⚡ Work and Energy – Class 9 Physics
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39 Terms
Work
Work is said to be done when a force is applied and there is displacement in the direction of force
Formula of work
W = F × d × cosθ
SI unit of work
Joule (J)
1 Joule
Work done when a force of 1 N displaces an object by 1 metre in the direction of force
Positive work
When displacement is in the direction of force (θ = 0°)
Negative work
When displacement is opposite to the direction of force (θ = 180°)
Zero work
When there is no displacement or force is perpendicular to displacement
Energy
Capacity to do work
SI unit of energy
Joule (J)
Kinetic energy
Energy possessed by a body due to its motion
Kinetic energy formula
KE = ½ mv²
Potential energy
Energy possessed by a body due to its position or configuration
Potential energy formula
PE = mgh
Work done by gravity
W = mgh
Law of conservation of energy
Energy can neither be created nor destroyed; it only transforms from one form to another
Total mechanical energy
Sum of kinetic and potential energy (remains constant in free fall)
Free fall energy conversion
Potential energy is converted into kinetic energy
Commercial unit of energy
Kilowatt hour (kWh)
1 kWh =
1000 watt × 3600 seconds = 3.6 × 10⁶ J
Power
Rate of doing work or energy transfer
Formula of power
P = Work done / Time taken
SI unit of power
Watt (W)
1 watt
Power of doing 1 joule of work in 1 second
1 kilowatt
1000 watts
Horsepower
1 HP = 746 W
Average power
Total work done / Total time taken
High power device
Can do more work in less time
Faster work
Indicates more power
Work in lifting object
Equal to its potential energy (mgh)
Work done by friction
Always negative (opposes motion)
Work done in circular motion
Zero (force is perpendicular to displacement)
Gravitational potential energy
PE = mgh (m = mass
Mass and kinetic energy
KE increases with square of velocity and linearly with mass
Mechanical energy
Kinetic energy + Potential energy
Joule to kWh conversion
1 kWh = 3.6 × 10⁶ J
Unit of power in practical use
Watt
Appliances rated in kW
Used to indicate power consumption capacity
Work done by constant force
Force × displacement in direction of force