Newton S Laws Of Motion(11th And 12th > Physics ) Questions and answers for exam Preparation

11th And 12th > Physics

NEWTON S LAWS OF MOTION MCQs

Total Questions : 30 | Page 1 of 3 pages

Question 1.

A force has been applied on a ball when it is

kicked
pushed
thrown
All of these

Discuss Question

Answer: Option D. -> All of these
:
D

A force has been applied on a ball when it is kicked, pushed, thrown or flicked

Question 2.

When a force is applied to a body its shape changes.

Always
Never
Maybe
All of these

Discuss Question

Answer: Option C. -> Maybe
:
C

Force on an object may change its shape, not always.

Question 3.

A rocket with a lift-off mass $3.5 \times 10^{4} k g$ is blasted upwards with an initial acceleration of $10 m / s^{2}$ . Then the initial thrust of the blast is

$1.75 \times 10^{5} N$
$3.5 \times 10^{5} N$
$7.0 \times 10^{5} N$
$14.0 \times 10^{5} N$

Discuss Question

Answer: Option C. ->

7.0 \times 10^{5} N

:
C

Initial thrust on the rocket $F = m (g + a) = 3.5 \times 10^{4} (10 + 10) = 7.0 \times 10^{5} N$

Question 4.

A force of 50 dynes is acted on a body of mass 5 g which is at rest for an interval of 3 seconds, then impulse is

$0.15 \times 10^{- 3} N - s$
$0.98 \times 10^{- 3} N - s$
$1.5 \times 10^{- 3} N - s$
$2.5 \times 10^{- 3} N - s$

Discuss Question

Answer: Option C. ->

1.5 \times 10^{- 3} N - s

:
C

$I m p u l s e = f o r c e \times t i m e = 50 \times 10^{- 5} \times 3 = 1.5 \times 10^{- 3} N - s$

Question 5.

A wagon weighing 1000 kg is moving with a velocity 50 km/h on smooth horizontal rails. A mass of 250 kg is dropped into it. The velocity with which it moves now is

12.5 km/hour
20 km/hour
40 km/hour
50 km/hour

Discuss Question

Answer: Option C. -> 40 km/hour
:
C

Initially the wagon of mass 1000 kg is moving with velocity of 50 km/h

So its momentum $= 1000 \times 50 \frac{k g \times k m}{h}$

When a mass 250kg is dropped into it. New mass of the system = 1000+250 = 1250 kg

Let v is the velocity of the system.

By the conservation of linear momentum : Initial momentum = Final momentum

$1000 \times 50 = 1250 \times v$

$v = \frac{50, 000}{1250} = \frac{40 k m}{h} .$

Question 6.

In a rocket of mass 1000 kg fuel is consumed at a rate of 40 kg/s. The velocity of the gases ejected from the rocket is $5 \times 10^{4}$ m/s. The thrust on the rocket is

$2 \times 10^{3} N$
$5 \times 10^{4} N$
$2 \times 10^{6} N$
$2 \times 10^{9} N$

Discuss Question

Answer: Option C. ->

2 \times 10^{6} N

:
C

Thrust on the rocket $F = \frac{u d m}{d t} = 5 \times 10^{4} (40) = 2 \times 10^{6} N$

Question 7.

If the force on a rocket moving with a velocity of 300 m/s is 210 N, then the rate of combustion of the fuel is

0.7 kg/s
1.4 kg/s
0.07 kg/s
10.7 kg/s

Discuss Question

Answer: Option A. -> 0.7 kg/s
:
A

Force on the rocket $= \frac{u d m}{d t} ∴$ Rate of combustion of fuel $(\frac{d m}{d t}) = \frac{F}{u} = \frac{210}{300} = 0.7 k g / s$

Question 8.

Two blocks are in contact on a frictionless table one has a mass m and the other 2 m as shown in figure. Force F is applied on mass 2m then system moves towards right. Now the same force F is applied on m. The ratio of force of contact between the two blocks will be in the two cases respectively.

1 : 1
1 : 2
1 : 3
1 : 4

Discuss Question

Answer: Option B. -> 1 : 2
:
B

When the force is applied on mass 2m contact force $f_{1} = \frac{m}{m + 2 m} g = \frac{g}{3}$

When the force is applied on mass m contact force $f_{2} = \frac{2 m}{m + 2 m} g = \frac{2}{3} g$

Ratio of contact forces $\frac{f_{1}}{f_{2}} = \frac{1}{2}$

Question 9.

Three blocks of masses 2 kg, 3 kg and 5 kg are connected to each other with light string and are then placed on a frictionless surface as shown in the figure. The system is pulled by a force F = 10 N,, then tension $T_{1} =$

Discuss Question

Answer: Option C. -> 8N
:
C

By comparing the above problem with general expression. $T_{1} = \frac{(m_{2} + m_{3})}{m_{1} + m_{2} + m_{2}} = \frac{(3 + 5) 10}{2 + 3 + 5} = 8 N e w t o n$

Question 10.

Three masses of 15 kg. 10 kg and 5 kg are suspended vertically as shown in the fig. If the string attached to the support breaks and the system falls freely, what will be the tension in the string between 10 kg and 5 kg masses. Take $g = 10 m s^{- 2}$ . It is assumed that the string remains tight during the motion