MotionMotion

The Need for SpeedThe Need for Speed

What is motion?What is motion?

• You are probably thinking that motion You are probably thinking that motion is when something is moving…is when something is moving…

• Unfortunately, that definition won’t do Unfortunately, that definition won’t do us much good. us much good.

• How do we know something is moving?How do we know something is moving?• How do we measure it?How do we measure it?• How do we tell fast motion from slow How do we tell fast motion from slow

motion?motion?

Well…Well…• An object is in motion when its distance An object is in motion when its distance

from another object is changing.from another object is changing.

• Huh?Huh?

• We can tell something is moving when we We can tell something is moving when we can measure it!can measure it!

• Motion is described relative to something Motion is described relative to something else.else.

• For example, even though you are in your For example, even though you are in your seat, you are moving 30k/s seat, you are moving 30k/s relativerelative to the to the sun!sun!

How do we measure How do we measure motion?motion?

• We measure motion based on how We measure motion based on how fast it is occurring, or better put, fast it is occurring, or better put, the the rate at which it occurs.rate at which it occurs.

• Rate tells us the amount of Rate tells us the amount of something that occurs or changes in something that occurs or changes in one unit of time.one unit of time.

• You are probably more familiar with You are probably more familiar with this concept when I talk about speed.this concept when I talk about speed.

The Need for Speed!The Need for Speed!

• Speed is one type of rate.Speed is one type of rate.

• The speed of an object is the distance The speed of an object is the distance the object travels per unit of time.the object travels per unit of time.

• Lets think of driving. Most streets in Lets think of driving. Most streets in Phoenix are 45 miles per hour. So, if I Phoenix are 45 miles per hour. So, if I drive the rate of 45 miles per hour, then drive the rate of 45 miles per hour, then I will travel 45 miles in one hour!I will travel 45 miles in one hour!

How do I calculate speed?How do I calculate speed?

• I am so glad you I am so glad you asked!asked!

• Speed=DistanceSpeed=Distance

TimeTimeS=DS=D

TTOR

A cheetah can maintain a constant speed of A cheetah can maintain a constant speed of 25m/s. At this rate, how far will it travel in 25m/s. At this rate, how far will it travel in 10 s?10 s?

• Remember the steps of problem Remember the steps of problem solving!solving!

• 1. Write the equation & rearrange.1. Write the equation & rearrange.

S=D/T or D=STS=D/T or D=ST

2. Write the knowns2. Write the knowns

S=25m/sS=25m/s

D= ?D= ?

T= 10sT= 10s

3. Convert3. Convert– We don’t need toWe don’t need to

4. Plug!4. Plug!

D=(25m/s) x (10s)D=(25m/s) x (10s)

5. Chug!5. Chug!

D=(25m/s) x (10s) =250mD=(25m/s) x (10s) =250m

6. Box Answer!6. Box Answer!

VelocityVelocity

• Society uses the terms “speed” and Society uses the terms “speed” and “velocity” interchangeably.“velocity” interchangeably.

• But…they are wrong.But…they are wrong.

• Velocity is speed with a given Velocity is speed with a given directiondirection

• So, speed is going 30m/hSo, speed is going 30m/h

• Velocity is going North 30m/hVelocity is going North 30m/h

““We’re not in Kansas We’re not in Kansas anymore…anymore…• If we lived in Kansas during Tornado If we lived in Kansas during Tornado

season, it really wouldn’t do us much season, it really wouldn’t do us much good to know the speed of a storm good to know the speed of a storm traveling with a tornado brewing inside it.traveling with a tornado brewing inside it.

• But…But…• It would be very beneficial to know the It would be very beneficial to know the

speedspeed of a tornado, and the of a tornado, and the directiondirection, , especially if it is headed toward our especially if it is headed toward our house.house.

The Rubber Hits the Road…The Rubber Hits the Road…

• Speed is distance over timeSpeed is distance over time

• Example: 65 m/hExample: 65 m/h

• Velocity is speed Velocity is speed and and directiondirection

• Example: 65 m/h SouthExample: 65 m/h South

MotionMotion

AccelerationAcceleration

AccelerationAcceleration

• Consider a car stopped at a stoplight. Consider a car stopped at a stoplight. As the light turns green, the driver As the light turns green, the driver presses the gas pedal and gradually presses the gas pedal and gradually increases speed, or increases speed, or acceleratesaccelerates..

• Acceleration is the rate at which Acceleration is the rate at which velocity changes.velocity changes.

• Remember, velocity is speed and Remember, velocity is speed and direction.direction.

Velocity changeVelocity change

• Velocity can either increase or Velocity can either increase or decreasedecrease

• The direction can also changeThe direction can also change

• Acceleration involves a change in Acceleration involves a change in either of these components.either of these components.

• Acceleration refers to increasing or Acceleration refers to increasing or decreasing speed and changing decreasing speed and changing direction.direction.

Increasing SpeedIncreasing Speed

• When an object increases speed, it When an object increases speed, it experiences acceleration.experiences acceleration.

• When a football is thrown, it When a football is thrown, it accelerates.accelerates.

• When a pitcher throws a ball, it When a pitcher throws a ball, it accelerates.accelerates.

• When a horse begins to run, it When a horse begins to run, it accelerates.accelerates.

Decreasing SpeedDecreasing Speed

• Objects that speed up, eventually Objects that speed up, eventually slow down.slow down.

• A ball that is thrown eventually rolls to A ball that is thrown eventually rolls to a stop.a stop.

• A car slows down as the driver steps A car slows down as the driver steps on the brake.on the brake.

• A runner finishing a race, eventually A runner finishing a race, eventually stops.stops.

Changing DirectionChanging Direction

• Acceleration is change in speed Acceleration is change in speed oror direction.direction.

• A car accelerates as it makes a turn.A car accelerates as it makes a turn.

• A Ferris Wheel at the fair is A Ferris Wheel at the fair is accelerating (direction is always accelerating (direction is always changing)changing)

• The Earth rotating around the sun.The Earth rotating around the sun.

Calculating AccelerationCalculating Acceleration

• To determine the acceleration of an To determine the acceleration of an object, you must calculate the change object, you must calculate the change in velocity during each unit of time. in velocity during each unit of time.

Final Velocity - Initial Final Velocity - Initial VelocityVelocity

Acceleration=Acceleration=

TimeTime

UnitsUnits

• If velocity is measured If velocity is measured meters/secondmeters/second

• And time is measured in seconds,And time is measured in seconds,

• Then, acceleration is measured in Then, acceleration is measured in meters per second per second.meters per second per second.

• Huh?Huh?

• Acceleration units are m/s2Acceleration units are m/s2

Remember when I said that Remember when I said that scientists are lazy?scientists are lazy?

• We can summarize the acceleration We can summarize the acceleration equation by the following:equation by the following:

Vf - ViVf - Vi

A=A=

tt

An Example:An Example:

• A roller coaster car rapidly picks up speed as it A roller coaster car rapidly picks up speed as it rolls down a slope. As it starts down the slope, its rolls down a slope. As it starts down the slope, its speed is 4 m/s. But 3 seconds later, at the bottom speed is 4 m/s. But 3 seconds later, at the bottom of the slope, its speed is 22m/s. What is its of the slope, its speed is 22m/s. What is its average acceleration?average acceleration?

• Relax! This is easy, just follow the steps for Relax! This is easy, just follow the steps for solving word problems!solving word problems!

Just follow the steps!Just follow the steps!

Step 1: Write the equationStep 1: Write the equation

Vf – ViVf – Vi

A=A=

tt

A roller coaster car rapidly picks up speed as it A roller coaster car rapidly picks up speed as it rolls down a slope. As it starts down the slope, rolls down a slope. As it starts down the slope, its speed is 4 m/s. But 3 seconds later, at the its speed is 4 m/s. But 3 seconds later, at the bottom of the slope, its speed is 22m/s. What is bottom of the slope, its speed is 22m/s. What is its average acceleration?its average acceleration?

Step 2: Write the knowns:

A=?

t= 3 s

Vf=22 m/s

Vi=4 m/s

Step 3: Convert

-Not needed

Step 4: Plug!

22 m/s – 4 m/s

A=

3s

Step 5: Chug! 18m/s

A= = 6 m/s2

3s

Step 6: Box Answer!