Work and Energy Test

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Work and Energy Test - 50

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Weekly Quiz Competition

Question 1
1 / -0

An object of mass $$5 \,kg$$ falls from a height of $$5 \,m$$ above the ground. The loss of potential energy of the mass is:

A
$$250 \,J$$

B
$$25 \,J$$

C
$$2.5 \,kJ$$

D
$$50 \,J$$

Solution

Given,
$$Mass = 5 \,kg$$
$$Height = 5 \,m$$
Let $$g = 10 \,ms^{-2}$$
Potential energy stored in a body of mass m and height h will be given by
$$= m \times g \times h$$
$$= 5 \,kg \times 10 \,ms^{-2} \times 5 \,m$$
$$= 250 \,J$$
When the mass falls completely, same potential energy is lost $$= 250 \,J$$
Question 2
1 / -0

A car is accelerated on a leveled road and attains a velocity $$4$$ times its initial velocity. In this process, the potential energy of the car:

A
Does not change

B
Becomes twice to that of initial

C
Becomes $$4$$ times that of initial

D
Becomes $$16$$ times that of initial

Solution

Potential energy depends on the height at which the object is situated. Where potential energy is measured by the product of the mass of the object, gravity, and height. In this case, there is no change in height of the object. Hence, there is no change in its potential energy.
Question 3
1 / -0

When a boy triples his speed, his kinetic energy becomes

A
half

B
double

C
nine times

D
no charge

Solution

Given that, speed of particle is becoming 3 times of its initial value while moving. ie $$v'=3v$$
As we know $$K.E = \dfrac{1}{2}$$ $$ mv ^{2}$$ ..........(1)
After velocity tripled $$K.E' = \dfrac{1}{2}$$ $$ m(3v) ^{2}=\dfrac92mv^2$$...........(2)
From equation 1 and 2
$$K.E' = 9 \times K.E$$
When a boy tripled his speed, his kinetic energy will becomes nine times.
Question 4
1 / -0

The $$S.I. $$ unit of work is

A
second

B
metre

C
joule

D
newton

Solution

Work $$=$$ Force $$\cdot$$ displacement

S.I Unit of force = N
S.I Unit of displacement = m
Hence Unit of Work $$=$$ N-m
N-m is known as Joule (J), Hence unit of work is Joule.
Option C
Question 5
1 / -0

Two coolies $$ A $$ and $$B$$ do same work in time $$ 1$$ minute and $$ 2 $$ minute respectively. The power spent is

A
same by both coolies

B
is more by coolie $$A$$ than by $$ B $$

C
is less by coolie $$A $$ than by $$B$$

D
nothing can be said.

Solution

Two coolies A and B do some work in time 1 minute and 2 minute respectively.

$$ P = \dfrac{W}{t}$$
$$ \implies P \propto \dfrac{1}{t}$$

$$t_A < t_B \implies P_A > P_B $$
The power spent is more by coolies $$A$$ than by $$ B $$.
Question 6
1 / -0

The decrease in the potential energy of a ball of mass $$ 20 kg $$ which falls from a height of $$ 50 cm $$ is

A
$$ 968 J $$

B
$$ 98 J $$

C
$$ 1980 J$$

D
None of these

Solution

Decrease in potential energy $$\Delta U =mg(h_2-h_1)$$
$$ \Delta U= mgh =20 \times 9.8 \times 0.5=98 J $$
Question 7
1 / -0

A body at rest may have

A
Energy

B
Momentum

C
Speed

D
Velocity

Solution

if Body is at rest , that means velocity is zero so speed is also zero.
$$v=0$$
$$KE=\dfrac{1}{2}mv^2=0$$
Body at rest may possess potential energy, due to it's height or other reason.
Question 8
1 / -0

If the stone is thrown up vertically and return to ground, its potential energy is maximum

A
During the upward journey

B
At the maximum height

C
During the return journey

D
At the bottom

Solution

Potential energy = $$ mgh $$
$$PE \propto h$$
Potential energy is maximum when $$ h $$ is maximum
Question 9
1 / -0

If velocity of a body is twice of previous velocity, then kinetic energy will become

A
$$ 2 $$ times

B
$$ \frac{1}{2} $$ times

C
$$ 4 $$ times

D
$$ 1$$ times

Solution

Kinetic energy=$$ \dfrac{1}{2} mv^2 \therefore K.E. \propto v^2 $$
$$v\rightarrow 2v$$
$$K \rightarrow 2^2=4K$$
If velocity is doubled then kinetic energy will become four times.
Question 10
1 / -0

A man does a given amount of work in 10 sec. Another man does the same amount of work in 20 sec. The ratio of the output power of first man to the second man is

A
$$ 1 $$

B
$$ 1 / 2 $$

C
$$ 2/ 1$$

D
None of these

Solution

$$ Power = \dfrac {W}{t} . $$ if W is constant then $$ P \propto \dfrac {1}{t} $$
i.e. $$ \dfrac {P_1}{P_2} = \dfrac {t_2}{t_1} = \dfrac {20}{10} = \dfrac {2}{1} $$

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Re-Attempt Weekly Quiz Competition

Work and Energy Test - 50

Result

Work and Energy Test - 50

Score

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SHARING IS CARING

Question 1

$$250 \,J$$

$$25 \,J$$

$$2.5 \,kJ$$

$$50 \,J$$

Solution

Question 2

Does not change

Becomes twice to that of initial

Becomes $$4$$ times that of initial

Becomes $$16$$ times that of initial

Solution

Question 3

half

double

nine times

no charge

Solution

Question 4

second

metre

joule

newton

Solution

Question 5

same by both coolies

is more by coolie $$A$$ than by $$ B $$

is less by coolie $$A $$ than by $$B$$

nothing can be said.

Solution

Question 6

$$ 968 J $$

$$ 98 J $$

$$ 1980 J$$

None of these

Solution

Question 7

Energy

Momentum

Speed

Velocity

Solution

Question 8

During the upward journey

At the maximum height

During the return journey

At the bottom

Solution

Question 9

$$ 2 $$ times

$$ \frac{1}{2} $$ times

$$ 4 $$ times

$$ 1$$ times

Solution

Question 10

$$ 1 $$

$$ 1 / 2 $$

$$ 2/ 1$$

None of these

Solution

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