Extra 20 important long-answer questions from the chapter-9 Force and Laws of Motion of Class 9 CBSE Science:-
1. Define force and explain its effects with examples.
Answer:
Force is a push or pull acting on an object, changing its state of motion or shape.
Effects of Force:
Change in speed: A moving object can speed up or slow down (e.g., applying brakes in a car).
Change in direction: A football changes direction when kicked.
Change in shape: A rubber band stretches when pulled.
Initiates motion: A stationary ball starts rolling when pushed.
Stops motion: A moving car stops when brakes are applied.
2. State and explain Newton’s First Law of Motion with examples.
Answer:
Newton’s First Law: An object remains at rest or in uniform motion unless acted upon by an external force.
Explanation:
A book on a table stays there unless pushed.
A moving car stops when brakes are applied due to friction.
Passengers in a bus lurch forward when it stops suddenly due to inertia of motion.
A coin placed on a card over a glass falls straight down when the card is removed.
A dust particle in a carpet moves when the carpet is beaten.
3. What is inertia? Explain its types with examples.
Answer:
Inertia
is the tendency of an object to resist a change in its motion.
Types of Inertia:
Inertia of Rest: A person in a stationary bus moves backward when the bus starts suddenly.
Inertia of Motion: A person leans forward when a moving bus stops suddenly.
Inertia of Direction: A person in a turning car moves sideways.
Example: When a tree is shaken, leaves remain still, but fruits fall due to inertia.
4. State and explain Newton’s Second Law of Motion with an example.
Answer:
Newton’s Second Law: The rate of change of momentum is directly proportional to the force applied.
Formula: F=maF = maF=ma
Explanation:
A heavier object requires more force to accelerate than a lighter object.
A football moves farther when kicked with more force.
A fast-moving cricket ball exerts more force on the bat.
A strong push on a swing makes it move faster.
A loaded truck requires more force to move than an empty truck.
5. Define momentum and explain its significance.
Answer:
Momentum (p) is the product of mass and velocity of an object.
Formula: p=mvp = mvp=mv
SI Unit: kg m/s
Significance:
A fast-moving car has high momentum and is harder to stop.
A bullet fired from a gun has high momentum despite low mass.
A moving train carries large momentum due to its high mass.
Goalkeepers use soft gloves to reduce the impact of a football's momentum.
Karate experts break bricks by applying a sudden force, transferring high momentum.
6. Derive the equation for force using Newton’s Second Law.
Answer:
Given: Force FFF is proportional to the rate of change of momentum.
Momentum: p=mvp = mvp=mv
Change in Momentum: Δp=m(v−u)\Delta p = m(v - u)Δp=m(v−u)
Force Equation: F=m(v−u)tF = \frac{m(v - u)}{t}F=tm(v−u) F=maF = maF=ma
Conclusion: The force acting on an object is the product of mass and acceleration.
7. What is impulse? Derive its relation with momentum.
Answer:
Impulse: The product of force and time for which the force acts.
Formula: Impulse=Ft=Impulse = Ft =Impulse=Ft= change in momentum
Derivation: Ft=m(v−u)Ft = m(v - u)Ft=m(v−u)
Example:
Catching a fast cricket ball with hands lowered reduces impulse force.
Airbags reduce impact force in accidents.
A person jumping from a height bends knees to reduce impulse force.
8. State and explain Newton’s Third Law with examples.
Answer:
Newton’s Third Law:
For every action, there is an equal and opposite reaction.
Examples:
A rocket moves upward as gases are expelled downward.
A gun recoils when a bullet is fired.
Walking is possible because the foot pushes the ground backward.
A swimmer moves forward by pushing water backward.
A bird flies by pushing air downward.
9. Why does a gun recoil when fired? Explain using the law of conservation of momentum.
Answer:
Gun and bullet system:
Before firing: p initial=0p_{initial} = 0pinitial=0
After firing: m1v1+m2v2=0m_1 v_1 + m_2 v_2 = 0m1v1+m2v2=0
The gun moves backward with velocity v2v_2v2 (recoil).
Conclusion:
The gun's recoil is due to the conservation of momentum.
10. Explain the law of conservation of momentum with examples.
Answer:
Law: The total momentum of a system remains constant unless an external force acts.
Examples:
Recoiling of a gun.
Rocket propulsion.
Billiard balls colliding.
A swimmer pushing off a boat.
Air escaping from a balloon.
11. Explain the concept of balanced and unbalanced forces with examples.
Answer:
Balanced Forces:
Two equal and opposite forces cancel each other.
The object remains at rest or moves with uniform velocity.
Example: A book resting on a table (gravity and normal force are balanced).
Example: A tug-of-war where both teams pull with equal force.
Balanced forces do not change the state of motion.
They can change the shape of an object (e.g., pressing a sponge).
Example: A stationary car parked on a road.
Unbalanced Forces:
Unequal forces acting on an object cause acceleration.
An object can speed up, slow down, or change direction.
Example: A moving car stops when brakes are applied.
Example: A ball rolling downhill due to gravitational force.
Example: Kicking a football applies an unbalanced force.
Example: A cyclist applying brakes to stop the cycle.
Unbalanced force causes a change in velocity.
12. Explain why a rocket is able to move in space using Newton’s Third Law of Motion.
Answer:
Rocket propulsion follows Newton’s Third Law:
Action Force: Expulsion of hot gases downward.
Reaction Force: Rocket moves upward in the opposite direction.
Rocket engines generate thrust by burning fuel.
The expelled gases exert equal and opposite force.
As fuel burns, the rocket’s mass decreases, increasing acceleration (Newton’s Second Law).
Multiple stages reduce weight, increasing efficiency.
Example: Space rockets, satellites, and missile launches.
14. Why does a skydiver experience weightlessness while falling? Explain in terms of Newton’s Laws.
Answer:
During Free Fall:
Skydivers initially accelerate downward due to gravity.
Air resistance acts in the opposite direction.
As velocity increases, air resistance also increases.
When air resistance equals gravitational force, acceleration stops (terminal velocity).
The skydiver feels weightless as only gravity acts.
In orbit, astronauts experience free fall continuously, creating the illusion of zero gravity.
Example: Astronauts on the International Space Station float due to continuous free fall.
15. How does Newton’s Second Law explain the functioning of seat belts in a car?
Answer:
Seat belts reduce impact force by increasing stopping time.
A fast-moving car has high momentum.
When brakes are applied, an unbalanced force stops the car.
Passengers continue moving forward due to inertia of motion.
Seat belts exert force to stop passengers gradually.
Increasing stopping time reduces force exerted on passengers.
Formula: F=ΔptF = \frac{\Delta p}{t}F=tΔp → Larger stopping time, lesser force.
Example: Airbags work similarly by increasing impact time.
16. Explain how a bicycle moves using Newton’s Laws of Motion.
Answer:
Newton’s First Law:
A stationary cycle remains at rest unless a force is applied by pedalling.
Newton’s Second Law:
2. The harder you pedal, the greater the acceleration.
3. Formula: F=
maF
=
maF
=ma → More force, greater speed.
Newton’s Third Law:
4. The tire pushes the road backward, and the road pushes the cycle forward.
5. The force exerted by legs is transferred to wheels, moving the bicycle.
6. Brakes apply frictional force to stop the cycle.
7. Example: Cycling uphill requires more force due to gravity.
17. Why do vehicles have a streamlined shape? Explain with Newton’s Laws.
Answer:
Aerodynamic Design:
Reduces air resistance (drag) while moving.
Newton’s First Law: Reducing drag helps maintain motion with less force.
Newton’s Second Law: More force applied → greater acceleration.
Newton’s Third Law: Air pushes back as vehicle moves forward.
Helps in fuel efficiency by reducing resistance.
Example: High-speed trains and airplanes have streamlined shapes.
Formula: F=maF = maF=ma, meaning less force is needed for motion.
18. How do boats move forward in water? Explain using Newton’s Third Law.
Answer:
Boats move due to action and reaction forces:
The oars push water backward (action).
Water pushes the boat forward (reaction).
The force exerted by water is equal and opposite to the force applied by oars.
Example: A swimmer moves forward by pushing water backward.
Jet skis propel water backward to move forward.
Fish swim by pushing water in the opposite direction.
Airplanes use the same principle by pushing air backward.
19. How does a helicopter take off and move in different directions?
Answer:
Helicopters use Newton’s Third Law:
The rotor blades push air downward (action).
Air pushes the helicopter upward (reaction).
The tail rotor controls direction by countering the torque effect.
Tilting the main rotor changes the direction of motion.
More power to the blades increases lift force.
Pilots adjust rotor angles to control ascent and descent.
Example: Drones use the same principle for movement.
