Usain Bolt is the world’s fastest man!!!. Physics of Motion We will look at: –Distance –Displacement –Speed First you need to realize that motion is relative…

Usain Bolt is the world’s fastest man!!!

Physics of Motion

We will look at:– Distance– Displacement– Speed

First you need to realize that motion is relative…

Motion is relative

What is meant by saying that motion is relative? For everyday motion, what is motion usually relative to?

Motion is Relative

Motion is relative to the observer’s position and their reference point– Sometimes called a “reference frame”

Consider the picture…

• If this man is driving at 15 mph, how fast is his coffee cup moving?

• Does the man feel like the cup is moving?

• Why?

Motion is relative

An object is in motion if it changes position relative to a stationary reference point.

Direction

We use N, S, E, W to describe the direction of movement.

Distance and DisplacementDistance measures the actual path an object takesDisplacement measures your overall distance from the starting point in a STRAIGHT LINE.DISPLACEMENT values must include a DIRECTION!Which color line represents distance?Displacement?

Distance vs. Displacement

http://physics.info/displacement/

Write in your own words- what is the difference between distance and

displacement?

Distance-

Displacement-

Speed

Describes how fast an object moves.We know some things move faster than others…but how do we measure it?What two quantities must you know to determine speed?

What two quantities must you know to determine speed?

Choose from: displacement, distance, time, velocity– Hint…what is speed measured in???

– Speed= distance/timeEx- miles/hour, m/s, etc.

There are three types of speed you must know…

Constant speedAverage speedInstantaneous speed

But first let’s look at some graphs…

If I wanted to graph speed, what should I label my axes???

So the slope of the line=SPEED

Dis

tance

Time

Speed

Constant Speed

When an object covers equal distances in equal amounts of timeEx- if a race car travels at a CONSTANT SPEED of 96m/s, it will travel a DISTANCE of 96 meters EVERY SECOND.

Constant Speed

What would a d-t graph look like for a constant speed?

But most objects do not travel at a constant speed.

The speed of an object can change from one minute to another.So we can use AVERAGE SPEED to describe its motion.Use this equation…

Average Speed = total Distance / total Time

Let’s try itA runner finished a 3 mile race in 22 minutes. He may not have run at the same pace the whole time, but you can still calculate his AVERAGE SPEED3 miles/22 minutes=.14 miles/min

PACE: 22 minutes / 3 miles = 7.33 min/mile

Instantaneous vs. Average speed

Average speed- overall distance over time the object traveledInstantaneous speed- measures speed over small time interval (at an instant)

Does a speedometer of a car read instantaneous or average speed?

What 2 controls on a car enable a change in speed?

Let’s use our math skills

Page 323…

Read through “MATH SKILLS”DO problems 1-3 “Practice Problems”

Answers to problems on 323

1. v=d/t110m/72 sec= 1.5 m/s toward shore

2. v=d/t38m/1.7 sec= 22m/s toward first base

3. d=vt=(12.0 km/hr)(5.00 hr)= 6.00 x 104 m

What if I want to describe speed AND direction?

For example…what if you wanted to find a plane. Knowing the speed would only tell you how far away to look but not in what direction. For that we need…

VELOCITY- the speed and direction of motion.

Let’s get back to the car example…

Name another control that enables a change in velocity.

In order to analyze such situations, we need to understand what are known as

vectors…All measured quantities can be

classified as being either a scalar or a vector.

Scalar

_________ only (size of the quantity ….a number)

Vector

_________ and _________

Magnitude Magnitu

deDirection

Speed is Velocity…kind of…• Velocity is the vector for the scalar of

speed.• The magnitude is the same!!! • Therefore in calculations, you can use speed or

velocity interchangeably

• Speed = distance / time

• Velocity = distance / time in a direction

Use the diagram to determine the resulting displacement and the distance traveled by

the skier during these three minutes.

Answer

The skier covers a distance of(180 m + 140 m + 100 m) = 420 m and has a displacement of 140 m, rightward.

What is the coach's resulting displacement and distance of

travel?

Answer

The coach covers a distance of(35 yds + 20 yds + 40 yds) = 95 yards and has a displacement of 55 yards, left.

Consider a car moving with a constant, rightward (+) velocity of 10 m/s.

How many meters will he travel in 5 seconds?

Constant velocity

Now consider a car moving with a rightward (+), changing velocity -

that is, a car that is moving rightward but speeding up or accelerating.

Acceleration

Distance time graphs

Draw 2 graphs– One showing a slow constant speed– One showing a faster constant speed

Slow, Rightward(+)Constant Velocity

Fast, Rightward(+)Constant Velocity

Slow, Leftward(-)Constant Velocity

Fast, Leftward(-)Constant Velocity

Consider a car moving with a constant, rightward (+) velocity - say of +10 m/s. A car moving with a constant velocity is a car with zero

acceleration.

Draw a graph!

Constant velocity = zero acceleration

Now consider a car moving with a rightward (+), changing velocity - that is, a car that is moving rightward but speeding up or accelerating. Since the car is

moving in the positive direction and speeding up, the car is said to have a positive acceleration.

Positive acceleration

Describe this graph!

Does the velocity of the wind affect such things as a sprinter’s speed or an airplane’s flight time?

Resultant Velocity animation

http://www.glenbrook.k12.il.us/GBSSCI/PHYS/mmedia/vectors/plane.html

A small airplane heads east with a speed of 200 mph with respect to the air (the “air speed”). This would be the plane’s speed if the air was NOT moving – no wind)

• If the wind/jet stream is moving east at 50 mph, what is the plane’s resulting velocity with respect to the ground (the “ground speed”)?

“Adding Vectors” Example:

with the wind

200 50

250 mph, east

• If, later, the airplane is flying west into the 50 mph wind with an “air speed” of 200 mph, now what is the plane’s resulting velocity with respect to the ground (the “ground speed”)?

against the wind

50

200

150 mph, west

1. Find the velocity in m/s of a swimmer who swims 110 m toward the shore in 72 s.

1.5 m/s toward the shore

1. Imagine that you could ride a baseball that is hit high enough and far enough for a home run. Using the baseball as a reference frame, what does the Earth appear to do?

1. Calculate the displacement in meters a cyclist would travel in 5.00 h at an average velocity of 12.0 km/h to the southwest.