus stops suddenly?

Section B (2 marks each)

6. A cricket player lowers his hands while catching a fast-moving ball. Explain why.
7. How is mass different from weight?
8. Define momentum. Give its SI unit.
9. Why is it easier to stop a moving bicycle than a moving truck, even if both are moving at the same speed?
10. What happens when you shake a branch of a tree? Which law explains this?

Section C (3 marks each)

11. Derive the equation: F = ma using Newton’s Second Law of Motion.
12. A truck of mass 3000 kg is moving with a velocity of 10 m/s. Calculate its momentum. If the velocity is doubled, how will the momentum change?
13. A force of 10 N is applied on a body of mass 2 kg for 3 seconds. Calculate the acceleration and final velocity of the body.

Section D (5 marks)

14. (a) State and explain Newton’s Three Laws of Motion with examples.
    (b) A bullet of mass 50 g is fired from a gun with a velocity of 500 m/s. If the mass of the gun is 5 kg, calculate the recoil velocity of the gun.

Answers

Section A (1 mark each)

1. Inertia is the tendency of a body to resist any change in its state of motion or rest.
2. Example of Newton’s Third Law of Motion: When we push a wall, the wall exerts an equal and opposite force on us.
3. The net force acting on a body moving with uniform velocity is zero.
4. Newton’s Third Law of Motion explains the working of a rocket.
5. When a moving bus stops suddenly, the passengers move forward due to inertia of motion.

Section B (2 marks each)

6. A cricket player lowers his hands while catching a fast-moving ball to increase the time of impact. This reduces the force exerted on the hands, preventing injury.
7. • Mass is the amount of matter in a body and is constant everywhere. (SI unit: kg)
   • Weight is the force exerted by gravity on a body (W = mg). It depends on gravitational acceleration. (SI unit: Newton)
8. Momentum is the product of an object’s mass and velocity. Mathematically, p = mv.
   • SI unit: kg·m/s
9. A moving truck has more mass than a bicycle. Since momentum (p = mv) depends on mass, the truck has higher momentum, making it harder to stop.
10. When we shake a tree, its trunk moves, but the leaves remain at rest due to inertia of rest, causing them to fall. This is explained by Newton’s First Law of Motion.

Section C (3 marks each)

11. Derivation of F = ma using Newton’s Second Law:

• Newton’s Second Law states that the rate of change of momentum is directly proportional to the applied force.
• Momentum, p = mv
• Change in momentum in time t: $$\text{Rate of change of momentum} = \frac{mv - mu}{t}$$
• Since acceleration a = (v - u) / t, $$F = m \cdot a$$ Thus, F = ma.

12. Given: Mass = 3000 kg, Velocity = 10 m/s

• Momentum, p = mv = 3000 × 10 = 30,000 kg·m/s
• If velocity doubles to 20 m/s, new momentum = 3000 × 20 = 60,000 kg·m/s
• Conclusion: When velocity is doubled, momentum also doubles.

13. Given: Force (F) = 10 N, Mass (m) = 2 kg, Time (t) = 3 s

• Acceleration, a = F/m = 10/2 = 5 m/s²
• Final velocity, v = u + at = 0 + (5 × 3) = 15 m/s
• Answer: Acceleration = 5 m/s², Final Velocity = 15 m/s

Section D (5 marks)

14. (a) Newton’s Three Laws of Motion:

• First Law (Law of Inertia): A body at rest remains at rest, and a body in motion continues to move with the same velocity unless acted upon by an external force.
  • Example: A book lying on a table stays there until someone moves it.
• Second Law (F = ma): The force applied on an object is equal to the rate of change of its momentum.
  • Example: A heavier object requires more force to push than a lighter one.
• Third Law (Action-Reaction): For every action, there is an equal and opposite reaction.
  • Example: A rocket moves upward due to the downward thrust of exhaust gases.

(b) Recoil velocity of a gun:

• Given:
  • Mass of bullet, m₁ = 50 g = 0.05 kg
  • Velocity of bullet, v₁ = 500 m/s
  • Mass of gun, m₂ = 5 kg
• By the law of conservation of momentum: $$m₁ v₁ + m₂ v₂ = 0$$
  
• $$(0.05)(500) + (5)(v₂) = 0$$
  
• $$25 + 5v₂ = 0$$
  
• $$v₂ = -\frac{25}{5} = -5 \text{ m/s}$$
  
• The negative sign indicates that the gun recoils in the opposite direction of the bullet.

Worksheet 2

Class 9 - Science

Chapter: Force and Laws of Motion

Maximum Marks: 30

Time: 1 Hour

Section A (1 mark each)

1. What is Newton’s First Law of Motion?
2. Name the physical quantity that determines the inertia of a body.
3. What is the SI unit of force?
4. Why does a person fall backward when a car starts suddenly?
5. Define balanced force with an example.

Section B (2 marks each)

6. A horse pulls a cart, but according to Newton’s Third Law of Motion, the cart pulls the horse with an equal and opposite force. Why does the cart move forward?
7. Why do athletes run a few steps before taking a long jump?
8. A car is moving with a uniform velocity. What can you say about the forces acting on it?
9. Why does a heavy object require more force to move than a lighter one?
10. A force of 50 N is applied to an object of mass 10 kg. What is the acceleration produced in the object?

Section C (3 marks each)

11. State and explain Newton’s Second Law of Motion with an example.
12. A ball of mass 2 kg is moving with a velocity of 5 m/s. Calculate its momentum. If the velocity doubles, what will be the new momentum?
13. A force of 20 N is applied to a body of mass 4 kg for 2 seconds. Find its acceleration and the final velocity, assuming it starts from rest.

Section D (5 marks each)

14. (a) Why do passengers in a moving bus tend to fall forward when the driver suddenly applies brakes? Which law explains this?
    (b) A machine gun fires 20 bullets per second, each of mass 50 g. If each bullet moves with a velocity of 400 m/s, calculate the force required to hold the gun.

Answers

Section A (1 mark each)

1. Newton’s First Law of Motion states that a body at rest will remain at rest, and a body in motion will continue to move with the same velocity unless acted upon by an external force.
2. The mass of a body determines its inertia. A heavier body has more inertia than a lighter one.
3. The SI unit of force is Newton (N).
4. A person falls backward when a car starts suddenly due to the inertia of rest. The body resists the sudden motion, causing the backward movement.
5. Balanced forces are equal forces acting in opposite directions that cancel each other out, resulting in no motion.
   • Example: A book resting on a table experiences gravitational force downward and an equal normal force upward, keeping it at rest.

Section B (2 marks each)

6. The cart moves forward because the force exerted by the horse is greater than the friction and other opposing forces acting on the cart. Newton’s Third Law of Motion applies to action-reaction forces on different objects, so the cart experiences a net forward force.
7. Athletes run before a long jump to gain momentum, which helps them cover a greater distance. According to Newton’s First Law, a body in motion remains in motion unless an external force acts on it.
8. If a car is moving with uniform velocity, the net force acting on it is zero. The forward force (engine force) is balanced by resistive forces (friction and air resistance).
9. A heavier object has greater inertia due to its larger mass. According to Newton’s Second Law (F = ma), more force is required to produce the same acceleration in a heavier object.
10. Given: F = 50 N, m = 10 kg
    • Acceleration, a = F/m = 50/10 = 5 m/s²
    • Answer: The acceleration produced is 5 m/s².

Section C (3 marks each)

11. Newton’s Second Law of Motion: The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction of the force.

    • Mathematically: F = ma
    • Example: If a football is kicked with a greater force, it accelerates more than when kicked lightly.

12. Given: Mass (m) = 2 kg, Velocity (v) = 5 m/s

    • Momentum, p = mv = 2 × 5 = 10 kg·m/s
    • If velocity doubles to 10 m/s, new momentum = 2 × 10 = 20 kg·m/s
    • Answer: The new momentum is 20 kg·m/s.

13. Given: F = 20 N, m = 4 kg, t = 2 s, Initial velocity (u) = 0

    • Acceleration, a = F/m = 20/4 = 5 m/s²
    • Final velocity, v = u + at = 0 + (5 × 2) = 10 m/s
    • Answer: Acceleration = 5 m/s², Final velocity = 10 m/s.

Section D (5 marks each)

14. (a) Explanation of sudden braking in a moving bus:
    • When the driver applies brakes suddenly, the lower part of the passenger’s body stops with the bus, but the upper part continues to move forward due to inertia of motion.
    • Newton’s First Law explains this: An object in motion remains in motion unless an external force acts on it.

(b) Force required to hold the machine gun:

• Given:
  • Number of bullets per second = 20
  • Mass of each bullet, m = 50 g = 0.05 kg
  • Velocity of each bullet, v = 400 m/s
• Momentum of one bullet, p = mv = 0.05 × 400 = 20 kg·m/s
• Change in momentum per second for 20 bullets: $$\text{Total momentum change per second} = 20 × 20 = 400 kg·m/s$$
• By Newton’s Second Law, force F = rate of change of momentum: $$F = 400 \text{ N}$$
• Answer: The force required to hold the gun is 400 N.