## Work Power and Energy Worksheet:-

## Introduction:-

### Today I am trying to explain an important topic 'work power and energy ' for on going school children of standard (ix) and standard (x) .Hope this will help them a lot.

To start my discussion first we will first know some basic definitions related to this topic.

## **What is force?

### Answer:- Force is an external agency which is when applied on a body changes the state of motion or state of rest of a body or tries to change the state of a body. It is denoted by 'F' ; It is a vector quantity and its C.G.S unit is dyne and S.I unit is Newton.

## ** what is distance?

### Anwer:- The total path (which may be curved or straight) covered by a moving body with respect to any interval of time is termed as distance in science. It has only magnitude but no direction,so it is a scaler quantity.

### Now we will come to our main point,

## **What is work?

### Answer:-

### If an external force is applied on a rigid body which is at rest and the point of contact of the force displaces i.e;if the body changes its state of rest and occurs a displacement, then in science it is called work done or work has been done. It is denoted by W. The C.G.S unit of work is erg and its S.I unit is Joule.

### Mathematically,

### work or work done = F× D

where, F= Applied external force.

and D = Distance/Displacement of the body.

## Problems related on work:-

## Question(i):- If an external force of 10N is applied on a body, and the body displaces

2000cm, in the direction of motion find the work which has been done.

### Answer:- ∵ Force (F) = 10 N

### Distance(D) = 2000 cm

= 2000/100 m [∵ 100 cm = 1m ]

= 20 m

∴ work or work done (W) = F× D

= (10 × 20) J

= 200 J

## Question(ii):- After application a force on a rigid body ,the body is displaced 5 m and the work is done 5 × 10¹⁰ erg, find the amount of force.

### Answer:- ∵ Work (W) = 5× 10¹⁰ erg

### = 5× 10¹⁰ /10⁷ Joule [ ∵ 10⁷ erg =1Joule]

= 5 × 10³ Joule

Distance = 5 m

∴ Amount of force (F) = W/D

= 5 × 10³/5

=10³ N

## Question(iii):- A body is at rest and a force of 20N applied on it and in the process a work of 60 J is done, find the distance the body is displaced.

### Answer:- ∵ Force (F) = 20 N

### Work (W) = 60 J

∴ Distance(D) = W/F

= 60/20

= 3 m

## **What is power?

### Answer:-

### The rate of doing or performing work in science is called power. It is denoted by P .

Mathematically, Power can be expressed as:-

P = W/t

where, P = Power

W = work done

and t = time

## Units of power:-

### Answer:-

In C.G.S system the unit of power is erg/sec and in M.KS system or S.I system the unit of powerp is Joule/sec, which is commonly known as watt. Its bigger unit which is commonly called commercial unit is Kwatt.

### Relation between watt and kilowatt.

### Answer:- Relation between watt and kilowatt is given by :-

1kilowatt = 10³ or 1000 watt

## Problems related on power:-

## Question(i):- A motor with 2HP can fill up a tank with water in 1 hr, find the work which is done by the motor in kilojoule.

### Answer:- ∵ Power (P) = 2 HP

### = (2× 746)watt [∵ 1 HP = 746 watt]

Time(t) = 1hr = (60 ×60)sec

= 3600 sec

∴ Work has been done by the motor (W)

= P × t

= 2× 746 ×3600

= 5371200 Joule

= 5371200/1000 KJ [∵ 1000joule = 1kilo Joule]

= 5371.2 KJ

## Question(ii):- A mechine can do work 60 J in 2 minutes, find its power .

### Answer:- Work(W) = 60 J

Time (t) = 2 minutes

= (2× 60)sec [ ∵ 1 minute = 60 sec]

∴ Power(P) of the mechine = W/t

= 60/(2×60)

= ½

=0.5 watt

## Question (iii):- If power 1Kwatt, and work 500 J, find time.

### Answer:- ∵ Power (P) = 1 Kw

= 1000 w [ ∵ 1000watt = 1 Kw]

Work (W) = 500 J

∴ Time ( t) = W/P

= 500/1000

= ½ sec.

= 0.5 sec

## **What is energy?

### Answer:- The capacity of doing or performing work is called energy. ∵ the ability of doing work is energy, so the units of energy are same as that of units of work i.e; erg (in C.G.S system) and Joule (in M.K.S or S.I system). Its commercial units are kilowatt-hour and units.

## Problems related on energy :-

## Question (i):-If an electric bulb of 100watt lights up for 2hrs daily for a month find the energy consumed by the bulb.

### Answer:-

Power (P) = 100 watt

= 100/1000 kw [∵ 1000 watt = 1 kw]

= 0.1 kw

∵ bulbs lights up in 1 day = 2 hrs

∴ bulbs lights up in 30days = (30×2)hrs

= 60 hrs

∴ Time (t) = 60 hrs

∴ Energy consumed by the bulb (W)

= P× t

= (0.1 × 60) kwt.hr

= 6 kwt.hr

= 6 units [ ∵ 1 kwt.hr = 1 unit]

## Question(ii):-During the Powerhouse lab, Ravi walks up the stairs, elevating his 100 kg body a vertical distance of 4meters in a time of 20 seconds at a constant speed.

(a)Determine the energy consumed by Rabi climbing the stair case.

## (b)Determine the power generated by Rabi.

### Answer :- ∵ Mass (m) = 100 kg

height (h) = 4 meters

Acceleration due to gravity (g)

= 10 m/s² [ let, us assumed]

Time (t) = 20 seconds

(a) ∴ Energy consumed by Rabi(W) = mgh

= 100 × 10 × 4

= 4000 J

(b) ∴ Power = W/t

= 4000/20

= 200 J/s

= 200 watt.

## (iii) Sumi has a weight 400N and she climbs a height 8 m in 20 sec, find her power.

### Answer:- ∵ Weight (w) = 400 N

Height (h) = 8 m

Time (t) = 20 sec

∴ Work has been done by her (W)

= w × h

= (400 × 8) J

∴ Her power = W/t

= (400 × 8)/20

= 20 × 8

= 160 watt

## (iv) An electric iron has power 1500 watt and it consumed an energy 15000 watt.hr

find the time taken by the iron to consume it.

### [Note:- Students are advised 1st realise the above process how to solve work,power and energy problems then try similar type of different problems]

## FREQUENTLY ASKED QUESTIONS:-
## What are the C.GS and S.I units of work, power and energy?

Answer: The C.G.S units of work,power and energy are :- erg, erg/sec and erg respectively.
The S.I units of work, power and energy are :- Joule, watt and joule respectively.

## what is kinetic energy formula?

Answer:- Kinetic energy formula is = ½ mv² where, m= mass of the moving body. v = velocity of the moving body.

## what is potential energy formula?

Answer :- Potential energy formula is = mgh
where , m = mass of the body
g = acceleration due to gravity.
h = height of the body from the ground.

## What is commercial unit of energy ?

Answer :- Commercial unit of energy is kwt.hr which also known as unit.

## What is the relation between erg and joule?

Answer:- The relation between erg and joule is,
10⁷ erg = 1 joule.

## What is the law of conservation of energy?

Answer :- The law of conservation of energy is :-
Energy can not be created nor it can be destroyed but it can be converted one form to another. This is known as law of conservation of energy.

## What are the C.GS and S.I units of work, power and energy?

Answer: The C.G.S units of work,power and energy are :- erg, erg/sec and erg respectively. The S.I units of work, power and energy are :- Joule, watt and joule respectively.

## what is kinetic energy formula?

Answer:- Kinetic energy formula is = ½ mv² where, m= mass of the moving body. v = velocity of the moving body.

## what is potential energy formula?

Answer :- Potential energy formula is = mgh where , m = mass of the body g = acceleration due to gravity. h = height of the body from the ground.

## What is commercial unit of energy ?

Answer :- Commercial unit of energy is kwt.hr which also known as unit.

## What is the relation between erg and joule?

Answer:- The relation between erg and joule is, 10⁷ erg = 1 joule.

## What is the law of conservation of energy?

Answer :- The law of conservation of energy is :- Energy can not be created nor it can be destroyed but it can be converted one form to another. This is known as law of conservation of energy.