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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.
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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 m_{1}v_{1}^{2} = 1/2 m_{2}v_{2}^{2}

► mv_{1}^{2} = 4m * v_{2}^{2}

► v_{1} : v_{2} = 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= p ^{2}/2m**

So, ΔKE = 2pΔp / 2m = pΔp / m

ΔKE / KE = (pΔp/m) * (2m/p^{2}) = 2Δp / p

Since the momentum p increases by 20%, so the final momentum becomes **1.2p**.

Hence, KE_{final} = (1.2p)^{2 }/ 2m = 1.44p^{2} / 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
**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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