11SCI - Mechanics

Finn LeSueur

2020

- Write the date in your books
- Discuss with the person next to you:
- What does the law of conservation of energy state?
- What unit is energy measured in?

- Define
**work**. - Name the unit of work and give its symbol.
- Use \(W = Fd\).

Write ngā whāinga ako in your book

The amount of energy transferred/transformed.

E.g. When you lift your backpack off the ground you are transferring some **chemical potential energy** in your muscles into **gravitational potential energy** in the backpack. You are doing **work** on your backpack.

Work has the formula

\[\begin{aligned} work &= force \times distance \\ W &= F \times d \end{aligned}\]Work is measured in Joules (J) because it is defined as **the amount of energy transferred/transformed**.

Sam has decided to take up weight lifting and starts by dead-lifting a \(20kg\) mass from the ground to a height of \(1m\). Calculate the work done for Sam to lift the weight.

- Formula
- Substitution
- Answer

Jack has gone hiking up Avalanche Peak. He has a mass of \(55kg\) and reaches a height of \(1833m\), starting from \(733m\). How much work has Jack done to reach the top?

- Formula
- Substitution
- Answer

- Get your mahi kāinga booklet and open it to Q31.
- Complete Q31
- What do you notice about the
*distance*value used? Is it horizontal or vertical? Why do you think that is?

- Formula
- Substitution
- Answer

We use the vertical distance, not the horizontal distance. Energy is not transferred/transformed when moving horizontally!

- Due Monday, October 19th
- Mahi Kāinga Booklet Q30, Q35

Recall that **work is the amount of energy transferred/transformed**. Two skiers ride the 6-chair. One descends to base via Jan’s Face and the other via Fascination/Broadway. Describe the change in their energies and how they are similar/different.

- Use \(W = Fd\) and \(Ep = mgh\) to determine amounts of energy transfer.

Write te whāinga ako in your book

- Work is
**the amount of energy transferred/transformed** - It is just a measure of energy
- In the vertical direction,
**it is exactly the same as**\(E_{p}\)! - You can use either formula

A rocket is launched with an acceleration of \(90ms^{-2}\). It has a mass of \(5kg\) and it reaches a height of 2000m. **How much work did the rocket do to get the rocket to this height?**

- Formula
- Substitution
- Answer

All energy is transformed from chemical potential **to gravitational potential**.

Open your mahi kāinga booklet to Q33 about Ian and Chris on a diving platform.

- Formula
- Substitution
- Answer

*How much work does Chris (48 kg) do when he climbs up the stairs to the 2m diving platform?*

*Ian’s mass is 52 kg. Why did Ian do more work climbing up the 5 m ladder compared to Chris climbing up the 2 m ladder?*

Because \(W=mgh\), if \(m\) increases, the work increases. Also if \(h\) increases, so does the work. Ian:

\[\begin{aligned} W = E_{p} &= m \times g \times h \\ &= 52 \times 10 \times 5 = 2600J \end{aligned}\]Get ready to Kahoot!

- Define
**power**. - Name the unit of power and give its symbol.
- Calculate the power of a device from given data.

Write the date and ngā whāinga ako in your book

The rate at which energy is transferred/transformed.

- A higher power means more energy is transferred each second.
- A lower power means less energy is transferred each second.

- Power is measured in Joules per second (\(J/s\)), which is also known as a Watt (\(W\))

You go to school but leave the light in your bedroom on all day. The lightbulb uses \(1,134,000J\) of energy while you are away at school for 7 hours. Calculate the **power** of the bulb.

- Formula
- Substitution
- Answer

Chris (48 kg) climbs 2m up some stairs to a diving board. He completes the climb in 5s. **What is Chris’ power?** *Hint: find work first, and then power.*

- Formula
- Substitution
- Answer

Mahi Kāinga booklet Q36. *Hint: find work first, and then power.*

- Formula
- Substitution
- Answer