9th Class Physics Unit No. 6 Work and Energy (Exercise Solution)
UNIT No. 6Work and Energy
(Exercise Solution)
6.1 Encircle the correct answer from the given choices.
i. The work done will be zero when the angle between the force and the distance is
ii. If the direction of motion of the force is perpendicular to the direction
of motion of the body, then work done will be
(a) Maximum
(b) Minimum
(c) zero
(d) None of the
above
iii. If the velocity of a body becomes double, then its kinetic energy
will
(a) remain the
same
(b) become
double
(c) become four
times
(d) become half
iv. The work done in lifting a brick of mass 2 kg through a height of 5
m above ground will be
(a) 2.5 J (b)
10 J (c)
50 J (d) 100 J
v. The kinetic energy of a body of mass 2 kg is 25 J. Its speed is
(a) 5 ms-1 (b) 12.5 ms-1 (c)
25 ms-1 (d) 50
ms-1
vi. Which one of the following converts light energy into electrical energy?
(a) electric
bulb
(b) electric
generator
(c) Photocell
(d) Electric
cell
vii. When a body is lifted through a height h, the work done on it appears
in the form of its:
(a) kinetic
energy
(b) potential
energy
(c) elastic
potential energy
(d) geothermal
energy
viii. The energy stored in coal is
(a) heat energy
(b) kinetic
energy
(c) chemical
energy
(d) nuclear
energy
ix. The energy stored in a dam is
(a) electric
energy
(b) potential
energy
(c) kinetic
energy
(d) thermal
energy
x. In Einstein's mass-energy equation, c is the
(a) speed of
sound
(b) speed of
light
(c) speed of
electron
(d) speed of
Earth
xi. Rate of doing work is called
(a) energy (b)
torque
(c) power (d) momentum
Answers of Multiple Choice Questions (MCQs)
|
i. |
(c) |
ii. |
(c) |
|
iii. |
(c) |
iv. |
(c) |
|
v. |
(a) |
vi. |
(c) |
|
vii. |
(c) |
viii. |
(c) |
|
ix. |
(b) |
x. |
(b) |
|
xi. |
(c) |
|
|
6.2 Define work. What is its SI unit?
Ans: Definition
“Work
is done when a force acting on a body displaces it in the direction of a force”
Work done = Force x displacement
W= FS
SI
unit of work is joule (J). It is defined as
“The amount of work is one joule when a force of one
newton displaces a body through one meter in the direction of force.”
6.3 When does a force do work? Explain.
Ans: Work is done when a force acting on a body displaces it in the direction of a force.
Work = (Component of force in the
direction od displacement)(displacement)
W = Fx S
W = (FcosѲ)S
W = FScosѲ
6.4 Why do we need energy?
Ans: We need energy to do work.
6.5 Define energy, give two types of mechanical energy.
Ans: Energy
“A
body possesses energy if it is capable to do work”
There
are two types of mechanical energy
1. Kinetic Energy
2. Potential Energy
“The energy possessed by a body due to its motion is
called its kinetic energy.”
\[K.E = \frac{1}{2}mv^{2}\]
Potential
Energy
“The
energy possessed by a body due to its position is known as its potential energy.”
P.E. = mgh
6.6 Define K.E. and derive its relation.
Ans: Definition
“The
energy possessed by a body due to its motion is called its kinetic energy.”
Derivation
Consider
a body of mass m moving with velocity v. The body stops after
moving through some distance S due to some opposing force such as force
of friction acting on it. The body possesses kinetic energy and is capable to
do work against opposing force F until
all of its kinetic energy is used up.
K.E. of the body = Work done by it due to motion
K.E. = FS ------------------- (1)
As
vi = v
vf = 0
By
Newton second law of motion
F= ma
\[a = \frac{F}{m}\]
Since
motion is opposed, hence, a is negative.
Using 3rd equation of motion
\[2aS = v_{f}^{2}-v_{i}^{2}\]
Putting
values in this equation
\[2(\frac{-F}{m})S = 0-v^{2}\]
\[\frac{2FS}{m} = v^{2}\]
\[ FS = \frac{1}{2}mv^{2} ---- (2)\]
From equation (1) putting FS = K.E in equation (2)
\[ KE = \frac{1}{2}mv^{2}\]
This
equation gives the K.E possessed by a body of mass m moving with
velocity v.
6.7 Define potential energy and derive its relation.
Ans: Definition
“The
energy possessed by a body due to its position is known as its potential
energy.”
Derivation
Let
a body of mass m be raised up through height h from
the ground. The body will acquire potential energy equal to the work done in
lifting it to height h. Thus
Potential energy = work done in lifting the body to
the height h
P.E. = Fh
Here
F = w, force is equal to the weight of the object
P.E. = wh
Here
weight of the body = w = mg
P.E. = mgh
This
equation gives the Potential Energy possessed by the body with respect to the
ground.
6.8 Why fossils fuels are non-renewable form of energy?
Ans: Fossil Fuels are called non-renewable forms of energy because it took millions of years for them to attain the present form. Once they used to get energy and consumed completely, they cannot be renewed or replenished in short duration
6.9 Which form of energy is most preferred and why?
Ans: Solar energy is the most preferred.
Explanation:
Solar energy is the energy coming
from the Sun and is used directly and indirectly. Sunlight does not pollute the
environment in any way. The sunrays are the ultimate source of life on Earth.
If we find a suitable method to use a fraction of the solar energy reaching the
Earth, then it would be enough to fulfil our energy requirement.
6.10 How is energy converted from one form to another? Explain.
Ans: Energy cannot be destroyed however it can be converted into some other forms.
Example
1. Rub your hands together quickly. You will feel them warm. You
have used your muscular energy in rubbing hands as a result heat is produced.
In the process of rubbing hands, mechanical energy is converted into heat
energy.
2. Processes in nature are the results of energy changes. For example, some of the heat energy from the Sun is taken up by water in the oceans. This increases the thermal energy. Thermal energy causes water to evaporate from the surface to form water vapours. These vapours rise up and form clouds. As they cool down, they form water drops and fall down as rain. Potential energy changes to kinetic energy as the rain falls. This rainwater may reach a lake or a dam. As the rainwater flows down, its kinetic energy changes into thermal energy while parts of the kinetic energy of flowing water is used to wash away soil particles of rocks known as soil erosion
6.11 Name the five devices that convert electrical energy into mechanical energy.
Ans: Following devices converts electrical energy to mechanical energy.
1. Electric motor
2. Drill machine
3. Electric fan
4. Elevator
5. Electric grinder
6.12 Name a device that converts mechanical energy into electrical energy.
Ans: Electric generator converts mechanical energy into electrical energy.
6.13 What is meant by the efficiency of a system?
Ans: Definition
“Efficiency of a system is the ratio of required form of energy obtained from a system as output to the total energy given to it as input.”
\[Efficiency = \frac{Required form of energy}{total input energy}\]
\[Efficiency = \frac{Required form of energy}{total input energy}X100\]
6.14 How can you find the efficiency of a system?
Ans: We can find efficiency of a system by the following relation6.15
What is meant by the term power?
Ans: Definition
“Power
is defined as the rate of doing work.”
Mathematically
\[Power = \frac{Work done}{Time Taken}\]
\[P = \frac{W}{t}\]
Power is a scalar quantity. SI unit of power is watt (W).
6.15 Define watt.
Ans: Definition
“The
power of a body is one watt if it does work at the rate of 1 joule per second.”
Mathematically
\[Power = \frac{Work done}{Time Taken}\]
\[P = \frac{W}{t}\]
.
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