1. When a horse pulls a cart, the cart pulls back on the horse with an equal force. Why does the system still move forward?
Correct Answer: (c) The net force on the horse-cart system is forward.
The action-reaction forces between horse and cart are internal to the system. The horse pushes backward on the ground, and the ground pushes forward on the horse (external force), creating net forward motion.
2. Swimming is possible on account of
Correct Answer: (c) Third law of motion
Swimming works by pushing water backward (action), which pushes the swimmer forward (reaction), demonstrating Newton's Third Law.
3. A man is at rest in the middle of a pond on perfectly smooth ice. He can get himself to the shore by making use of Newton's
Correct Answer: (c) Third law
He can throw an object in one direction (action), which will push him in the opposite direction (reaction).
4. A swimmer pushes the water backward with their hands. What force propels them forward?
Correct Answer: (b) The water's reaction force
According to Newton's Third Law, the action (pushing water backward) results in an equal and opposite reaction (water pushing swimmer forward).
5. When a rocket pushes exhaust gases downward, the gases push the rocket upward. This is an example of:
Correct Answer: (c) Newton's Third Law
This is a classic example of action-reaction pairs described in Newton's Third Law.
6. A cannon after firing recoils due to
Correct Answer: (c) Newton's third law of motion
The cannon exerts a forward force on the projectile (action), and the projectile exerts an equal backward force on the cannon (reaction), causing recoil.
7. Which of the following is not an action-reaction pair?
Correct Answer: (b) Earth's gravity pulling a book down and the table pushing the book up.
These are not action-reaction pairs. The reaction to Earth's gravity on the book is the book's gravity pulling Earth upward. The table's normal force is a separate interaction.
8. Newton's third law of motion leads to the law of conservation of
Correct Answer: (d) Momentum
Newton's Third Law implies that internal forces cancel out, leading to conservation of momentum in a closed system.
9. A man is carrying a block of a certain substance (of density 1000 kg/m³) weighing 1 kg in his left hand and a bucket filled with water and weighing 10 kg in his right hand. He drops the block into the bucket. How much load does he carry in his right hand now?
Correct Answer: (c) 11 kg
The block's weight is now part of the bucket's contents (10 kg + 1 kg = 11 kg). The density information tells us the block will sink but doesn't affect the total weight.
10. A man is standing on a balance and his weight is measured. If he takes a step in the left side, then weight
Correct Answer: (c) Remains same
The scale measures normal force, which equals weight (mg) as long as there's no vertical acceleration. Horizontal movement doesn't affect the reading.
11. A man is standing at a spring platform. Reading of spring balance is 60 kg wt. If man jumps outside platform, then reading of spring balance
Correct Answer: (a) First increases then decreases to zero
When jumping, the man pushes down harder initially (increasing reading), then the reading drops to zero when contact is lost.
12. A cold soft drink is kept on the balance. When the cap is open, then the weight
Correct Answer: (b) Decreases
When opened, CO₂ gas escapes, reducing the total mass of the system (bottle + contents).
13. A bird is sitting in a large closed cage which is placed on a spring balance. It records a weight of 25 N. The bird (mass m = 0.5 kg) flies upward in the cage with an acceleration of 2 m/s². The spring balance will now record a weight of
Correct Answer: (c) 26 N
The bird's upward acceleration requires additional force: \( F = ma = 0.5 \times 2 = 1 \) N.
This force is exerted on the cage, adding to the weight: \( 25 N + 1 N = 26 N \).
This force is exerted on the cage, adding to the weight: \( 25 N + 1 N = 26 N \).
14. A light spring balance hangs from the hook of the other light spring balance and a block of mass M kg hangs from the former one. Then the true statement about the scale reading is
Correct Answer: (b) Both the scales read M kg each
Each spring balance supports the entire weight of the mass M below it, so both show M kg. The upper one supports M kg (block + lower scale), and the lower one supports just the block (M kg).
15. A machine gun fires 20 bullets per second into a target. Each bullet weighs 150 gms and has a speed of 800 m/sec. Find the force necessary to hold the gun in position
Correct Answer: (d) 2400 N
Force = rate of change of momentum
\( F = n \times m \times v \) where n = bullets/second, m = mass, v = velocity
\( F = 20 \times 0.150 \times 800 = 2400 \) N
\( F = n \times m \times v \) where n = bullets/second, m = mass, v = velocity
\( F = 20 \times 0.150 \times 800 = 2400 \) N
16. A book is lying on the table. What is the angle between the action of the book on the table and the reaction of the table on the book
Correct Answer: (d) 180°
Action and reaction forces are equal in magnitude but opposite in direction (180° apart).
17. A person pushes a wall but it doesn't move. Which statement is correct?
Correct Answer: (c) The wall exerts an equal and opposite force back on the person.
Newton's Third Law applies to all forces, whether objects move or not. The wall pushes back with equal force, but the net force is zero so there's no movement.
18. If a book rests on a table, the table exerts an upward normal force on the book. What is the reaction force to this?
Correct Answer: (b) The book pushing down on the table.
The normal force (table on book) and the book's force on the table are the action-reaction pair. The book's weight is a separate force (Earth's gravity on book).
19. When a horse pulls a wagon, the force that causes the horse to move forward is the force
Correct Answer: (a) The ground exerts on it
The horse pushes backward on the ground, and the ground pushes forward on the horse (reaction force). This external force propels the horse-wagon system forward.
20. Two ice skaters push off each other. Skater A (50 kg) moves left at 2 m/s. If Skater B (100 kg) moves right, what is their speed?
Correct Answer: (b) 1 m/s
Conservation of momentum: \( m_1v_1 = m_2v_2 \)
\( 50 \times 2 = 100 \times v \)
\( v = 1 \) m/s
\( 50 \times 2 = 100 \times v \)
\( v = 1 \) m/s
21. Newton's Third Law states that:
Correct Answer: (b) Every action has an equal and opposite reaction.
This is the direct statement of Newton's Third Law of Motion.
22. A student attempts to pull himself up by tugging on his hair. He will not succeed
Correct Answer: (d) As the force applied is internal to the system.
Internal forces cannot produce net motion of the system. For movement, an external force is needed (like pushing against the ground).
23. A man is standing at the centre of frictionless pond of ice. How can he get himself to the shore
Correct Answer: (b) By spitting horizontally
By spitting horizontally (action), he receives an equal and opposite reaction force that propels him toward the shore.
24. In the above ques., the height to which the lift takes the passenger is
Correct Answer: (a) 3.6 meters
This appears to reference a previous question about acceleration and height calculation, though the full context isn't provided here.
25. Why can't a fish swim in empty space?
Correct Answer: (b) There is no water to push backward for forward motion.
Swimming requires a medium to push against (action) to generate forward motion (reaction). In empty space, there's no medium to interact with.
26. A balloon moves forward when air escapes backward. How does this demonstrate Newton's Third Law?
Correct Answer: (b) The action (air pushed backward) causes reaction (balloon moves forward).
This is a direct application of Newton's Third Law - the balloon exerts force on the air (action), and the air exerts equal force on the balloon in the opposite direction (reaction).