2. Work and Energy
- ‘Work is said to be done when a force applied on an object causes displacement of the object.’
- Work = force × displacement
- W = Fs
- W = F s cosθ
- cos 30° = √3/2 cos 45° = 1/√2 cos 60° = 1/2
Units of Work / Energy :
1. SI : 1 Joule : If a force of 1 newton displaces an object through 1 metre in the direction of the force, the amount of work done on the object is 1 joule.
- 1 joule = 1 newton x 1 metre
- 1 J = 1 N x 1 m
2. CGS :1 erg : If a force of 1 dyne displaces an object through 1 centimetre in the direction of the force, the amount of work done is 1 erg.
- 1 erg = 1 dyne x 1 cm
- 1 newton = 10⁵dyne 1 m = 10² cm
- 1 joule = 10⁷ erg
Types of Work :
1. Positive work :
When the force and the displacement are in the same direction (θ =0°) the work done by the force is positive.
2. Zero Work :
When the force and the displacement are in the same direction (θ =90°) the work done by the force is Zero.
3. Negative work :
When the force and the displacement are in the same direction (θ =180°) the work done by the force is Negative
+ Point :
Q.01 : Why work done by the gravitational force is zero when an artificial satellite is moving around the earth in a circular orbit.
Ans : Gravitational force acting on the satellite (along the radius of the circle) and its displacement (along the tangent to the circle) are perpendicular (θ =90°) to each other, the work done by the gravitational force is zero.
The National Physical Laboratory, New Delhi
- It was conceptualized in 1943.
- Its Functions under the Council of Scientific and Industrial Research.
Energy :
• The capacity of a body to perform work is called its energy.
• The units of work and energy are the same.
• The unit in SI system is joule while that in cgs system is erg.
Forms of energy :
Mechanical, Heat, Light, Sound, Electro-magnetic, Chemical, Nuclear and Solar.
Two forms of mechanical energy
1. Potential energy
2. Kinetic energy.
1. Kinetic energy :
• ‘The energy which an object has because of its motion is called its kinetic energy’.
• The work done by a force to displace a stationary object through a distance s is the kinetic energy gained by the object.
• Kinetic energy = work done on the object
Potential energy :
‘The energy stored in an object because of its specific state or position is called its potential energy.’
Work = force x displacement
• W = mg × h
• W = mgh
• The amount of potential energy stored in the object because of its displacement
• P.E. = mgh (W = P.E.)
Transformation of energy :
• Energy can be transformed from one type to another.
• For example, the exploding firecrackers convert the chemical energy stored in them into light, sound and heat energy.
• Free fall
• If we release an object from a height, it gets pulled towards the earth because of the gravitational force.
• An object falling solely under the influence of gravitational force is said to be in free fall or to be falling freely.
• Let us look at the kinetic and potential energies of an object of mass m, falling freely from height h, when the object is at different heights.
• Every object has potential energy when it is at a height above the ground and it keeps getting converted to kinetic energy as the object falls towards the ground.
• On reaching the ground, all the potential energy gets converted to kinetic energy.
• But at any point during the fall the total energy remains constant.
Power :
• ‘Power is the rate at which work is done.’
• In SI system the unit of work is J, so the unit of power is J/s. This is called watt
• 1 watt = 1 joule / 1 second
• 1 horse power = 746 watt.
• The unit of energy for commercial use is kilo watt hour / Unit
• 1000 joule work performed in 1 second is 1 kilowatt power
James Watt :
• The steam engine was invented in 1781 by the Scottish scientist James Watt (1736 – 1819).
• First to use the term ‘horse-power’.