Work, Energy and Power Test

Result

Work, Energy and Power Test - 5

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Weekly Quiz Competition

Question 1
1 / -0

A body of a mass 10 kg falls from a building 400 m height. What is the difference between the energy possessed by the body at any instance and initial energy?

A
4 × 10³ J

B
0 J

C
4 J

D
None of these

Solution

Using conservation of energy , total mechanical energy of the system will remain conserved
So ,energy at any instant is always equal to the initial energy
Hence the difference between the energy at any instant and the initial energy is always zero.
Question 2
1 / -0

Find the kinetic energy of a car having mass 1000 kg and is moving with a speed of 40 m/s.

A
8 10⁵ kJ

B
8 kJ

C
800 kJ

D
8000 J

Solution

Kinetic energy of the body = mv²
= 1000 40 40
= 500 1600
= 80 10⁴
= 8 10⁵ J
Question 3
1 / -0

What should be the speed of athlete who has total weight of 300 N so that he gets 450 J of kinetic energy? Take g = 10 m/s²

A
5.4 m/s

B
4.4 m/s

C
3.4 m/s

D
None of these

Solution

It is given that W = 300 N, g = 10 m/s²
K. E = 450 J, g = 10 m/s²
or W = mg
or 300 = mg
or 300 = m × 10
m = 30 kg
or K. E = mv²
450 = × 30 ×v²
v² = 30
v =
v = 5.4 m/s
Question 4
1 / -0

For the same kinetic energy of a body, what should be the change in its mass if its velocity is tripled?

A
Mass should be times the initial mass

B
Mass should be 9 times the initial mass

C
Mass should be 3 times the initial times

D
None of these

Solution

Let initial mass m₁ = m, velocity = v₁ = v
Final mass m₂, velocity = v₂ = 3v
For the same kinetic energy
m₁v₁² = m₂v₂²
× m × v² = m₂× 9v²
m₂ =
Question 5
1 / -0

Calculate the momentum of the body having kinetic energy equal to 2 kJ and mass equal to 100 gram.

A
20 kg cm/sec

B
20 kg m/sec

C
40 kg m/sec

D
None of these

Solution

Since we know that
K. E = mv² or p = mv = momentum
or K. E =
or 2 × 10³ =
p² = 2 × 10³ × 2 × 0.1
p² = 400 or 20 kgm/sec
Question 6
1 / -0

What is the potential energy of a spring which is being compressed by a external body having mass 100g? It is known that on the removal of external body, spring moves with a speed of 1ms¹.

A
50 × 10^-3 J

B
5 × 10^-3 J

C
500 × 10^-3 J

D
None of these

Solution

m = 100g = 100 × 10^-3 kg
Velocity v = 1 m/sec
On the removal of external body, spring tries to want to its initial state by pushing to the external body.
Therefore potential energy of spring
= kinetic energy of the external body
U = mv² = × 100 × 10^-3 × 1
= 50 × 10^-3 J
Question 7
1 / -0

Power cannot be measure in

A
kilowatt hour

B
Joule/sec

C
horse power

D
Watt

Solution

kilowatt hour is not the unit of power all others are unit of power.
Question 8
1 / -0

At which point(s) potential energy of the simple pendulum is maximum?

A
at the equilibrium

B
at turning points of oscillation

C
at remain same always

D
cannot be determined

Solution

Potential energy of the simple pendulum is maximum at turning points of oscillation - as height is maximum at these points. Since potential energy = mgh, therefore potential energy of simple pendulum is maximum at turning points.
Question 9
1 / -0

What type of energy conversion takes place in electric cell?

A
Chemical to Mechanical

B
Electric to Chemical

C
Chemical to Electrical

D
Electrical to Mechanical

Solution

From an electric cell, when current is drawn. The chemical energy stored in it changes to electrical energy.
Question 10
1 / -0

What kind of energy conversion takes place in photoelectric cell?

A
Electrical to light energy

B
Light to electrical energy

C
Electrical to Mechanical energy

D
Light to Mechanical energy

Solution

In photoelectric cell, light energy gets converted into electrical energy.
Question 11
1 / -0

Which of the following is used to change heat energy into electrical energy?

A
Thermocouple

B
Thermostat

C
Hydrometer

D
None of these

Solution

Thermocouple is used to change heat energy into electrical energy
When two junctions of thermocouple are kept at different temperature, current start flowing through the thermocouple.
Question 12
1 / -0

Which of the following is true about simple pendulum?

A
at end points P. E = mgh, K. E = mgh

B
at centre (mid points), P. E = 0, K. E = mgh

C
at centre (mid points), P. E = K. E = mgh

D
none of these

Solution

Potential energy of simple pendulum is zero at centre and its kinetic energy is equal to mgh.
At peak points, K. E = 0 and P. E = mgh
Question 13
1 / -0

What will be the K. E of the body if it is moving with 30 m/s?

It is known that same body has K. E equal to 40 J when moving at 10 m/s.

A
36 J

B
360 J

C
720 J

D
1440 J

Solution

mv² = 40 J when v = 10 m/s when v = 10 m/s
or × m × 100 = 40
or m =
m = kg or 800 gm
K. E at 30 m/s
= × × 30 × 30
=
= 8 × 45 = 360 J