📘 Chapter 9 – Force and Laws of Motion

🧠 Detailed & Easy Notes

🔹 1. Force

A push or pull acting on an object.
It can change:
✔ Shape
✔ Speed
✔ Direction of motion

🔹 2. Types of Force

Balanced Force: No motion/change in motion (equal and opposite).
Unbalanced Force: Motion occurs (net force ≠ 0).

🔹 3. Newton’s First Law of Motion (Law of Inertia)

An object continues to stay at rest or in uniform motion unless an external unbalanced force acts on it.
📌 Inertia: Tendency of object to resist change in its state.
🔸 Inertia depends on mass.

🔹 4. Newton’s Second Law of Motion

Force = mass × acceleration
📌 F = ma
📌 Unit of Force: Newton (N)
1 N = Force to accelerate 1 kg of mass by 1 m/s²

🔹 5. Momentum (p)

Product of mass and velocity
📌 p = mv
Unit: kg·m/s
Momentum is a vector quantity.

🔹 6. Newton’s Third Law of Motion

For every action, there is an equal and opposite reaction.
⚡ Action = Reaction (but acts on different objects)

🔹 7. Law of Conservation of Momentum

Total momentum before collision = Total momentum after collision
p₁ + p₂ = p₁’ + p₂’
Used in collisions, rocket propulsion, etc.

✅ MCQs

The SI unit of force is:
a) Dyne
b) kg·m/s
c) Newton
d) Pascal
Force = mass × __________
a) distance
b) velocity
c) acceleration
d) time
An object remains at rest unless:
a) It is heavy
b) It is pulled
c) Unbalanced force acts
d) It wants to move
Which of the following is a vector quantity?
a) Distance
b) Speed
c) Momentum
d) Mass
A body of mass 2 kg moving with velocity 3 m/s. Momentum is:
a) 2 kg·m/s
b) 5 kg·m/s
c) 6 kg·m/s
d) 8 kg·m/s
Inertia depends on:
a) Volume
b) Velocity
c) Mass
d) Time
What causes acceleration in a body?
a) Balanced force
b) Friction
c) Unbalanced force
d) Gravity only
When two bodies collide, total momentum:
a) Increases
b) Decreases
c) Remains same
d) Doubles
Action and reaction forces:
a) Act on same object
b) Act on different objects
c) Cancel each other
d) Are unbalanced
Rocket works on:
a) Gravity
b) First law
c) Third law
d) Second law
A man walking on ground applies force on:
a) Ground
b) Air
c) Himself
d) Shoes
Force produces:
a) Inertia
b) Mass
c) Acceleration
d) Pressure
If mass = 4 kg, acceleration = 2 m/s². Force = ?
a) 2 N
b) 6 N
c) 8 N
d) 10 N
Change in momentum is caused by:
a) Mass only
b) Velocity only
c) Force
d) Inertia
Greater mass means:
a) Greater inertia
b) Smaller force
c) No motion
d) Smaller inertia

✅ Assertion & Reason

A: A heavy object has more inertia.
R: Inertia depends on mass.
a) A and R true, R explains A
b) A and R true, R doesn’t explain A
c) A true, R false
d) A false, R true

A: Force is needed to maintain uniform motion.
R: Newton’s First Law says motion continues unless disturbed.
a) A and R true, R explains A
b) A and R true, R doesn’t explain A
c) A true, R false
d) A false, R true

A: Momentum is scalar quantity.
R: It has magnitude and direction.
a) A and R true, R explains A
b) A and R true, R doesn’t explain A
c) A true, R false
d) A false, R true

A: Newton’s third law involves forces on two different objects.
R: Forces are always equal and opposite.
a) A and R true, R explains A
b) A and R true, R doesn’t explain A
c) A true, R false
d) A false, R true

A: Balanced forces change the speed of an object.
R: Net force is zero in balanced condition.
a) A and R true, R explains A
b) A and R true, R doesn’t explain A
c) A true, R false
d) A false, R true

✅ Case-Based Study

📘 Passage:
A cricket player lowers his hands while catching a fast ball. This increases the time of contact with the ball and reduces the impact of force.

Why does player lower hands?
a) To jump higher
b) To increase force
c) To reduce force
d) For fun
What happens to time of contact?
a) Increases
b) Decreases
c) No change
d) Becomes zero
What physical quantity is involved here?
a) Energy
b) Force
c) Power
d) Acceleration
What happens to momentum?
a) Increases
b) Conserved
c) Decreases
d) Becomes zero
Reducing impact force relates to:
a) Newton’s 1st Law
b) Newton’s 2nd Law
c) Newton’s 3rd Law
d) Conservation of Energy

✅ Reason-Based Questions

Why is force needed to stop a moving body?
Because it resists change in motion due to inertia.
Why is Newton’s first law called law of inertia?
Because it explains how inertia keeps object at rest or in motion.
Why is a heavier object harder to move?
Because more mass means more inertia.
Why is seat belt important in a moving car?
To protect us from sudden jerks due to inertia.
Why does a rocket move upward?
Because gases come out downward (action-reaction pair).
Why do action and reaction not cancel each other?
Because they act on different bodies.
Why is momentum conserved in collision?
Because there is no external force acting.
Why does a football stop on its own?
Because of friction (unbalanced force).
Why do we apply brakes to stop bicycle?
To apply external unbalanced force.
Why do athletes take long jump with a run-up?
To gain momentum before jumping.
Why is impulse large in case of collision?
Because force acts for a short time.
Why do objects fall under gravity?
Because gravity applies unbalanced force downward.
Why do glass pieces scatter when dropped?
Because they retain motion due to inertia.
Why is acceleration zero in balanced force?
Because net force is zero.
Why do we push ground backward while walking?
Because ground gives us forward reaction.

✅ Final Answer Key

MCQs:
1–c, 2–c, 3–c, 4–c, 5–c, 6–c, 7–c, 8–c, 9–b, 10–c, 11–a, 12–c, 13–c, 14–c, 15–a

Assertion & Reason:
16–a, 17–d, 18–d, 19–a, 20–d

Case-Based:
21–c, 22–a, 23–b, 24–b, 25–b