Describing Motion

Chapter 2

1. Speed
• How fast you are going ?
1. Average Speed
• The distance traveled divided by the time of travel
• Average Speed = distance traveled/time of travel
• s = d/t
• 55 mph
1. Instantaneous Speed
• How fast you are going at a given instant
• Gives little indication how long it will take to cover a certain distance.
• The rate at which distance is being covered at a given instant in time.
1. Velocity
• Involves the direction of motion as well as how fast the object is going (velocity = speed and direction)
• Vector quantity
• Velocity = distance/time (v = d/t)
• 55 mph due east
• A force is required to change the velocity (friction)
1. Instantaneous Velocity
• The velocity of an object at a particular instant in time
1. Average Velocity
• The velocity of an object over a certain time period

C) Application: Racing Car Tires

1. Acceleration
• Vector quantity
• The rate at which velocity changes
• Acceleration = velocity/time (a = v/t)
1. Instantaneous acceleration
• The rate at which velocity is changing at a given instant in time.

1. Average acceleration
• The change in velocity divided by the time required to produce that change

a = v/t = (vf – vi)/t

t = 5 s

v1 = 0 m/s v2 = 20 m/s

a = v/t = (vf – vi)/t

a = (20 m/s – 0 m/s)/5 s

a = 4 m/s2

1. Direction of an acceleration
• Velocity is increasing

v1 v2

a

• Velocity is decreasing

v1 v2

a

• Example

The velocity of a car decreases from 30 m/s to

15 m/s in a time of 3 seconds. What is the average acceleration of the car?

v1 = 30 m/s v2 = 15 m/s

a = vf – vi/t = (15 m/s – 30 m/s)/3s = -5m/s2

a m/s2

1. Graphing Motion
• Graph => Provides an overview of what happened.
 Time Distance 0 sec 0 cm 5 sec 4.1 cm 10 sec 7.9 cm 15 sec 12.1 cm 20 sec 16.0 cm 25 sec 16.0 cm 30 sec 18.0 cm 35 sec 21.0 cm

1. Uniform Acceleration
• The simplest form of accelerated motion
• Occurs whenever there is a constant force acting on an object.
1. How does velocity vary with uniform acceleration?
2. v = vo + at

3. How does distance vary with time?

* If the object starts from rest:

d = 1/2 at2

* If the object is moving before it begins to accelerate:

d = vot + ½ at2