Work, Energy and Simple Machines — Part 2: Conservation, power and machines: visual lesson, CBSE Class 9 Science

Lesson

Conservation, power and simple machines

Part 2: energy never disappears, how fast work is done, and how machines make jobs easier.

In this lesson you will

Describe energy transformations and apply conservation of mechanical energy
Calculate power in watts and energy in kilowatt hours
Classify levers and calculate mechanical advantage
Calculate mechanical advantage and efficiency of pulley systems

Table of four ideas of this part with a one-line statement of each
Idea	In one line
Conservation of energy	energy changes form but the total stays the same
Power	work done per second
Simple machine	a device that makes work easier by changing force or direction
Efficiency	how much of the input is useful output

Concept

Forms and transformations of energy

Law of conservation of energy. Energy can neither be created nor destroyed; it can only change from one form into another. The total energy of an isolated system stays constant. Some energy often changes to heat that spreads into the surroundings, so we cannot easily use it again.

Table of seven devices or events and the energy change in each
Device or event	Energy change
Falling mango	potential → kinetic
Ceiling fan	electrical → kinetic (and a little heat)
Solar cell on a roof	light → electrical
Hydroelectric dam	potential of water → kinetic → electrical
Burning cooking gas	chemical → heat and light
Rubbing your palms	kinetic → heat
Loudspeaker	electrical → sound

Diagram

Mechanical energy of a falling body

PE falls and KE rises by the same amount; the total stays constant.

Mechanical energy is the sum of kinetic and potential energy. For a freely falling body, with air resistance ignored, the total stays constant: as the height falls, PE falls by exactly the amount KE rises. For the 0.40 kg mango, $\text{PE} + \text{KE} = 19.6$ J at every height. At the ground $\tfrac{1}{2}mv^2 = mgh$ , so $v = \sqrt{2gh}$ for a fall from rest.

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