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

12th Grade > Physics

NEWTON S LAWS OF MOTION MCQs

Total Questions : 30 | Page 2 of 3 pages

Question 11. 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 12. 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

1.75×105 N
3.5×105 N
7.0×105 N
14.0×105 N

Discuss Question

Answer: Option C. -> 7.0×105 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 13. A block A of mass 7 kg is placed on a frictionless table. A thread tied to it passes over a frictionless pulley and carries a body B of mass 3 kg at the other end. The acceleration of the system is

(g i v e n g = 10 m s^{- 2})

100 ms−2
3 ms−2
10 ms−2
30 ms−2

Discuss Question

Answer: Option B. -> 3 ms−2
:
B

a = [\frac{m_{2}}{m_{1} + m_{2}}] g = [\frac{3}{7 + 3}] 10 = 3 m / s^{2}

Question 14. A car moving at a speed of 30 kilo meters per hour is brought to a halt in 8 metres by applying brakes. If the same car is moving at 60 km. per hour, it can be brought to a halt with same breaking power in

8 metres
16 metres
24 metres
32 metres

Discuss Question

Answer: Option D. -> 32 metres
:
D

F r o m v^{2} = u^{2} - 2 a s 0 = u^{2} - 2 a s s = \frac{u^{2}}{2 a} \Rightarrow s α u^{2} (i f a = c o n s t a n t) \frac{s_{2}}{s_{1}} = {(\frac{u_{2}}{u_{1}})}^{2} = {(\frac{60}{30})}^{2} = 4 \Rightarrow s_{2} = 4 s_{1} = 4 \times 8 = 32 m e t r e s

Question 15. A light string passing over a smooth light pulley connects two blocks of masses

m_{1}

and

m_{2}

(vertically). If the acceleration of the system is g/8 then the ratio of the masses is

8 : 1
9 : 7
4 : 3
5 : 3

Discuss Question

Answer: Option B. -> 9 : 7
:
B

a = (\frac{m_{2} - m_{1}}{m_{1} - m_{2}}) g = \frac{g}{g}

; by solving

\frac{m_{2}}{m_{1}} = \frac{9}{7}

Question 16. 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 17. 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 18. 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 19. With what minimum acceleration can a fireman slides down a rope while k breaking strength of the rope is

\frac{2}{3}

his weight

23g
g
13g
Zero

Discuss Question

Answer: Option C. -> 13g
:
C
When fireman slides down, Tension in the rope T = m(g – a)
For critical condition

m (g - a) = \frac{2}{3} m g \Rightarrow m g - m a = \frac{2}{3} m g ∴ a = \frac{g}{3}

Question 20. The ratio of the weight of a man in a stationary lift and when it is moving downward with uniform acceleration ‘a’ is 3:2. The value of ‘a’ is (g – Acceleration due to gravity on the earth)

Discuss Question

Answer: Option B. -> g3
:
B