Uniform Motion, Speed and Graphs

Introduction to Physics

Physics!

What is it?Physics is the science of matter and energy and

their interactionsWhy is it important to us?

All around us! What will we be learning?

Energy! And how it relates to movement, forms of energy, thermodynamics

Topics for Today:

Physics IntroUniform MotionAverage SpeedScalar vs Vector Quantities (intro)

Some definitions

Motion: Move away from a reference point Reference point: where we make our observations in

relation to Uniform motion:

Object travelling at a constant rate of motion in a straight line

Is it possible to maintain uniform motion? Ex. Why is it hard for a car travelling at 100 km/hr to

maintain uniform motion?

Uniform Motion

Is it possible to maintain uniform motion? Ex. Why is it hard for a car travelling at 100 km/hr

to maintain uniform motion?

Objects can’t maintain the motion due to external factors, like friction and wind resistance when driving.

Average Speed

Average speed is uniform motion that involves traveling a certain distance in a specific time

Direction of movement is not involved. It is like looking at the speedometer of your car to see how fast you are going. It doesn’t tell you what direction you are going.

  There are two ways to calculate average speed

Using a formula (today’s topic) Graphing data (tomorrow’s topic)

Scalar vs Vector Quantities

There are two types of quantities that existScalar quantities – only indicates

magnitude (how much)  Speed  Temperature

Vector quantity – indicates magnitude and direction

 Velocity Acceleration

Average Speed

Average speed is an example of a scalar quantity because it only indicates the magnitude (how much) of motion, not the direction. Quantities that also indicate a direction are called vector quantities and we will start dealing with them tomorrow.

Average Speed

Uniform motion in a specific time Using a formula:

Average speed = distance travelled time elapsed

v = ∆d (∆ means “change in”)∆t

v= average speedd= distancet= time

Formula for Speed: 3 ways

Example 1:

Amber just got her driver's license and decided that a nice way to celebrate that milestone in life would be to grab the keys to her mom's vehicle and drive to Edmonton with her friends. Now, if Edmonton is 150 km away and it

took her 80 minutes to travel that far, what was her average speed?

Example 1 Answers

What do we know?Distance = 150 km Time = 80 minutesGreat! What do we need? Velocity

Example 2

Amber remembered that she needed to get home to Athabasca ASAP as her parents didn't know that she borrowed their vehicle... Fortunately, she had only gotten to Clyde, which is 80km from her home. How long (in hours and in minutes) would it take her to get home if she was forced to stay at the speed limit (as a RCMP vehicle was casually tailing her as she drove north)?

Example 2

What do we know?d=80 kmv=100km/hWhat do we need? Time

Converting Units

The System International (SI) units for speed is m/s

More commonly we measure speed in km/h

To convert from m/s to km/h or the other way:

1km = m h = s

Convert 5m/s to km/hConvert 15km/h to m/s

Converting Units Continued

Topics Covered

How to interpret graphs Distance - time graphsSpeed – Time graphs

Graphs

Two types of graphs: Distance-time graph Speed-time graph **Pay attention to which type of graph you are

working with!

Making a graph

Important points to remember: Time always goes on the x-axis (what is the x-

axis?) Label both y and x-axis (with correct units too!) Make graphs a suitable size (not too small: make a

scale) Graphs must always have a title Use an appropriate scale for values Line of best fit: use a ruler

When looking at graphs

The first piece of information is the found on the x axis, then the second piece of information is found on the y axis.

For example:(0,0); (2.0,10)

Time (s) 0 2.0Distance Travelled (m) 0 10

Average Speed by Graphing Distance An average speed graph plots distance vs. time

Time is the manipulated variable and is plotted on the “x” axis

Distance is the responding variable and is plotted on the “y” axis

A “best fit” line goes through the majority of data points on the graph  When the line is straight, we assume that the motion being represented

is uniform.

The steeper the slope of the line, the faster the object is traveling.  Average speed can be determined by finding the slope of the line that

you have drawn

Before Getting Started

Before Getting Started

Distance-time graph

Deal with an object’s change in distance an object travels compared to time

Use our reference point Distance always increases from that point

Slope of line (line of best fit) = average speed of an object How do we calculate slope?

Rise/Run For these graphs,

change in distance/change in time

Distance Time Graphs

Rise/Run For these graphs,

change in distance/change in time v=∆d/∆t

Slope= Rise/Run Slope (v) = ∆d/∆t ∆d = df - di ∆t = tf - ti

Practice: Calculating Slope

Time (s) 0 2.0 4.0 6.0 8.0

Distance Travelled (m)

0 10 20 30 40

Albert was riding on his scooter. His friend recorded his speed.

Calculating Slope

Practice: Distance-time graph

p. 130 Example

Interpreting Distance – Time Graphs

Distance - Time Graph

0

5

10

15

20

25

30

35

0 2 4 6 8 10

Time (seconds)

Dis

tanc

e (m

)

Practice

How do we calculate slope? What are the units of the slope? What is this unit the same as? What does a horizontal line on the graph

mean? The object has stopped moving (not changing

distance)

Your Assignment