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
