Laws of Motion — DPP 2

20 NEET questions from Laws of Motion, with answers and explanations. Attempt it as a timed test to get scoring, accuracy breakdown and mistake tracking.

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1
Two blocks A and B of masses 5 kg and 3 kg respectively rest on a smooth horizontal surface with B over A. The coefficient of friction between A and B is 0.5. The maximum horizontal force (in kg wt.) that can be applied to A, so that there will be motion of A and B without relative slipping, is
  1. A 1.5
  2. B 2.5
  3. C 4
  4. D 5
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Correct answer: C

Laws of Motion – NEET-12136
2
A horizontal force 10 N is necessary to just hold a block stationary against a wall. The coefficient of friction between the block and the wall is 0.2, the weight of the block is
Laws of Motion – NEET-12140
  1. A 20 N
  2. B 50 N
  3. C 100 N
  4. D 2 N
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Correct answer: D

Laws of Motion – NEET-12140 Normal reaction N = 10 newton
In the vertical direction frictional force will balance its weight
f = mg = W
μN =(0.2)(10) = 2 newton
3
When an object is at rest
  1. A Force is required to keep it in rest state
  2. B No force is acting on it
  3. C A large number of forces may be acting on it which balance each other
  4. D It is in vacuum
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Correct answer: C

Object can be at rest only if net force acting on it is zero.
$\vec{F}_{n e t}=\vec{F}_{1}+\vec{F}_{2}+\vec{F}_{3}+\vec{F}_{4}+\ldots \ldots \ldots . . \vec{F}_{n}=0$
4
A metallic chain 1 m long lies on a horizontal surface of a table. The chain starts sliding on the table if 25 cm (or more of it) hangs over the edge of a table. The correct value of the coefficient of friction between the table and the chain is
  1. A $\frac { 1 } { 3 }$
  2. B $\frac { 2 } { 3 }$
  3. C $\frac { 1 } { 4 }$
  4. D $\frac { 1 } { 5 }$
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Correct answer: A

Laws of Motion – NEET-12133 $\begin{array}{c}\frac{M}{4} g=\frac{\mu 3 M}{4} g \\ \mu=1 / 3\end{array}$
5
In the following figure, the object of mass m is held at rest by a horizontal force as shown. The force exerted by the string on the block is
Laws of Motion – NEET-12138
  1. A F
  2. B mg
  3. C F + mg
  4. D $\sqrt { F ^ { 2 } + m ^ { 2 } g ^ { 2 } }$
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Correct answer: D

Laws of Motion – NEET-12138
6
From Newton's second law of motion, it can be inferred that
  1. A No force is required to move a body uniformly along straight line
  2. B Accelerated motion is always due to an external force
  3. C Inertial mass of a body is equal to force required per unit acceleration in the body
  4. D All of these
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Correct answer: D

By Newton's second law
$\vec{F}=m \vec{a}$ ....(i)
for (i) Uniform motion means body is moving with constant velocity.
By (i) it can be said that only for accelerated motion force is required (2) is true using (i)
(3) Using (i) $\vec{a}=\frac{\vec{F}}{m}$ so this is true
7
Which of the following is self-adjusting force?
  1. A Static friction
  2. B Limiting friction
  3. C Kinetic friction
  4. D Rolling friction
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Correct answer: A

Static friction is self adjusting force. Its value varies from $0 \leq f_{s} \leq \mu_{s} N$
8
If a block moving up an inclined plane at $30 ^ { \circ }$ with a velocity of 5 m/s, stops after 0.5 s, then the coefficient of friction will be nearly
  1. A 0.5
  2. B 0.6
  3. C 0.9
  4. D 1.1
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Correct answer: B

Using v = u + at
Retardation will be provided by friction as well as gravitational force
$\begin{array}{l}a=\frac{u}{t} \\ g \sin 30^{\circ}+\mu g \cos 30^{\circ}=\frac{5}{0.5}=10 \\ \mu=\frac{1}{\sqrt{3}} \cong 0.6\end{array}$
9
Newton's first law is applicable
  1. A In all reference frames
  2. B Only in inertial reference frames
  3. C Only in non-inertial reference frames
  4. D None of these
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Correct answer: B

Newton's law is applicable only in inertial reference frames.
10
A cubical block rests on a plane of μ = √3. The angle through which the plane be inclined to the horizontal so that the block just slides down will be
  1. A $30 ^ { \circ }$
  2. B $45 ^ { \circ }$
  3. C $60 ^ { \circ }$
  4. D $75 ^ { \circ }$
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Correct answer: C

Laws of Motion – NEET-12127
11
A bullet of mass 40 g is fired from a gun of mass 10 kg. If velocity of bullet is 400 m/s, then the recoil velocity of the gun will be
  1. A 1.6 m/s in the direction of bullet
  2. B 1.6 m/s opposite to the direction of bullet
  3. C 1.8 m/s in the direction of bullet
  4. D 1.8 m/s opposite to the direction of bullet
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Correct answer: B

Laws of Motion – NEET-12116
12
A man of mass 50 kg carries a bag of weight 40 N on his shoulder. The force with which the floor pushes up his feet will be
  1. A 882 N
  2. B 530 N
  3. C 90 N
  4. D 600 N
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Correct answer: B

Laws of Motion – NEET-12120 $\begin{aligned} \mathrm{N} & =m_{1} g+m_{2} g \\ & =50(9.8)+40 \\ & =490+40=530 \mathrm{~N}\end{aligned}$
13
It is difficult to move a cycle with brakes on because
  1. A Rolling friction opposes motion on road
  2. B Sliding friction opposes motion on road
  3. C Rolling friction is more than sliding friction
  4. D Sliding friction is more than the rolling friction
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Correct answer: D

Sliding friction > Rolling friction
14
A child weighing 25 kg slides down a rope hanging from a branch of a tall tree. If the force of friction acting against him is 200 N , the acceleration of child is $\left( g = 10 \mathrm {~m} / \mathrm { s } ^ { 2 } \right)$
  1. A $22.5 \mathrm {~m} / \mathrm { s } ^ { 2 }$
  2. B $8 \mathrm {~m} / \mathrm { s } ^ { 2 }$
  3. C $5 \mathrm {~m} / \mathrm { s } ^ { 2 }$
  4. D $2 \mathrm {~m} / \mathrm { s } ^ { 2 }$
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Correct answer: D

Laws of Motion – NEET-12129 $\begin{array}{l}m g-f_{s}=m a \\ 250-200=25 a \\ a=2 \mathrm{~m} / \mathrm{s}^{2}\end{array}$
15
In a rocket, fuel burns at the rate of 2 kg/s. This fuel gets ejected from the rocket with a velocity of 80 km/s. Force exerted on the rocket is
  1. A 16,000 N
  2. B 1,60,000 N
  3. C 1600 N
  4. D 16 N
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Correct answer: B

For variable mass system
$F=\frac{u d m}{d t}=80 \times 10^{3} \times 2=1,60,000 \mathrm{~N}$
16
In the figure shown, the coefficient of static friction between the block A of mass 20 kg and horizontal table is 0.2. What should be the minimum mass of hanging block just beyond which blocks start moving?
Laws of Motion – NEET-12135
  1. A 2 kg
  2. B 3 kg
  3. C 4 kg
  4. D 5 kg
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Correct answer: C

Tension produced in the string should be just greater than the frictional force acting on the 20 kg block
$\begin{aligned} T & =m_{B} g \\ T & >\mu m_{A} g \\ m_{B} g & >(0.2)(20(g) \\ m_{B} & >4 \mathrm{~kg}\end{aligned}$
17
Maximum force of friction is called
  1. A Limiting friction
  2. B Static friction
  3. C Sliding friction
  4. D Rolling friction
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Correct answer: A

Limiting friction is maximum force of friction.
18
In the figure given below, with what acceleration does the block of mass m will move? (Pulley and strings are massless and frictionless)
Laws of Motion – NEET-12075
  1. A $\frac{g}{3}$
  2. B $\frac{2g}{5}$
  3. C $\frac{2g}{3}$
  4. D $\frac{g}{2}$
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Correct answer: C

Laws of Motion – NEET-12075
19
A block of mass m is released on a smooth inclined plane of inclination θ with the horizontal. The force exerted by the plane on the block has a magnitude
  1. A mg
  2. B $\frac { m g } { \cos \theta }$
  3. C mg tan θ
  4. D mg cos θ
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Correct answer: D

Laws of Motion – NEET-12121 Force exerted by the plane on the block will be N
N = mgcos θ
20
Mr. A, B, and C are trying to put a heavy piston into a cylinder at a mechanical workshop in railway yard. If they apply forces $F _ { 1 } , F _ { 2 }$ and $F _ { 3 }$ respectively on ropes then for which set of forces at that instant, they will be able to perform the said job?
Laws of Motion – NEET-12139
  1. A $\sqrt { 3 } F _ { 1 } = F _ { 2 } + 2 F _ { 3 }$
  2. B $2 F _ { 1 } = F _ { 2 } + F _ { 3 }$
  3. C $2 F _ { 2 } = \sqrt { 3 } F _ { 1 } - \frac { F _ { 3 } } { 2 }$
  4. D $F _ { 3 } = 2 F _ { 1 } - \sqrt { 3 } F _ { 2 }$
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Correct answer: A

Piston is vertically above the cylinder so to drop it inside the cylinder, Net horizontal force must be zero on the piston. So,
$\begin{array}{l}F_{1} \sin 60=F_{2} \cos 60+F_{3} \\ F_{1} \frac{\sqrt{3}}{2}=\frac{F_{2}}{2}+F_{3} \\ \sqrt{3} F_{1}=F_{2}+2 F_{3}\end{array}$

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