Assertion A: A 5 kg block on a rough horizontal table (coefficient of static friction 0.4, g = 10 m/s²) is pushed horizontally with 15 N and stays at rest, so the friction on it is 15 N. Reason R: Static friction adjusts itself to the applied force, up to its limiting value μₛN. Choose the correct answer.

Both A and R are correct and R is the correct explanation of A

Both A and R are correct but R is not the correct explanation of A

A is not correct but R is correct

A is correct but R is not correct

Laws of Motion

28 NEET-UG practice questions on Laws of Motion, part of Physics. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Easy
What does the area under a force-time graph of a body represent?
1. APower
2. BImpulse
3. CAcceleration
4. DWork
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Answer: B · Impulse
Impulse is the integral of F dt, which is the area under the F-t graph, and it equals the change in momentum.
- A: Power is a rate, not an area under this graph.
- C: Acceleration is force divided by mass, not an area.
- D: Work is the area under a force-displacement graph.
Remember this
Area under F-t gives impulse and change in momentum; area under F-x gives work.
Question 2 · Hard
A 4 kg block on a rough horizontal table (coefficient of kinetic friction 0.25) carries a 2 kg block on top of it. A light string from the 4 kg block passes over a smooth pulley at the table's edge and supports a hanging 6 kg block. The system accelerates and the 2 kg block does not slip. Taking g = 10 m/s², find the friction force acting on the 2 kg block.
1. A7.5 N
2. B5 N
3. C10 N
4. D15 N
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Answer: A · 7.5 N
Friction from the table = 0.25 × 60 = 15 N, so a = (60 − 15)/12 = 3.75 m/s². The only horizontal force on the 2 kg block is friction, so f = 2 × 3.75 = 7.5 N. Check: T = 6(10 − 3.75) = 37.5 N, and 37.5 − 15 = 6 × 3.75.
- B: This applies μN with the table's coefficient to the upper block.
- C: This uses a = 5 m/s², which ignores friction at the table.
- D: This is the friction from the table on the lower block.
Remember this
A block carried without slipping feels only the friction needed for its acceleration, not μN.
Question 3 · Medium
Two blocks of 3 kg and 2 kg hang from the ends of a light string passing over a smooth, light pulley. Taking g = 10 m/s², what is the tension in the string once they are released?
1. A24 N
2. B20 N
3. C30 N
4. D25 N
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Answer: A · 24 N
a = (3 − 2)(10)/(3 + 2) = 2 m/s². For the lighter block, T = 2(10 + 2) = 24 N. Check with the heavier block: T = 3(10 − 2) = 24 N.
- B: This is the weight of the lighter block alone, ignoring its acceleration.
- C: This is the weight of the heavier block alone.
- D: This is the mean of the two weights, which is not what the equations of motion give.
Remember this
Tension lies between the two weights and equals 2m₁m₂g/(m₁ + m₂).
Question 4 · Medium
A curved road of radius 200 m is banked so that tan θ = 0.2. At what speed can a car take this curve with no help from friction? Take g = 10 m/s².
1. A28.3 m/s
2. B20 m/s
3. C44.7 m/s
4. D6.3 m/s
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Answer: B · 20 m/s
With no friction, the horizontal component of the normal force supplies the centripetal force, which gives tan θ = v²/(rg). So v² = 200 × 10 × 0.2 = 400 and v = 20 m/s.
- A: This uses v² = 2rg tan θ, an extra factor of 2.
- C: This uses √(rg), leaving out tan θ.
- D: This leaves out g, using √(r tan θ).
Remember this
The optimum banking speed is v = √(rg tan θ), independent of the mass of the car.
Question 5 · Medium
Assertion A: A 5 kg block on a rough horizontal table (coefficient of static friction 0.4, g = 10 m/s²) is pushed horizontally with 15 N and stays at rest, so the friction on it is 15 N. Reason R: Static friction adjusts itself to the applied force, up to its limiting value μₛN. Choose the correct answer.
1. ABoth A and R are correct but R is not the correct explanation of A
2. BA is not correct but R is correct
3. CBoth A and R are correct and R is the correct explanation of A
4. DA is correct but R is not correct
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Answer: C · Both A and R are correct and R is the correct explanation of A
The limiting friction is 0.4 × 50 = 20 N, more than the 15 N push, so the block stays at rest. Balance of forces makes the friction exactly 15 N, and this happens because static friction is self-adjusting. So A is true and R explains it.
- A: R is the very reason the friction takes the value 15 N.
- B: A is correct, since 15 N is below the limiting 20 N.
- D: R is a correct statement of static friction.
Remember this
Below the limit, static friction equals the applied force; μₛN is only its maximum value.
Question 6 · Medium
Friction is absent in the arrangement drawn, and the string and pulley are light. Once it is released, what tension does the string carry? Take g = 10 m/s².
Figure: MedExams
1. A20 N
2. B5 N
3. C30 N
4. D15 N
Answer and explanation in LearnFree account
Question 7 · Hard
Block A rests on block B, and the 42 N force acts on B alone. With g = 10 m/s² and equal static and kinetic friction coefficients, what are the accelerations of A and B?
Figure: MedExams
1. AA: 5 m/s², B: 10.5 m/s²
2. BA: 7 m/s², B: 7 m/s²
3. CA: 5 m/s², B: 8 m/s²
4. DA: 8 m/s², B: 5 m/s²
Answer and explanation in LearnFree account
Question 8 · Medium
Given below are two statements: Statement I: For a 2 kg block on a level surface with the friction graph shown (g = 10 m/s²), the coefficients of static and kinetic friction are 0.6 and 0.4. Statement II: Once the block in the graph slides, the friction force keeps growing as the applied force grows. Choose the correct answer.
Figure: MedExams
1. ABoth Statement I and Statement II are correct
2. BBoth Statement I and Statement II are incorrect
3. CStatement I is correct but Statement II is incorrect
4. DStatement I is incorrect but Statement II is correct
Answer and explanation in LearnFree account
Question 9 · Easy
Net force on a 2 kg trolley, initially at rest on a straight track, varies with time as plotted. What is its speed at t = 4 s?
Figure: MedExams
1. A10 m/s
2. B40 m/s
3. C5 m/s
4. D20 m/s
Answer and explanation in LearnFree account
Question 10 · Hard
A 5 kg block rests on a rough horizontal floor with coefficient of static friction 0.75. It is pulled by a rope inclined at 37° above the horizontal. Taking g = 10 m/s², sin 37° = 0.6 and cos 37° = 0.8, what tension in the rope just starts the block moving?
1. A46.9 N
2. B107 N
3. C37.5 N
4. D30 N
Answer and explanation in LearnFree account
Question 11 · Medium
A shell of mass 2 kg at rest bursts into two fragments of masses 0.5 kg and 1.5 kg. The lighter fragment flies off at 12 m/s. How much kinetic energy is released in the explosion?
1. A24 J
2. B36 J
3. C48 J
4. D12 J
Answer and explanation in LearnFree account
Question 12 · Easy
Why does a fielder draw his hands back while catching a fast cricket ball?
1. AIt reduces the change in momentum of the ball
2. BIt increases the time of impact and so reduces the force on his hands
3. CIt shortens the time of impact so the catch is quicker
4. DIt increases the impulse given to the ball
Answer and explanation in LearnFree account
Question 13 · Medium
Given below are two statements: Statement I: A person standing on a weighing scale in a lift that is moving upward with constant velocity reads his true weight. Statement II: When the lift accelerates downward with an acceleration less than g, the scale reads more than his true weight. Choose the correct answer.
1. AStatement I is incorrect but Statement II is correct
2. BBoth Statement I and Statement II are correct
3. CBoth Statement I and Statement II are incorrect
4. DStatement I is correct but Statement II is incorrect
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Question 14 · Hard
A block is projected up a rough incline of angle 37° with a speed of 10 m/s. The coefficients of static and kinetic friction are both 0.5. Taking g = 10 m/s², sin 37° = 0.6 and cos 37° = 0.8, with what speed does the block return to its starting point?
1. AZero, as it stays at rest at the top
2. B2√5 m/s
3. C10 m/s
4. D2√15 m/s
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Question 15 · Medium
A 4 kg block on a horizontal table is tied by a light string, passing over a smooth pulley at the table's edge, to a 1 kg block hanging freely. The coefficient of kinetic friction between the 4 kg block and the table is 0.1. Taking g = 10 m/s², what is the tension in the string while the blocks move?
1. A8.8 N
2. B4.8 N
3. C10 N
4. D8 N
Answer and explanation in LearnFree account
Question 16 · Easy
The cable of a lift snaps and the lift falls freely. What does a weighing machine on the lift floor read for a man of mass m standing on it?
1. Amg
2. B2mg
3. Cmg/2
4. DZero
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Question 17 · Medium
A 0.2 kg ball strikes a wall normally at 10 m/s and rebounds along the same line at 8 m/s. The contact lasts 0.02 s. What average force does the wall exert on the ball?
1. A3.6 N
2. B180 N
3. C100 N
4. D20 N
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Question 18 · Medium
Given below are two statements: Statement I: When a small horizontal force is applied to a box resting on a rough horizontal floor and the box does not move, the friction on it equals μₛN. Statement II: For a given pair of surfaces, the coefficient of kinetic friction is generally less than the coefficient of static friction. Choose the correct answer.
1. AStatement I is correct but Statement II is incorrect
2. BBoth Statement I and Statement II are correct
3. CStatement I is incorrect but Statement II is correct
4. DBoth Statement I and Statement II are incorrect
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Question 19 · Medium
A person of mass m stands on a weighing machine placed on the floor of a lift. Match List-I with List-II. List-I (Motion of the lift) A. At rest B. Accelerating upward at a C. Accelerating downward at a (a < g) D. Falling freely List-II (Reading of the machine) I. m(g + a) II. Zero III. mg IV. m(g − a) Choose the correct answer from the options given below.
1. AA-III, B-I, C-IV, D-II
2. BA-III, B-I, C-II, D-IV
3. CA-I, B-III, C-IV, D-II
4. DA-III, B-IV, C-I, D-II
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Question 20 · Medium
A gun of mass 5 kg fires a bullet of mass 50 g with a speed of 200 m/s. With what speed does the gun recoil?
1. A0.2 m/s
2. B200 m/s
3. C2 m/s
4. D20 m/s
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Question 21 · Medium
A curved road of radius 30 m is to be banked so that a car moving at 15 m/s needs no friction to go round it. Taking g = 10 m/s², what should the banking angle θ be?
1. Acos⁻¹(3/4)
2. Bsin⁻¹(3/4)
3. Ctan⁻¹(3/4)
4. Dtan⁻¹(4/3)
Answer and explanation in LearnFree account
Question 22 · Hard
Blocks of 3 kg and 5 kg hang at the two ends of a light string passing over a light frictionless pulley, which is held by a rope tied to the ceiling. Taking g = 10 m/s², what force does the ceiling rope exert while the blocks move?
1. A60 N
2. B37.5 N
3. C80 N
4. D75 N
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Question 23 · Easy
On which property of a body does its inertia depend?
1. AIts shape
2. BThe net force on it
3. CIts mass
4. DIts velocity
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Question 24 · Medium
A block rests on a rough plank. The plank is tilted slowly, and the block just begins to slide when the plank makes 30° with the horizontal. What is the coefficient of static friction between them?
1. A√3
2. B1/√3
3. C1/2
4. D√3/2
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Question 25 · Easy
According to Newton's third law, action and reaction forces are equal and opposite. Why do they never cancel each other?
1. AThey are unequal in magnitude
2. BThey act at different instants
3. CThey act in the same direction
4. DThey act on two different bodies
Answer and explanation in LearnFree account

3 more Laws of Motion questions

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Laws of Motion

What does the area under a force-time graph of a body represent?

Two blocks of 3 kg and 2 kg hang from the ends of a light string passing over a smooth, light pulley. Taking g = 10 m/s², what is the tension in the string once they are released?

A curved road of radius 200 m is banked so that tan θ = 0.2. At what speed can a car take this curve with no help from friction? Take g = 10 m/s².

Friction is absent in the arrangement drawn, and the string and pulley are light. Once it is released, what tension does the string carry? Take g = 10 m/s².

Block A rests on block B, and the 42 N force acts on B alone. With g = 10 m/s² and equal static and kinetic friction coefficients, what are the accelerations of A and B?

Net force on a 2 kg trolley, initially at rest on a straight track, varies with time as plotted. What is its speed at t = 4 s?

A 5 kg block rests on a rough horizontal floor with coefficient of static friction 0.75. It is pulled by a rope inclined at 37° above the horizontal. Taking g = 10 m/s², sin 37° = 0.6 and cos 37° = 0.8, what tension in the rope just starts the block moving?

A shell of mass 2 kg at rest bursts into two fragments of masses 0.5 kg and 1.5 kg. The lighter fragment flies off at 12 m/s. How much kinetic energy is released in the explosion?

Why does a fielder draw his hands back while catching a fast cricket ball?

A block is projected up a rough incline of angle 37° with a speed of 10 m/s. The coefficients of static and kinetic friction are both 0.5. Taking g = 10 m/s², sin 37° = 0.6 and cos 37° = 0.8, with what speed does the block return to its starting point?

The cable of a lift snaps and the lift falls freely. What does a weighing machine on the lift floor read for a man of mass m standing on it?

A 0.2 kg ball strikes a wall normally at 10 m/s and rebounds along the same line at 8 m/s. The contact lasts 0.02 s. What average force does the wall exert on the ball?

A gun of mass 5 kg fires a bullet of mass 50 g with a speed of 200 m/s. With what speed does the gun recoil?

A curved road of radius 30 m is to be banked so that a car moving at 15 m/s needs no friction to go round it. Taking g = 10 m/s², what should the banking angle θ be?

Blocks of 3 kg and 5 kg hang at the two ends of a light string passing over a light frictionless pulley, which is held by a rope tied to the ceiling. Taking g = 10 m/s², what force does the ceiling rope exert while the blocks move?

On which property of a body does its inertia depend?

A block rests on a rough plank. The plank is tilted slowly, and the block just begins to slide when the plank makes 30° with the horizontal. What is the coefficient of static friction between them?

According to Newton's third law, action and reaction forces are equal and opposite. Why do they never cancel each other?