# Energy Transformations

11SCI - Mechanics

2020

## Akoranga 21: Mahi Tuatahi:

1. Write the date in your book
2. Brainstorm on the board what types of energy are there?

## Ngā Whāinga Ako

1. Identify energy changes.
2. Use $$E_{k} = \frac{1}{2}mv^{2}$$
3. Explain the conservation of mechanical energy in free fall situations (e.g. ball sports)

## Types of Energy

• Gravitational potential energy
• Heat energy
• Sound energy
• Light energy
• Elastic potential energy
• Electrical energy etc.

### What do they have in common?

• All energy is measured in Joules ($$J$$).
• A transfer of energy is called work, measured in $$J$$.

## Conservation of Energy

Energy cannot be created or destroyed, only transformed.

• This means that gravitational potential energy can be transformed into kinetic energy, and kinetic energy into other forms of energy.
• It is always taken from somewhere, never created from nothing.

Written as an equation, this can mean:

\begin{aligned} E_{k} &= E_{p} \\ \frac{1}{2}mv^{2} &= mgh \end{aligned}

In fact, we can make any two energy equations equal to each other! E.g. Spring potential energy and kinetic.

• We can convert one type of energy into another through mathematics!
• This is only true in a frictionless world where no energy is lost through heat/sound/light

### Pātai

Mr Le Sueur has mass 71kg and has climbed a tree 4.5m tall to jump into a lake. How fast will he be traveling when he hits the water?

• Starts with $$E_{p}$$, so calculate it
• Then transformed all to $$E_{k}$$, so put known values into $$E_{k}=\frac{1}{2}mv^{2}$$

#### Whakatika

\begin{aligned} E_{p} &= mgh = 71 \times 10 \times 4.5 = 3195J \\ E_{k} &= E_{p} \\ E_{k} &= \frac{1}{2}mv^{2} \\ 3195 &= \frac{1}{2} \times 71 \times v^{2} \\ \frac{3195 \times 2}{71} = 90 &= v^{2} \\ \sqrt{90} = 9.49ms^{-1} &= v \end{aligned}

## Tūhura: Marble Drop

1. Come over to the demo bench to see the setup
2. Open Google Classroom and do the tūhura!

# Akoranga 22 Mahi Tuatahi

Read page 55 and 56 of your sciPAD to remind yourself of some types of energy and how it is transformed between different types.

Then answer Question 1 on page 59.

In Physics we live in an idealised world where friction does not exist and energy is transformed with 100% efficiency.

This means that 100% of elastic potential energy is converted into kinetic energy, in the case of the rubber band.

It means that 100% of gravitational potential energy is converted into kinetic energy in the case of a sky diver.

# Akoranga 23 Mahi Tuatahi:

1. Write the date in your books
2. Open the Quizlet on Google Classroom and complete “Match” three times
3. Get ready to play the Quizlet Live

## Ngā Whāinga Ako

1. Identify energy changes.
2. Use $$E_{k} = \frac{1}{2}mv^{2}$$
3. Explain the conservation of mechanical energy in free fall situations (e.g. ball sports)

Write ngā whāinga ako in your books

## Conservation of Energy

Energy cannot be created or destroyed, only transformed

We investigated this last term by calculating the gravitational potential energy of a marble at the top of a tube and predicting how fast it would be moving at the bottom of the tube if all of the $$E_{p}$$ was transformed into kinetic energy $$E_{k}$$.

Energy cannot be created or destroyed, only transformed

• This means that we can say $$E_{k} = E_{p}$$
• In words, all kinetic energy is transformed into potential energy, or vice versa.

### Pātai: Mahi Kāinga Booklet Q40

• Open mahi kāinga booklet 40
• 40a is an achieved level question
• 40b is an excellence level question
• start by identifying the change in energy
• then use it to calculate the speed if all of that energy is transformed into kinetic energy