WORK AND ENERGY CLASS 9
In this article you will get CBSE Class 9 Science, chapter notes on Work and Energy. In these notes a brief and simple explanation of all important topics is given which will help students make an easy and quick revision of the chapter during the examinations.
Work (W)
Work is defined as a force acting upon an object to cause a displacement
It is expressed as the product of force and displacement in the direction of force.
W=F x s
Here, W= work done on an object
F = Force on the object
s = Displacement of the object
The unit of Work is Newton metre (Nm) or joule (J).
1 Joule is defined as the amount of work done by force of 1 N when displacement is 1 m.
Sign Conventions for Work Done
- when both the force and the displacement are in the same direction, positive work is done.
W = F x s - when force acts in a direction opposite to the direction of displacement, the work done is negative.
W= − F x s
Angle between force and displacement is 180o.
- If force and displacement are inclined at an angle less than 180o, then work done is given as:
W= Fs cosθ
- If force and displacement act at an angle of 90° then work done is zero.
CBSE Class 9 Science Syllabus (Latest)
Necessary Conditions for Work to be done
Two conditions need to be satisfied for work to be done:
- Force should act on the object.
- Object must be displaced.
Energy
The capacity of a body to do work is called the energy of the body.
Unit of energy = Joules
1KJ = 1000 J
Forms of Energy
The various forms of energy are potential energy, kinetic energy, heat energy, chemical energy, electrical energy and light energy.
Kinetic Energy
- It is the energy possessed by a body due to its motion. Kinetic energy of an object increases with its speed.
- Kinetic energy of body moving with a certain velocity = work done on it to make it acquire that velocity
Derivation
Let an object of mass m, starts from rest and attains a uniform velocity v, after a force F is applied on it. Let during this period the object be be displaced by distance s.
Potential Energy
The energy possessed by a body due to its position or shape is called its potential energy.
For Example:
- Water stored in a dam has large amount of potential energy due to its height above the ground.
- A stretched rubber band possesses potential energy due to its distorted shape.
Types of Potential Energy
On the basis of position and change in shape of object, potential energy is of two type:
1. Gravitational Potential Energy:
It is the energy possessed by a body due to it position above the ground.
2. Elastic Potential Energy:
It is the energy possessed by a body due to its change in shape.
Expression for Potential Energy
The potential energy (Ep) is equal to the work done over an object of mass ‘m’ to raise it by a height ‘h’.
Thus, Ep = mgh, where g = acceleration due to gravity.
Law of Conservation of Energy
It states that energy can neither be created nor destroyed, but it can be transformed from one form to another.
The total energy before and after the transformation remains the same.
Proof of Law of Conservation of Energy
Let a body of mass m falls from a point A, which is at a height h from the ground as shown in the following figure:
At point A,
Kinetic energy Ek = 0
Potential energy Ep = mgh
Total energy, EA = Ep + Ek
⟹ EA = mgh + 0
⟹ EA = mgh
During the fall, after moving a distance x from A, the body has reached at B.
At point B,
Let the velocity at this point be v.
We know, v2 = u2 + 2as
⟹ v2 = 0 + 2ax = 2ax [As, velocity at A, u = 0]
Also, Kinetic energy, Ek = 1/2 mv2
⟹ Ek =1/2 m × 2gx
⟹ Ek = mgx
Potential energy, Ep = mg(h – x)
So, total energy, EB = Ep + Ek
⟹ EB = mg (h − x) + mgx
⟹ EB = mgh – mgx + mgx
⟹ EB = mgh
At the end the body reaches the position C on ground.
At point C,
Potential energy, Ep = 0
Velocity of the body is zero here.
So, v2 = u2 + 2as
⟹ v2 = 0 + 2gh = 2gh
Kinetic energy, Ek = 1/2 mv2
⟹ Ek = 1/2 x m x 2gh = mgh
Total energy at C
EC = Ep + Ek
EC = 0 + mgh
EC = mgh
Hence, energy at all points remains same.
Power
The time rate of doing work is defined as power (P).
Power= work/time
Unit of power
- sI unit of Power is Joule per second or Js1.
- 1 watt is the power when 1J of work is done in 1s.
- The bigger unit of power is Kilowatt and represented by kW.
1kW = 1000W - Some another units to measure power are:
1 Megawatt = 106 watt
1 horse power = 746 watt
Commercial unit of energy
- Commercial unit of energy is kilo watt hour (kWh)
- The unit kilowatt-hour means one kilowatt of power supplied for one hour.
1 kWh = 1 kW x 1 h
= 1000 W x 60 x 60 s
= 1000 Js-1 x 3600 s
= 3.6 x 106 J
1 unit = 1 kilowatt hour = 3.6x106 J.
Question 1.
A light and a heavy object have the same momentum. Find out the ratio of their kinetic energies. Which one has a larger kinetic energy?
Question 2.
Calculate the kinetic energy of a car of mass 500kg moving with a velocity of 36km/h. Find the kinetic energy if the velocity of car doubles?
Question 3.
An electric heater of 1000 W is used for two hours in a day? What is the cost of using it for a month of 28 days, if one unit costs 3.00 rupees?
Question 4.
A crane pulls up a car weighing 500kg to a vertical height of 4m. Calculate the work done by the crane?
Given g=10m/s2
Question 5
A lamp consumes 500 J of electrical energy in 20 sec. What is the power of the lamp?
Question 7.
A geyser of 2.5kW is used for eight hours daily? Calculate the monthly consumption of electrical energy units. Also calculate the cost of electrical units consumption in a month if rate per unit is 3.50 rupees?
Question 8.
A 1.5 m high person is holding a 20 kg trunk on his head and is standing at a roadways bus-terminal. How much work is done by the person?
Question 9.
A body of mass 2kg is moving with a speed of 20m/s Find the kinetic energy. (400J)
Question 10
Calculate the change that should be affected in the velocity of a body to maintain the same KE , if mass of the body is increased to 4 times (half the original velocity)
Question 11.
A weighting 50kg runs up a hill rising himself vertically 10m in 20Sec. Calculate power. given g = 9.8m-1 (245w)
Question 12.
A athlete weighing 60kg runs up a staircase having 10 steps each of 1m in 30 sec. Calculate power (g = 9.8ms-1 ) (ans: 200W)
Question 13.
Calculate the time taken 60 w bulb to consume 3000 J of energy . (50sec. )
Question 14.
An electric kettle of 500W is used to heat water everyday for 2 hours . Calculate the number of unit of electrical energy consumed y it in 10 days.
Question 15.
Calculate the cost of using a 2kwh immersion rod for heating water in a house for one hour each day for 60 days if the rate is Rs. 1.50 per unit kWh. (Rs. 180)
Question 16.
The power of a heart which beats 72 times in a minute is 1.2kW. Calculate the work done by heart for each beat. ( 1kJ)
Question 17.
An electric kettle of 500W is used to heat water everyday for 2 hours . Calculate the number of unit of electrical energy consumed y it in 10 days
Question 18
A force of 10N causes a displacement or 2m in a body in its own direction. Calculate the work
done by force. 20j)
Question 19
How much force is applied on the body when 150joule of work is done in displacing the body
through a distance of 10m in the direction of force?(15 N)
Question 20.
A body of 5kg raised to 2m find the work done(98j)
Question 21.
A work of 4900j is done on road of mass 50 kg to lift it to a certain height. Calculate the height
through which the load is lifted. (10m)
Question 22.
An engine work 54,000J work by exerting a force of 6000N on it. What is the displacement of
the force . (10m)
Match the column
Column A
(Energy Conversion)
| Column B
(Converters)
|
Chemical energy into Mechanical energy
| Electric Generator
|
Heat energy into Mechanical engine
| Electric Motor
|
| Mechanical energy into Electrical energy | Car engine
|
Electrical energy to Mechanical energy
| Steam engine
|
Light energy into electrical energy
| Electric bulb
|
Electrical energy into light energy
| Solar Cell
|
Fill in the blanks
- 1 Kilo Watt hour of energy is equal to ________ Joule
- Power is the rate of doing ______. And its unit is ___________
- Work done by the force can be _______ and ______.
- The water stored in the reservoir of the tank possessed _______ energy
- The total energy of the swinging pendulum remains ______ at all the points
- When the body falls freely towards earth ,potential energy of the body _______while kinetic energy of the body______. The total remains ______at all the point during the motion
ANSWE
- 3.6X106
- Work, watt
- Negative,zero,positive
- Potential
- Constant
- Decrease, increase, constant
Important Questions for CBSE Class 9 Science Chapter 11 Work, Power and Energy
IMPORTANT QUESTIONS
1-MARK QUESTIONS
Question.1 Does work done depend upon the velocity of the body. [SAII-2014]
Answer. No.
Question.2 State the law of conservation of energy. [SAII-2014]
Answer. It states that energy can neither be created nor destroyed. It can only change its form.
Question.3 In a tug-of-war one team gives way to the other. What work is being done and by whom ? [SAII-2014]
Answer.
The winning team does work. The work is equal to the product of the resultant force and the displacement undergone by the losing team.
Question.4 What will cause greater change in kinetic energy of a body? Changing its mass or changing its velocity ?
Answer. Changing its velocity.
Question.5 List two essential conditions for work to be done. [SAII-2010]
Answer. (i) A force must act and (ii) There should be displacement in the body.
Question.6 When is 1 joule of work said to be done ?
Answer. When a force of 1 newton acting on a body displaces it in its own direction.
Question.7 What is the SI unit of work done and power ?
AnswQuestion.15 Is it possible that some force is acting on a body but still the work done is zero ?
Answer. Yes, when force acts at an angle of 90° with the displacement.
Question 8 What is the work done on a body moving in a circular path ?
Answer. Zero, because force and displacement are perpendicular to each other.
Question 9 Does every change in energy of the body involve work ?
Answer. Yes.
Question.10 What is the work done in the situation shown below ?
Answer. Zero
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