Albert Teen

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Speed and Velocity

# Speed and Velocity

### Speed and Velocity

Speed is a measurement that describes how fast an object is travelling.

Units for speed

1

Speed is...

2

We can measure speed using different units. Which are the three units in the picture?

3

Miles, kilometres and metres are all measures of ______________.

4

Hours and seconds are both measures of ______________.

5

All units for speed include a measure of _____________ and a measure of ___________.

6

What is the SI unit for speed?

Measuring Speed

1

It's sports day and your friend is about to start a race. They've asked you to find out how fast they cycle. What two quantities do you need to measure?

2

To measure these quantities, you'll need...

3

The race begins and you start your stopwatch. The track is $400 \space m$ long and your friend crosses the finish line in $50s$. Now you have to calculate their speed; how many metres did they cycle per second?

4

You worked out you speed like this $\frac{400 \space m}{50 \space s}=8 \space m/s$. What is the correct formula for speed then?

5

So the formula for speed is $speed = \frac{distance}{time}$

This gives you units that include both measures of distance and measures of time, like $m/s$, $km/h$ or $mph$

6

Here we can see a graph with the measurements from the race.

At time $50s$ your friend reached the distance of $400m$.

7

At what point did your friend start the race?

8

Your friend finished the race at $(50, 400)$ and you calculated the speed at which they cycled to be $8\:m/s$. Can you guess what part of the graph might represent the speed?

9

In any graph, the slope of the line is calculated by $\frac{change\:in\:y}{change\:in\:x}$.

For a distance-time graph, this is the same as $\frac{change\:in\:distance}{change\:in\:time}$ which is just the speed!

Vectors and Scalars

1

When we talk about speed, we are giving magnitude to how fast an object is travelling. What type of quantity is this?

2

When we talk about speed in a given direction, we call it velocity. This is...

3

We talk about velocity when we care about what direction a moving object is travelling in.

Speed simply gives us the magnitude of the velocity.

Summary!

1

The speed of an object is how fast an object is going

By that we mean how far that object has gone in a given time, for example $10 \space m$ in $5 \space s$

2

You calculate speed like this $speed = \frac{distance}{time}$

For example $\frac{10 \space m}{5 \space s}=2 \space m/s$

3

That means that units for speed have a measure of distance and a measure of time

For example $m/s$, $km/h$ or $mph$

4

Speed is a scalar quantity.

Velocity is a vector quantity.