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12th Grade > Physics

NEWTON S LAWS OF MOTION MCQs

Total Questions : 30 | Page 1 of 3 pages
Question 1. 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 5×104 m/s. The thrust on the rocket is
  1.    2×103 N
  2.    5×104 N
  3.    2×106 N
  4.    2×109 N
 Discuss Question
Answer: Option C. -> 2×106 N
:
C
Thrust on the rocket F=udmdt=5×104(40)=2×106N
Question 2. 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
  1.    0.15×10−3N−s
  2.    0.98×10−3N−s
  3.    1.5×10−3N−s
  4.    2.5×10−3N−s
 Discuss Question
Answer: Option C. -> 1.5×10−3N−s
:
C
Impulse=force×time=50×105×3=1.5×103Ns
Question 3. In the above problem, if the speed of m2 is 50 cm/s the distance covered by m1 in 0.4 s will be
  1.    40 cm
  2.    20 cm
  3.    10 cm
  4.    80 cm
 Discuss Question
Answer: Option A. -> 40 cm
:
A
Distance covered by m2=50cm/s×0.4s=20cmSince the m1 mass cover double distance therefore S=2×20=40cm
Question 4. A man of mass m stands on a create of mass M. He pulls on a light rope passing over a smooth light pulley. The other end of the rope is attached to the crate. For the system to be in equilibrium, the force exerted by the men on the rope will be
A Man Of Mass M Stands On A Create Of Mass M. He Pulls On A ...
  1.    (M+m)g
  2.    12(M+m)g
  3.    Mg
  4.    mg
 Discuss Question
Answer: Option B. -> 12(M+m)g
:
B
From the free body diagram of man and crate system: For vertical equilibrium
2T = (M + m)g
T=(M+m)2g
A Man Of Mass M Stands On A Create Of Mass M. He Pulls On A ...
Question 5. A mass M is suspended by a rope from a rigid support at P as shown in the figure. Another rope is tied at the end Q, and it is pulled horizontally with a force F. If the rope PQ makes angle θ with the vertical then the tension in the string PQ is
A Mass M Is Suspended By A Rope From A Rigid Support At P As...
  1.    F sin θ
  2.    Fsin θ
  3.    C cos θ
  4.    Fcos θ
 Discuss Question
Answer: Option B. -> Fsin θ
:
B
From the figure
For horizontal equilibrium
Tsinθ=F
T=Fsinθ
A Mass M Is Suspended By A Rope From A Rigid Support At P As...
Question 6. A cricket ball of mass 150 gm is moving with a velocity of 12 m/s and is hit by a bat so that the ball is turned back with a velocity of 20 m/s. The force of blow acts for 0.01s on the ball. The average force exerted by the bat on the ball is
  1.    480 N
  2.    600 N
  3.    500 N
  4.    400 N
 Discuss Question
Answer: Option A. -> 480 N
:
A
v1=12 m\s and v2=+20m/s [because direction is reversed]
m=150gm=0.15kg,t=0.01sec
Force exerted by the bat on the ball F=m[v2v1]t=0.15[20(12)0.01]=480Newton
Question 7. Three weights W, 2W and 2W are connected to identical springs suspended form rigid lkhorizontal rod. The assembly of the rod and the weights fall freely. The positions of the weights from the rod are such that
  1.    3W will be farthest 
  2.    W will be farthest
  3.    All will be at the same distance
  4.    2 W will be farthest
 Discuss Question
Answer: Option C. -> All will be at the same distance
:
C
For W, 2W, 3W apparent weight will be zero because the system is falling freely. So there will be no extension in any spring i.e. the distances of the weight from the rod will be same.
Question 8. 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
  1.    12.5 km/hour
  2.    20 km/hour
  3.    40 km/hour
  4.    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 =1000×50kg×kmh
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
1000×50=1250×v
v=50,0001250=40kmh.
Question 9. 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=10ms2 . It is assumed that the string remains tight during the motion
Three Masses Of 15 Kg. 10 Kg And 5 Kg Are Suspended Vertical...
  1.    300 N
  2.    250 N
  3.    50 N
  4.    Zero
 Discuss Question
Answer: Option D. -> Zero
:
D
In the condition of free fall, tension becomes zero.
Question 10. 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 T1=
Three Blocks Of Masses 2 Kg, 3 Kg And 5 Kg Are Connected To ...
  1.    1N
  2.    5N
  3.    8N
  4.    10N
 Discuss Question
Answer: Option C. -> 8N
:
C
By comparing the above problem with general expression. T1=(m2+m3)m1+m2+m2=(3+5)102+3+5=8Newton

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