Force & Newton’s Laws
1. A passenger in a moving bus tends to fall forward when the driver suddenly applies the brakes. This is due to:
(A) Friction
(B) Inertia of motion
(C) Centripetal force
(D) Gravitational force
Answer: (B) Inertia of motion
Explanation: The passenger’s body tends to continue moving forward due to inertia when the bus suddenly stops.
2. When a stationary bus suddenly starts moving, passengers tend to fall backward. This is an example of:
(A) Inertia of rest
(B) Inertia of motion
(C) Centrifugal force
(D) Conservation of energy
Answer: (A) Inertia of rest
Explanation: The lower part of the body moves with the bus while the upper body tends to remain at rest.
3. Dust is removed from a carpet by beating it with a stick because of:
(A) Friction
(B) Inertia
(C) Gravitation
(D) Centripetal force
Answer: (B) Inertia
Explanation: When the carpet is suddenly moved, the dust tends to remain at rest and separates from the carpet.
4. A person jumping from a moving bus may fall forward. Which law explains this phenomenon?
(A) Newton’s first law
(B) Newton’s second law
(C) Newton’s third law
(D) Law of gravitation
Answer: (A) Newton’s first law
Explanation: The person’s body continues moving forward with the bus due to inertia.
5. When a gun is fired, the gun moves backward. This phenomenon is explained by:
(A) Newton’s first law
(B) Newton’s second law
(C) Conservation of momentum
(D) Conservation of energy
Answer: (C) Conservation of momentum
Explanation: The forward momentum of the bullet is balanced by an equal and opposite momentum of the gun.
6. A rocket moves upward because:
(A) Air pushes the rocket upward
(B) Earth repels the rocket
(C) Gases are expelled downward
(D) Gravity pushes it upward
Answer: (C) Gases are expelled downward
Explanation: The rocket pushes gases downward, and the gases exert an equal and opposite force on the rocket.
7. Walking is possible mainly because:
(A) We push the ground backward and the ground pushes us forward
(B) We push the ground forward
(C) Gravity pulls us forward
(D) Air pushes us forward
Answer: (A) We push the ground backward and the ground pushes us forward
Explanation: This is an application of Newton’s third law of motion.
8. A swimmer moves forward in water by:
(A) Pushing water forward
(B) Pushing water backward
(C) Pulling himself forward using gravity
(D) Reducing his mass
Answer: (B) Pushing water backward
Explanation: The swimmer pushes water backward, and water exerts an equal and opposite force forward.
Friction & Everyday Applications
9. The main purpose of treads on automobile tyres is to:
(A) Increase the weight of the vehicle
(B) Reduce friction
(C) Increase friction with the road
(D) Reduce pressure
Answer: (C) Increase friction with the road
Explanation: Treads provide better grip and reduce the possibility of skidding.
10. Why are lubricants used between moving machine parts?
(A) To increase friction
(B) To reduce friction
(C) To increase pressure
(D) To increase weight
Answer: (B) To reduce friction
Explanation: Lubricants form a thin layer between surfaces and reduce direct contact.
11. A matchstick catches fire when rubbed against the striking surface mainly because of:
(A) Gravity
(B) Friction
(C) Centripetal force
(D) Buoyancy
Answer: (B) Friction
Explanation: Friction converts mechanical energy into heat, raising the temperature sufficiently to ignite the match head.
12. It is difficult to walk on a very smooth, wet floor because:
(A) Gravity disappears
(B) Friction decreases
(C) Friction increases
(D) Atmospheric pressure increases
Answer: (B) Friction decreases
Explanation: Water reduces friction between the feet and the floor, making slipping more likely.
Circular Motion
13. The force required to keep an object moving in a circular path is called:
(A) Centrifugal force
(B) Centripetal force
(C) Gravitational force
(D) Frictional force
Answer: (B) Centripetal force
Explanation: Centripetal force acts toward the centre of the circular path.
14. The direction of centripetal force acting on a body moving in a circular path is:
(A) Away from the centre
(B) Tangential to the path
(C) Towards the centre
(D) Vertically upward
Answer: (C) Towards the centre
15. A stone tied to a string is whirled in a circle. If the string suddenly breaks, the stone moves:
(A) Towards the centre
(B) Away from the centre along the radius
(C) Along the tangent to the circular path
(D) Vertically downward immediately
Answer: (C) Along the tangent to the circular path
Explanation: At the instant the string breaks, the stone continues in the direction of its instantaneous velocity, which is tangential to the circle.
16. Why are roads banked at curves?
(A) To reduce the weight of vehicles
(B) To provide the necessary centripetal force
(C) To increase gravitational force
(D) To eliminate friction completely
Answer: (B) To provide the necessary centripetal force
Explanation: Banking helps provide the inward force needed for a vehicle to negotiate a curve safely.
17. When a car takes a sharp turn, passengers tend to lean outward. This is mainly due to:
(A) Centripetal force
(B) Inertia
(C) Gravity
(D) Magnetism
Answer: (B) Inertia
Explanation: The body tends to continue moving in its original straight-line direction while the car changes direction.
18. A cyclist bends his body inward while taking a turn mainly to:
(A) Increase his mass
(B) Maintain balance and provide the required centripetal effect
(C) Reduce gravitational force
(D) Increase air resistance
Answer: (B) Maintain balance and provide the required centripetal effect
Centrifugal Effect — Very Important
19. The spin-drying process in a washing machine is commonly explained using:
(A) Centripetal effect
(B) Centrifugal effect
(C) Gravitational force
(D) Magnetic force
Answer: (B) Centrifugal effect
Explanation: During rapid rotation, water tends to move outward relative to the rotating drum and escapes through the holes.
20. A cream separator separates cream from milk using:
(A) Gravitational force
(B) Centrifugal effect
(C) Magnetic force
(D) Electrostatic force
Answer: (B) Centrifugal effect
21. When a wet umbrella is rotated rapidly and water droplets fly outward, the phenomenon is associated with:
(A) Centrifugal effect
(B) Buoyancy
(C) Elasticity
(D) Atmospheric pressure
Answer: (A) Centrifugal effect
22. Mud is thrown outward from the tyres of a rapidly rotating bicycle wheel mainly because of:
(A) Centripetal force
(B) Centrifugal effect
(C) Gravitational force
(D) Magnetic force
Answer: (B) Centrifugal effect
Momentum & Force
23. A cricket player moves his hands backward while catching a fast-moving ball in order to:
(A) Increase the momentum of the ball
(B) Increase the time of impact and reduce the force
(C) Increase the force of impact
(D) Stop the ball instantly
Answer: (B) Increase the time of impact and reduce the force
Explanation: Increasing the stopping time reduces the average force acting on the hands.
24. Why are airbags used in automobiles?
(A) To increase the momentum of passengers
(B) To decrease the stopping time
(C) To increase the time over which passengers are brought to rest
(D) To increase the speed of the vehicle
Answer: (C) To increase the time over which passengers are brought to rest
Explanation: Increasing the collision time reduces the average impact force.
25. A person jumps onto a sand bed rather than a concrete floor because sand:
(A) Increases the person’s momentum
(B) Increases the stopping time and reduces impact force
(C) Increases gravitational force
(D) Eliminates inertia
Answer: (B) Increases the stopping time and reduces impact force
26. A heavy truck and a small car move at the same velocity. Which has greater momentum?
(A) Small car
(B) Truck
(C) Both have equal momentum
(D) Cannot be determined
Answer: (B) Truck
Explanation: Momentum depends on mass and velocity. At the same velocity, the greater mass has greater momentum.
Conceptual & Exam-Trap Questions
27. If the net force acting on an object is zero, the object:
(A) Must be at rest
(B) Must be accelerating
(C) May be at rest or moving with constant velocity
(D) Must move in a circle
Answer: (C) May be at rest or moving with constant velocity
28. An object moving in a circular path at constant speed is:
(A) Not accelerating
(B) Accelerating because its direction changes
(C) Not under any force
(D) Moving with constant velocity
Answer: (B) Accelerating because its direction changes
Explanation: Velocity is a vector quantity. Even if speed remains constant, a change in direction means velocity changes.
29. In uniform circular motion, the velocity of an object:
(A) Remains constant
(B) Changes continuously
(C) Becomes zero
(D) Always points toward the centre
Answer: (B) Changes continuously
Explanation: The direction of velocity changes continuously, although its magnitude (speed) remains constant.
30. If the speed of an object moving in a circular path is doubled while the radius remains constant, the required centripetal force becomes:
(A) Half
(B) Double
(C) Four times
(D) Unchanged
Answer: (C) Four times
Explanation: Centripetal force is proportional to the square of speed.
Most Important Applications to Memorize
For competitive exams, these 30 associations are particularly high-yield:
Bus suddenly stops → Inertia
Bus suddenly starts → Inertia
Beating carpet → Inertia
Seat belt → Inertia
Gun recoil → Conservation of momentum
Rocket → Conservation of momentum/Newton’s third law
Walking → Newton’s third law
Swimming → Newton’s third law
Rowing boat → Newton’s third law
Tyre treads → Friction
Lubricants → Reduce friction
Matchstick → Friction
Banking of roads → Centripetal force
Turning vehicle → Centripetal force
Satellite → Gravitational centripetal force
Stone on string → Centripetal force
Broken string → Tangential motion
Washing machine → Centrifugal effect
Cream separator → Centrifugal effect
Rotating umbrella → Centrifugal effect
Mud from rotating tyre → Centrifugal effect
Catching a ball → Increase stopping time
Airbag → Increase collision time
Sand/mattress → Increase stopping time
Circular motion → Continuous change in velocity