Laws of Motion Test

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Laws of Motion Test - 14

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Question 1
1 / -0

When a train stops suddenly, passengers in the running train feel an instant jerk in the forward direction because

A
The back of seat suddenly pushes the passengers forward

B
Inertia of rest stops the train and takes the body forward

C
Upper part of the body continues to be in the state of motion whereas the lower part of the body in contact with seat remains at rest

D
Nothing can be said due to insufficient data

Solution

When a train stops suddenly, passengers in the running train feel an instant jerk in the forward direction because upper part of the body continues to be in the state of motion whereas the lower part of the body in contact with seat remains at rest.
Question 2
1 / -0

A man getting down a running bus falls forward because

A
Due to inertia of rest, road is left behind and man reaches forward

B
Due to inertia of motion upper part of body continues to be in motion in forward direction while feet come to rest as soon as they touch the road

C
He leans forward as a matter of habit

D
Of the combined effect of all the three factors stated in

Solution

A man getting down a running bus falls forward because due to inertia of motion upper part of body continues to be in motion in forward direction while feet come to rest as soon as they touch the road.
Question 3
1 / -0

A particle is moving with a constant speed along a straight-line path. A force is not required to

A
Increase its speed

B
Decrease the momentum

C
Change the direction

D
Keep it moving with uniform velocity

Solution

Particle will move with uniform velocity due to inertia.
Question 4
1 / -0

A bird weighs 2 kg and is inside a closed cage of 1 kg. If it starts flying, then what is the weight of the bird and cage assembly?

A
1.5 kg

B
2.5 kg

C
3 kg

D
4 kg

Solution

When the bird flies, it pushes air down to balance its weight. So the weight of the bird and closed cage assembly remains unchanged.
Question 5
1 / -0

Newton's second law gives the measure of

A
Acceleration

B
Force

C
Momentum

D
Angular momentum

Solution

\(\vec F = m \vec a\)
Question 6
1 / -0

A force of 100 dynes acts on box of 5 gm for 10 sec. The velocity produced is

A
0.2 cm/sec

B
20 cm/sec

C
200 cm/sec

D
2000 cm/sec

Solution

a = \(\frac{F}{m} = \frac{100}{5} = 20 cm/s^2\)

v = at = 20 x 10 = 200 cm/s
Question 7
1 / -0

A Diwali rocket is ejecting 0.05 kg of gases per second at a velocity of 400 m/sec. The accelerating force on the rocket is

A
20 dynes

B
20 N

C
22 dynes

D
1000 N

Solution

F = u \((\frac{dm}{dt})\)

= 400 x 0.05

= 20 N
Question 8
1 / -0

If the tension in the cable of 1000 kg elevator is 1000 kg weight, the elevator

A
Is accelerating upwards

B
Is accelerating downwards

C
May be at rest or accelerating

D
May be at rest or in uniform motion

Solution

Since T = mg, it implies that elevator may be at rest or in uniform motion.
Question 9
1 / -0

A machine gun is mounted on a 2000 kg car on a horizontal frictionless surface. At some instant, the gun fires bullets of mass 10 gm with a velocity of 500 m/sec with respect to the car. The number of bullets fired per second is ten. The average thrust on the system is

A
550 N

B
50 N

C
250 N

D
250 dyne

Solution

u = velocity of bullet

\(\frac{dm}{dt}\) = Mass thrown per second by the machine gun

= (Mass of bullet) × (Number of bullet fired per second)

= 10g ×10 bullet/sec =100 g/sec = 0.1 kg/sec

\(\therefore\) Thrust = \(\frac{udm}{dt}\)

= 500 × 0.1

= 50 N
Question 10
1 / -0

Gravels are dropped on a conveyor belt at the rate of 0.5 kg/sec. The extra force required in newtons to keep the belt moving at 2 m/sec is

A
1

B
3

C
5

D
0.5

Solution

Opposing force

F = \(u(\frac{dm}{dt})\) = 2 x 0.5 = 1 N

\((As, F = m \frac{du}{dt})\)

So same amount of force is required to keep the belt moving at 2 m/s.
Question 11
1 / -0

When we jump out of a boat standing in water it moves

A
Forward

B
Backward

C
Sideways

D
None of the above

Solution

Because for every action there is an equal and opposite reaction takes place.
Question 12
1 / -0

A ball of mass 150g starts moving with an acceleration of 20 m/\(s^2\). When hit by a force, which acts on it for 0.1 sec. The impulsive force is

A
0.5 N-s

B
0.1 N-s

C
0.3 N-s

D
1.2 N-s

Solution

Impulse = (Force) x (time) = m a t

= (0.15) × (20) × (0.1) = 0.3N-s
Question 13
1 / -0

A bullet of mass 0.1 kg is fired with a speed of 100 m/sec; the mass of gun is 50 kg. The velocity of recoil is

A
0.2 m/sec

B
0.1 m/sec

C
0.5 m/sec

D
0.05 m/sec

Solution

According to principle of conservation of linear momentum

\(m_Gv_G = m_Bv_B\)

\(v_G = \frac{m_Bv_B}{m_G} = \frac{0.1 \times 10^2}{50}\)

= 0.2 m/s
Question 14
1 / -0

Assertion: When a bicycle is in motion, the force of friction exerted by the ground on the two wheels is always in forward direction.

Reason: The frictional force acts only when the bodies are in contact.

(A) If both assertion and reason are true and the reason is the correct explanation of the assertion.

(B) If both assertion and reason are true but reason is not the correct explanation of the assertion.

(C) If assertion is true but reason is false.

(D) If the assertion and reason both are false.

(E) If assertion is false but reason is true.

A
A

B
B

C
C

D
D

Solution

When a bicycle is in motion, two cases may arise:

(i) When the bicycle is being pedalled. In this case, the applied force has been communicated to rear wheel. Due to which the rear wheel pushes the earth backwards. Now the force of friction acts in the forward direction on the rear wheel but front wheel move forward due to inertia, so force of friction works on it in backward direction.

(ii) When the bicycle is not being pedalled: In this case both the wheels move in forward direction, due to inertia. Hence force of friction on both the wheels acts in backward direction.
Question 15
1 / -0

Assertion: No force is required by the body to remain in any state.

Reason: In uniform linear motion, acceleration has a finite value.

(A) If both assertion and reason are true and the reason is the correct explanation of the assertion.

(B) If both assertion and reason are true but reason is not the correct explanation of the assertion.

(C) If assertion is true but reason is false.

(D) If the assertion and reason both are false.

A
A

B
B

C
C

D
D

Solution

Force is required to change the state of the body. In uniform motion body moves with constant speed so acceleration should be zero.
Question 16
1 / -0

A block of weight 10 N resting on a horizontal surface. The coefficient of static friction between the block and the surface \(\mu_s\) = 0.4. A force of 3.5 N will keep the block in uniform motion, once it has been set in motion. A horizontal force of 3 N is applied to the block, and then the block will

A
Move over the surface with constant velocity

B
Move having accelerated motion over the surface

C
Not move

D
First it will move with a constant velocity for some time and then will have accelerated motion

Solution

\(F_l = \mu_s R\) = 0.4 x mg

= 0.4 x 10 = 4N

i.e. minimum 4N force is required to start the motion of a body. But applied force is only 3N. So the block will not move.

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Re-Attempt Weekly Quiz Competition

Laws of Motion Test - 14

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Laws of Motion Test - 14

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SHARING IS CARING

Question 1

The back of seat suddenly pushes the passengers forward

Inertia of rest stops the train and takes the body forward

Upper part of the body continues to be in the state of motion whereas the lower part of the body in contact with seat remains at rest

Nothing can be said due to insufficient data

Solution

Question 2

Due to inertia of rest, road is left behind and man reaches forward

Due to inertia of motion upper part of body continues to be in motion in forward direction while feet come to rest as soon as they touch the road

He leans forward as a matter of habit

Of the combined effect of all the three factors stated in

Solution

Question 3

Increase its speed

Decrease the momentum

Change the direction

Keep it moving with uniform velocity

Solution

Question 4

1.5 kg

2.5 kg

3 kg

4 kg

Solution

Question 5

Acceleration

Force

Momentum

Angular momentum

Solution

Question 6

0.2 cm/sec

20 cm/sec

200 cm/sec

2000 cm/sec

Solution

Question 7

20 dynes

20 N

22 dynes

1000 N

Solution

Question 8

Is accelerating upwards

Is accelerating downwards

May be at rest or accelerating

May be at rest or in uniform motion

Solution

Question 9

550 N

50 N

250 N

250 dyne

Solution

Question 10

1

3

5

0.5

Solution

Question 11

Forward

Backward

Sideways

None of the above

Solution

Question 12

0.5 N-s

0.1 N-s

0.3 N-s

1.2 N-s

Solution