Case Study 1: Balanced vs. Unbalanced Forces
Rahul placed a book on a table. The weight of the book acts downward, and the table exerts an equal upward force. Hence, the book remains at rest. Later, he pushed the book with his hand, and it started moving.
Questions:
- The force acting downwards on the book is:
(a) normal force (b) muscular force (c) gravitational force (d) none
Answer: (c) - The upward force is:
(a) gravitational force (b) normal reaction (c) friction (d) none
Answer: (b) - In the first case, the book does not move because forces are:
(a) unbalanced (b) balanced (c) variable (d) none
Answer: (b) - When Rahul pushes, the book moves due to:
(a) balanced force (b) unbalanced force (c) inertia (d) none
Answer: (b) - Net force in balanced condition = ?
(a) 0 (b) equal to weight (c) infinite (d) variable
Answer: (a)
Case Study 2: Inertia (Newton’s First Law)
While travelling in a bus, when it suddenly starts, passengers tend to fall backward, and when it suddenly stops, they fall forward.
Questions:
- The property of a body to resist change in state is called:
(a) force (b) momentum (c) inertia (d) pressure
Answer: (c) - Falling backward on sudden start is due to inertia of:
(a) rest (b) motion (c) direction (d) none
Answer: (a) - Falling forward on sudden stop is due to inertia of:
(a) rest (b) motion (c) direction (d) none
Answer: (b) - This law is also known as:
(a) Newton’s first law (b) Newton’s second law (c) Newton’s third law (d) none
Answer: (a) - Inertia depends on:
(a) velocity (b) mass (c) acceleration (d) force applied
Answer: (b)
Case Study 3: Newton’s Second Law (Force & Acceleration)
A cricket player lowers his hands while catching a fast ball. This increases the time of impact and reduces the force exerted on his hands.
Questions:
- Force = ?
(a) mass × acceleration (b) momentum × velocity (c) energy × displacement (d) none
Answer: (a) - Reducing force is possible by:
(a) increasing time of impact (b) decreasing time (c) increasing acceleration (d) none
Answer: (a) - Momentum of ball = ?
(a) mass × acceleration (b) mass × velocity (c) velocity / time (d) none
Answer: (b) - Which law is applied here?
(a) First law (b) Second law (c) Third law (d) Gravitation
Answer: (b) - SI unit of force:
(a) joule (b) newton (c) pascal (d) watt
Answer: (b)
Case Study 4: Newton’s Third Law
When a swimmer pushes water backward with his hands, water pushes him forward with an equal and opposite force, enabling him to move.
Questions:
- This is an example of:
(a) Newton’s first law (b) second law (c) third law (d) none
Answer: (c) - Action = ?
(a) swimmer pushes water (b) water pushes swimmer (c) both (d) none
Answer: (a) - Reaction = ?
(a) water pushes swimmer (b) swimmer pushes water (c) both (d) none
Answer: (a) - Action and reaction forces are:
(a) equal and opposite (b) unequal (c) in same direction (d) none
Answer: (a) - Action and reaction act on:
(a) same body (b) different bodies (c) no body (d) none
Answer: (b)
Case Study 5: Conservation of Momentum
Two skaters standing still on ice push each other and move in opposite directions with equal and opposite momenta.
Questions:
- Principle applied = ?
(a) conservation of energy (b) conservation of momentum (c) inertia (d) none
Answer: (b) - Momentum before push = ?
(a) 0 (b) finite (c) infinite (d) none
Answer: (a) - Momentum after push = ?
(a) equal and opposite (b) zero (c) unequal (d) none
Answer: (a) - Law of conservation of momentum states:
(a) total momentum of system remains constant if no external force acts (b) energy remains constant (c) mass remains constant (d) none
Answer: (a) - This law is related to:
(a) Newton’s first law (b) Newton’s second law (c) Newton’s third law (d) all
Answer: (d)
Case Study 6: Force and Friction
A block is placed on a rough surface. When a small force is applied, it doesn’t move. On increasing the force, it starts sliding.
Questions:
- Friction acts in direction:
(a) same as motion (b) opposite to motion (c) perpendicular (d) none
Answer: (b) - Maximum force before sliding starts is called:
(a) kinetic friction (b) static friction (c) rolling friction (d) none
Answer: (b) - Once motion starts, friction is:
(a) static (b) kinetic (c) rolling (d) none
Answer: (b) - Friction is a:
(a) contact force (b) non-contact force (c) gravitational (d) none
Answer: (a) - Lubrication reduces:
(a) force (b) mass (c) friction (d) acceleration
Answer: (c)
Case Study 7: Rocket Propulsion
A rocket works on the principle of expelling gases backward at high velocity. The reaction force pushes the rocket upward.
Questions:
- Principle used in rocket propulsion = ?
(a) inertia (b) Newton’s second law (c) Newton’s third law (d) conservation of energy
Answer: (c) - Gases expelled backward = ?
(a) action (b) reaction (c) both (d) none
Answer: (a) - Upward thrust on rocket = ?
(a) action (b) reaction (c) none (d) inertia
Answer: (b) - Conservation of momentum applies when:
(a) no external force acts (b) gravity absent (c) friction absent (d) none
Answer: (a) - Rocket’s velocity increases because of:
(a) thrust (b) inertia (c) pressure (d) mass
Answer: (a)
Case Study 8: Accidents and Seatbelts
During a car accident, passengers are thrown forward if not wearing seatbelts. Seatbelts prevent injuries by providing opposing force to stop their motion.
Questions:
- Phenomenon = ?
(a) inertia of rest (b) inertia of motion (c) Newton’s third law (d) none
Answer: (b) - Seatbelt provides:
(a) friction (b) external opposing force (c) normal reaction (d) none
Answer: (b) - Force acting during collision is:
(a) small (b) very large (c) negligible (d) none
Answer: (b) - Law involved = ?
(a) Newton’s first (b) Newton’s second (c) Newton’s third (d) all
Answer: (d) - Seatbelts work by:
(a) reducing mass (b) increasing time of impact (c) reducing time (d) none
Answer: (b)
Case Study 9: Free Fall and Momentum
A ball of mass 2 kg falls from a height and hits the ground with velocity 10 m/s.
Questions:
- Initial momentum = ?
(a) 0 (b) 5 kg m/s (c) 20 kg m/s (d) none
Answer: (a) - Final momentum before hitting = ?
(a) 5 kg m/s (b) 10 kg m/s (c) 20 kg m/s (d) 25 kg m/s
Answer: (c) - Change in momentum = ?
(a) 0 (b) 10 (c) 20 (d) none
Answer: (c) - Force on ground due to ball is equal and opposite to:
(a) action of ground (b) weight of ball (c) reaction of ball (d) none
Answer: (c) - Law involved = ?
(a) momentum conservation (b) energy conservation (c) inertia (d) none
Answer: (a)
Case Study 10: Tug of War
In a tug of war, two teams pull a rope in opposite directions. The team exerting greater force wins.
Questions:
- If both forces equal, the rope:
(a) moves left (b) moves right (c) remains stationary (d) breaks
Answer: (c) - If Team A > Team B, motion is towards:
(a) Team A (b) Team B (c) both (d) none
Answer: (a) - Type of forces acting:
(a) balanced (b) unbalanced (c) variable (d) none
Answer: (b) - Net force = ?
(a) sum (b) difference (c) product (d) none
Answer: (b) - Tug of war illustrates:
(a) Newton’s first law (b) Newton’s second law (c) Newton’s third law (d) both a and b
Answer: (d)