Motion of an object Average Speed Velocity Acceleration

Objects move in relation to a reference point.

Reference point

The tree is the reference point since it stays in place.

When an object changes position over time relative to a reference point, the object is in motion.

When the motorcycle changes position over time relative to the tree, the motorcycle is in motion.

You can describe the direction of the object’s motion with the reference direction…

North, south, east, west, up, or down.

The speed of an object is the distance traveled by an object divided by the time taken to travel that distance.

S = distance/time (S = d/t)

If the motorcycle traveled 10 meters and it took it 2 seconds, how fast is the motorcycle traveling?

S = d/t S = 10 meters/ 2 seconds

S = 5 m/s

Don’t forget units!

Think to yourself about how you walk class to class….is it at a constant pace?

Most often, objects do not travel at a constant speed, so we need to calculate the average speed.

Average speed = total distance/total time

Mr. Summers swims a distance from one end of a 50 m pool to the other end in a time of 25 s. What is Mr. Summer’s average speed?

S = d/t S = 50 m/ 25 s S = 2 m/s

Distance (m) Time (s)5 1

10 215 320 4

Create a graph of this data. Label your y-axis as “distance” and your x-axis as “time”.

Since the graph is comparing distance to time, then the line represents SPEED!

S = d/t

Time (s)

Speed of object

Picture two birds leaving a tree at the same time.

They both fly at 10 km/h for 5 min. Why don’ they end up at the same place?

They went in different directions!

The speed of an object in a particular direction is called velocity.

The speeds of the birds were the same , but they had different velocities.

Which is the correct way to describe the velocity of a car traveling on I-5?

“A car’s velocity is 60 m/h”

“A car’s velocity is 60 m/h south.”

Picture sitting in a car at a stop light, when it turns green what happens to the car?

“Speeding up” is commonly described by using the word acceleration.

However, in science it means much more…

Acceleration is the rate at which velocity changes over time…..

So velocity changes when speed changes, direction changes, or both.

A car accelerates when it increases it’s speed from 25 to 35 m/h.

A car accelerates when it turns a corner.

A car accelerates when it puts it’s brakes on for a stop sign.

Positive acceleration – increase in velocity.

Negative acceleration – decrease in velocity (also called deceleration)

List two examples of each in your compbook. Share with your neighbor.

Acceleration is also how fast velocity changes. …•The faster the velocity changes, the faster the acceleration.

Since acceleration is the rate at which velocity changes over time…..

Acceleration = final velocity – starting velocity

time it takes to change velocity

A = Vf – Vi

t

Mrs. Rhoades is riding her bike south downhill. At the top of the hill she is going 1 m/s south and at the bottom she is going 5 m/s south . It took her 4 sec to go travel the entire hill. What was Mrs. Rhoades acceleration down the hill?

A = Vf – Vi

t A = 5 m/s – 1 m/s = 1 m/s/s South 4 s

A plane passes over point A at a velocity of 240 m/s north. 40 seconds later, it passes point B at a velocity of 260 m/s north. What is the plane's average acceleration?

A = Vf – Vi

t A = 260 m/s – 240 m/s = 0.5 m/s/s

north40 s

Create a graph of this data. Label your y-axis as “Velocity” and

your x-axis as “time”.

Velocity (m/s) Time (s)5 12 23 34 4

Since the graph is comparing velocity to time, then the line represents acceleration!

A = Vf – Vi

t

Time (s)

Acceleration of object

Think about a person spinning around on their feet… are they accelerating?

Yes! An object traveling in a circular motion is always changing its direction, therefore always changing velocity. So its accelerating!

The acceleration that occurs in a circular motion is called centripetal acceleration!

Centripetal acceleration happens on a Ferris wheel or as the moon orbits the earth.