12PHYS - Mechanics

Finn Le Sueur

2021

- Review basic speed and acceleration calculations.

Write the date and te whāinga ako in your book

- Andy can run \(100m\) in \(11.9\) seconds
- Bob can run \(100m\) in \(10.8\) seconds
- Chris can run \(100m\) in \(12.4\) seconds

- Andy can run \(100m\) in \(11.9\) seconds
- Bob can run \(100m\) in \(10.8\) seconds
- Chris can run \(100m\) in \(12.4\) seconds

Bob because he ran \(100m\) in the shortest time.

- Aaron can run \(534m\) in \(1 minute\)
- Billy can run \(510m\) in \(1 minute\)
- Cameron can run \(452m\) in \(1 minute\)

- Aaron can run \(534m\) in \(1 minute\)
- Billy can run \(510m\) in \(1 minute\)
- Cameron can run \(452m\) in \(1 minute\)

Aaron because he ran the furthest in \(1 minute\).

- Ash can run \(0.3km\) in \(45 seconds\)
- Bailey can run \(420m\) in \(1 minute\)
- Caleb can run \(510m\) in \(1.5 minutes\)

Write this equation in your book and give the unit for each letter in the equation.

- \(ms^{-1}\)
- It stands for
**meters per second** - E.g. the speed of sound is \(343ms^{-1}\)
*Sound travels \(330m\) in one second*

Ash runs \(315m\) in \(45s\). Calculate his average speed in **meters per second**.

**Knowns:****Unknowns:****Formula:****Substitute:****Solve:**

Ash runs \(315m\) in \(45s\). Calculate his average speed in **meters per second**.

- A skydiver (freefall) = \(53ms^{-1}\)
- A handgun bullet = \(660ms^{-1}\)
- A car on the road = \(50km/hr\)
- A flying airplane = \(1100kmh^{-1}\)
- Light = \(300,000,000\)

Pātai: In pairs, convert the speed of an airplane to meters per second.

A car is moving at a speed of \(10ms^{-1}\). How far does the car travel in \(12s\)?

**Knowns:****Unknowns:****Formula:****Substitute:****Solve:**

A car is moving at a speed of \(10ms^{-1}\). How far does the car travel in \(12s\)?

\[\begin{aligned} v &= 10ms^{-1}, t=12s \newline d &= ? \newline v &= \frac{d}{t} \newline 10 &= \frac{d}{12} \newline 10 \times 12 &= d = 120m \end{aligned}\]A man is running at a speed of \(4ms^{-1}\). How long does he take to run \(100m\)?

**Knowns:****Unknowns:****Formula:****Substitute:****Solve:**

A man is running at a speed of \(4ms^{-1}\). How long does he take to run \(100m\)?

\[\begin{aligned} v &= 4ms^{-1}, d=100m \newline t &= ? \newline v &= \frac{d}{t} \newline 4 &= \frac{100}{t} \newline 4 \times t &= 100 \newline t &= \frac{100}{4} = 25s \end{aligned}\]Velocity may refer to **average velocity** or **instantaneous velocity**.

The formula \(v = \frac{d}{t}\) can only be used to calculate **average velocity** or when **the velocity is constant**.

- Review basic acceleration calculations.
- Cover a basic introduction vectors.

Write the date and ngā whāinga ako in your book

\[\begin{aligned} & a = \frac{\Delta v}{t} \newline & \Delta v = \text{ change in speed} \newline & t = \text{ time} \newline & a = \text{ acceleration} \end{aligned}\]The rate of change in speed

*meters per second squared OR meters per second per second*

For example, \(a=12ms^{-2}\) means that the velocity is increased by \(12ms^{-1}\) every second.

This is the difference between the **initial** and the **final** value.

A man initially walking at \(2.0ms^{-1}\) notices that his house is on fire so he speeds up to \(11ms^{-1}\) in \(1.3s\).

- Calculate the change in speed
- Calculate his acceleration

A cyclist who has been travelling at a steady speed of \(4ms^{-1}\) starts to accelerate. If he accelerates at \(2.5ms^{-2}\), how long will he take to reach a speed of \(24ms^{-1}\)?

**K,U,F,S,S**

- A car initially moving at \(12.7ms^{-1}\) accelerates at \(1.3ms^{-2}\) for
**one minute**. What is the car’s final speed? - A car decelerates at \(1.8ms^{-2}\) for \(9.4s\) to stop. What was the car’s initial speed?

**Whakatika 1**

**Whakatika 2**

Discuss with the person next to you, the relevance of the positive and negative signs.