Motion The Need for Speed

What is motion? You are probably thinking that motion is when something is moving… You are probably thinking that motion is when something is moving… Unfortunately, that definition won’t do us much good. Unfortunately, that definition won’t do 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 motion? How do we tell fast motion from slow motion?

Well… An object is in motion when its distance from another object is changing. An object is in motion when its distance from another object is changing. Huh? Huh? We can tell something is moving when we can measure it! We can tell something is moving when we can measure it! Motion is described relative to something else. Motion is described relative to something else. For example, even though you are in your seat, you are moving 30k/s relative to the sun! For example, even though you are in your seat, you are moving 30k/s relative to the sun!

How do we measure motion? We measure motion based on how fast it is occurring, or better put, the rate at which it occurs. We measure motion based on how fast it is occurring, or better put, the rate at which it occurs. Rate tells us the amount of something that occurs or changes in one unit of time. Rate tells us the amount of something that occurs or changes in one unit of time. You are probably more familiar with this concept when I talk about speed. You are probably more familiar with this concept when I talk about 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 object travels per unit of time. The speed of an object is the distance the object travels per unit of time. Lets think of driving. Most streets in Phoenix are 45 miles per hour. So, if I drive the rate of 45 miles per hour, then I will travel 45 miles in one hour! Lets think of driving. Most streets in Phoenix are 45 miles per hour. So, if I drive the rate of 45 miles per hour, then I will travel 45 miles in one hour!

How do I calculate speed? I am so glad you asked! I am so glad you asked! Speed=Distance Speed=Distance Time TimeS=D T OR

A cheetah can maintain a constant speed of 25m/s. At this rate, how far will it travel in 10 s? Remember the steps of problem solving! Remember the steps of problem solving! 1. Write the equation & rearrange. 1. Write the equation & rearrange. S=D/T or D=ST S=D/T or D=ST 2. Write the knowns S=25m/s D= ? T= 10s

3. Convert 3. Convert –We don’t need to 4. Plug! D=(25m/s) x (10s) D=(25m/s) x (10s) 5. Chug! D=(25m/s) x (10s) =250m 6. Box Answer!

Velocity Society uses the terms “speed” and “velocity” interchangeably. Society uses the terms “speed” and “velocity” interchangeably. But…they are wrong. But…they are wrong. Velocity is speed with a given direction Velocity is speed with a given direction So, speed is going 30m/h So, speed is going 30m/h Velocity is going North 30m/h Velocity is going North 30m/h

“We’re not in Kansas anymore… If we lived in Kansas during Tornado season, it really wouldn’t do us much good to know the speed of a storm traveling with a tornado brewing inside it. If we lived in Kansas during Tornado season, it really wouldn’t do us much good to know the speed of a storm traveling with a tornado brewing inside it. But… But… It would be very beneficial to know the speed of a tornado, and the direction, especially if it is headed toward our house. It would be very beneficial to know the speed of a tornado, and the direction, especially if it is headed toward our house.

The Rubber Hits the Road… Speed is distance over time Speed is distance over time Example: 65 m/h Example: 65 m/h Velocity is speed and direction Velocity is speed and direction Example: 65 m/h South Example: 65 m/h South

Motion Acceleration

Acceleration Consider a car stopped at a stoplight. As the light turns green, the driver presses the gas pedal and gradually increases speed, or accelerates. Consider a car stopped at a stoplight. As the light turns green, the driver presses the gas pedal and gradually increases speed, or accelerates. Acceleration is the rate at which velocity changes. Acceleration is the rate at which velocity changes. Remember, velocity is speed and direction. Remember, velocity is speed and direction.

Velocity change Velocity can either increase or decrease Velocity can either increase or decrease The direction can also change The direction can also change Acceleration involves a change in either of these components. Acceleration involves a change in either of these components. Acceleration refers to increasing or decreasing speed and changing direction. Acceleration refers to increasing or decreasing speed and changing direction.

Increasing Speed When an object increases speed, it experiences acceleration. When an object increases speed, it experiences acceleration. When a football is thrown, it accelerates. When a football is thrown, it accelerates. When a pitcher throws a ball, it accelerates. When a pitcher throws a ball, it accelerates. When a horse begins to run, it accelerates. When a horse begins to run, it accelerates.

Decreasing Speed Objects that speed up, eventually slow down. Objects that speed up, eventually slow down. A ball that is thrown eventually rolls to a stop. A ball that is thrown eventually rolls to a stop. A car slows down as the driver steps on the brake. A car slows down as the driver steps on the brake. A runner finishing a race, eventually stops. A runner finishing a race, eventually stops.

Changing Direction Acceleration is change in speed or direction. Acceleration is change in speed or direction. A car accelerates as it makes a turn. A car accelerates as it makes a turn. A Ferris Wheel at the fair is accelerating (direction is always changing) A Ferris Wheel at the fair is accelerating (direction is always changing) The Earth rotating around the sun. The Earth rotating around the sun.

Calculating Acceleration To determine the acceleration of an object, you must calculate the change in velocity during each unit of time. To determine the acceleration of an object, you must calculate the change in velocity during each unit of time. Final Velocity - Initial Velocity Final Velocity - Initial VelocityAcceleration= Time Time

Units If velocity is measured meters/second If velocity is measured meters/second And time is measured in seconds, And time is measured in seconds, Then, acceleration is measured in meters per second per second. Then, acceleration is measured in meters per second per second. Huh? Huh? Acceleration units are m/s2 Acceleration units are m/s2

Remember when I said that scientists are lazy? We can summarize the acceleration equation by the following: We can summarize the acceleration equation by the following: Vf - Vi Vf - Vi A= A= t

An Example: A roller coaster car rapidly picks up speed as it rolls down a slope. As it starts down the slope, its speed is 4 m/s. But 3 seconds later, at the bottom of the slope, its speed is 22m/s. What is its average acceleration? A roller coaster car rapidly picks up speed as it rolls down a slope. As it starts down the slope, its speed is 4 m/s. But 3 seconds later, at the bottom of the slope, its speed is 22m/s. What is its average acceleration? Relax! This is easy, just follow the steps for solving word problems! Relax! This is easy, just follow the steps for solving word problems!

Just follow the steps! Step 1: Write the equation Vf – Vi Vf – Vi A= A= t

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