Test: Work & Energy


10 Questions MCQ Test Engineering Mechanics | Test: Work & Energy


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This mock test of Test: Work & Energy for JEE helps you for every JEE entrance exam. This contains 10 Multiple Choice Questions for JEE Test: Work & Energy (mcq) to study with solutions a complete question bank. The solved questions answers in this Test: Work & Energy quiz give you a good mix of easy questions and tough questions. JEE students definitely take this Test: Work & Energy exercise for a better result in the exam. You can find other Test: Work & Energy extra questions, long questions & short questions for JEE on EduRev as well by searching above.
QUESTION: 1

When a body falls freely under gravity, then the work done by the gravity is:

Solution:
  • If a force acting on a body has a component in the direction of displacement, then the work done by the force is positive.
  • Hence when a body falls freely under the influence of gravity the work done by gravity is positive.
QUESTION: 2

Raindrops fall on the earth with certain kinetic energy on the earth. From where do they get this energy?

Solution:

Every object that tends to fall or does fall on earth's surface does have a gravitational acceleration which is possessed by them due to their gravitational potential energy.

QUESTION: 3

Time rate at which work is done by a force is:

Solution:

We know that Work done, W = F.s
where F is force and s is displacement due to that force.
Thus rate of work done is: 

dW/dt = d(F.s)/dt

So as F is constant we get, 
dW/dt = F.d(s)/dt = F.v = P (Power)

QUESTION: 4

There are two bodies X and Y with equal kinetic energy but different masses m and 4m respectively. The ratio of their linear momentum is:

Solution:

X and Y have equal kinetic energy but their masses are m and 4m respectively.
► 1/2 m1v12 = 1/2 m2v22  
► mv12 = 4m * v22  
► v1 : v2 = 2 : 1
Hence the ratio of their linear momentum is:
m1v1 : m2v2 = m * 2v : 4m * v = 1 : 2

QUESTION: 5

A truck moves at a steady speed on a horizontal road. What is the work done by the road on the truck? 

Solution:

The work done is zero because while the truck does have a displacement over time, the force of the road (the normal force) is perpendicular and so the dot product of the two is zero.

QUESTION: 6

Output of a truck is 4500 J and its efficiency is 50%, the input energy provided to the truck is:

Solution:

► η = work output / heat input​
► η = 50% = 50/100 = 1/2
► 1/2 = 4500 / Heat input
► Heat input = 9000 J

QUESTION: 7

By how much does kinetic energy increase if the momentum is increased by 20%?

Solution:

The kinetic energy is given by: 

KE= p2/2m

So, ΔKE = 2pΔp​ / 2m = pΔp / m​
ΔKE / KE ​= (pΔp/m)​ * (2m/p2)​ = 2Δp / p
Since the momentum p increases by 20%, so the final momentum becomes 1.2p.
Hence, KEfinal​ = (1.2p)/ 2m​ = 1.44p2 / 2m​ = 1.44KE
So, % change in KE = 44%

QUESTION: 8

When a body slides against a rough horizontal surface, the work done by friction is:

Solution:
  • If a force acting on a body has a component in the opposite direction of displacement, the work done is negative.
  • So when a body slides against a rough horizontal surface, its displacement is opposite to that of the force of friction. The work done by the friction is negative.
QUESTION: 9

 A machine gun fires 60 bullets per minute, with a velocity of 700 m/s. If each bullet has a mass of 50g, find the power developed by the gun.

Solution:

Mass of the bullets = 60×50 = 3000g = 3kg
v = 700m/s t = 1min = 60s
Power = W/t = (Kinetic energy)/t = 12250W.

QUESTION: 10

 If a force acts perpendicular to the direction of motion of a body, what is the amount of work done?

Solution:

If a force acts perpendicular to the direction of a body, the amount of work done is zero because there is no displacement in the direction of a force.

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