Welcome to PhysicsWelcome to Physics

Main Principal #1: Physics is the study of moving objects!

We want to be able to predict how objects will move in all situations

But here’s the problem:

Q: How do you know if an object

is actually “moving”? A: When it’s “changing position” (travelling

a certain distance)?If a person is “standing still”, we assume he/she is “not moving”

But what about the planet that person is standing on?

Is the person therefore “moving” anyway?

So how do we tell if something really is “moving”?

It is moving around a star!!!

Oh boy; welcome to physics!!!

We use a principal called: “Point of Reference”

The hands on a clock don’t seem to move; but they are constantly

moving very slowly!

Are the passengers in a boat moving compared to a person standing on the shore?

Or is the person standing on the shore moving compared to the boat in the water?

Fwew! It can be tricky!

“PoR” means: what appears to be moving to you?

It can be tricky to really know what is moving and what isn’t!

Main Principal #2 & 3:

• #2: Basically, (if you really think about it) #2: Basically, (if you really think about it) all all objectsobjects are are movingmoving some amount, some amount, all the all the timetime..

• #3: We #3: We comparecompare the motion of the object that the motion of the object that appears to moveappears to move to a “ to a “stationarystationary” object ” object (one that doesn’t appear to move).(one that doesn’t appear to move).

An object is moving “slowly” if it moves a short distance in a long time.

An object is moving “quickly” if it moves a long distance in a short time.

There are several ways we use to describe the motion of an object:

“Velocity” and “Speed”

• Think: What are two ways to increase speed (go faster)?– Increase distance traveled in the same amount of time – Decrease the time it took to go the original distance

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***Since they are almost the exact same thing, “Speed” and “Velocity” are used for the same motion

“Speed” means distance travelled in a certain amount of time

“Velocity” is the speed in a certain direction

Speed =Formula for Speed:

Graphs can help us Interpret what has actually happened!

The shape of the line and steepness of the The shape of the line and steepness of the “slope” can tell us what has happened and “slope” can tell us what has happened and

how fast it occurred!how fast it occurred!

That’s right! We are NOT done with graphs!!!

We are only just beginning!!!

Graphing Velocity

What you want to be able to do is know what the graph is telling

you by looking at the line…

This is NOT the graph for your notes...

Time

Interpreting Velocity on a Graph

Think of the Velocity graph this way: Which one of these balls will fall faster?

5 m

15 m

10 m

15 s10 s5 s

More DistanceIn Less Time

(Faster)

The graph with more steepness is the faster velocity. The less steep, the slower.

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cetanDisVelocity

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Less DistanceIn More Time

(Slower)

You can use the slopeof a line to find Velocity

Draw this graph in your notes…

Interpreting Speed on a Graph:

5 m

15 m

10 m

15 s10 s5 s

It can help to think about the speed of a

rolling ball on the slope

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Time

Stopped moving!Started slowly…

Now it’s moving fast!

As a group, describe how fast the object is probably moving as you travel along the line (left to right)

(After you try it together, click to see the real motion)

Q: HOW CAN YOU TELL?!?!

This ball would roll slowly…

This ball won’t roll at all!

This ball would roll fast!

Q: What should you learn from this? Q: What should you learn from this? A: Steep lines on motion graphs, mean an object is moving quickly! Non-steep lines mean the object is moving slowly!

Draw this graph in your notes…

Time (min)

Dist.(meters)

1

2

3

4

5

100

220

300

430

500

But this graph is not the whole story!!! It all can change if you pick different variables to graph!

100 m/min

110 m/min

100 m/min

107 m/min

100 m/min

Speed

Graphing Speed:The two lines look different, but still describe the same objects motion!

What should we learn? WATCH YOUR VARIABLES CAREFULLY!

Time (min)

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)S

peed

(m

/min

)

This is basically the line of the data you see in the table…Let’s dim that last line but keep it here to compare to the new line…

Now we graph the line of this data…

Houston; um…Where

are we going?

• Think: How would you describe the motion of floating balloons?

• Do they move in a “constant” speed or direction? (“constant” means: “staying the same”)

The movement of a balloon is a great example of an object

Accelerating!

Balloons just kinda move all over the place. Sometimes they go straight up, sometimes they fly more sideways. CRAZY!

What is “Acceleration”?

• A: The rate at which velocity changes!

– In other words: How fast something speeds up, slows down, or even changes direction.

Speeding up: Slowing down: Changing direction:

You probably think it simply means to “speed up.” Well yes, and a bit more:

Acceleration = A change in Velocity

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VelocityonAccelerati

You already learned the Velocity equation:

Here is the equation for Acceleration:

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Means “change”

So how do you do that?

Final Velocity – Start Velocity

Final Time – Start Time

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Like this:

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Interpreting Acceleration on a Graph

5 m/s

15 m/s

10 m/s

15 s10 s5 s

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tanc

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This is the line of an object with an “constant velocity”

Compare it to the line of an object with a changingVelocity = Acceleration!

It is also possible to have an object with a negative acceleration

We can call this “Deceleration”

Notice the downward curveIt is a nice straight line!

Straight lines on a graph mean “constant”

Curved lines mean a changehas occurred = Acceleration!

Vel

ocity

Notice also, the different Y-axis variable!Notice also, the different Y-axis variable!

Acceleration Graphing Practice

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VelocityonAccelerati

Time Dist. Acc.

1

2

3

4

5

2 m

8 m

18 m

32 m

50 m

2 m/s2

2 m/s2

2 m/s2

2 m/s2

2 m/s2

Think: Why is the acceleration staying the same if the car is speeding up more and more?

Now try and graph both the Speed line AND the Acceleration line. Compare them! Do this on the very back page of your packet!