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

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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 Displacement
Distance measures the actual path an object takes Displacement 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

Write in your own words- what is the difference between distance and 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/time Ex- miles/hour, m/s, etc.

There are three types of speed you must know…
Constant speed Average speed Instantaneous 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 Distance Speed Time

Constant Speed When an object covers equal distances in equal amounts of time Ex- 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 it A 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 SPEED 3 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 traveled Instantaneous 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/t 110m/72 sec= 1.5 m/s toward shore
38m/1.7 sec= 22m/s toward first base 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 Magnitude Direction

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 m 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.

How many meters will he travel in 5 seconds?
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

A small airplane heads east with a speed of 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”)? 200 50 250 mph, east with the wind

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”)? 200 50 150 mph, west against the wind

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

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?

Calculate the displacement in meters a cyclist would travel in 5
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.