How can we describe the various motions in this situation?

Chapter 2 Describing Motion

I. SpeedA. 70 miles per hour

1) In 1 hour, we would travel 70 miles

2) Speed has a number (70) and a unit (miles per hour)

3) Miles per hour = miles/hour = mph = miles hours (distance/time)

B. Average Speed = distance traveled divided by time of travel

1)

2) Average speed is a rate

C. Converting units of speed: always distance/time

1) Common Speeds

t

dsspeed

time

distance

dollar

centsor

game

runsor

gal

milesor

y

x

m/h Km/h m/s

20 32 9

40 64 18

60 97 27

80 130 36

100 160 45

2) Examples:

a. How fast in Km/h is 70 m/h?

-Conversion Factor: 1 Km = 0.621 m

-Calculation:

b. What is the conversion factor of Km/h to m/s?

-1Km/h = 0.278m/s

Km/h 112.70.621m

1Km

h

70m

11

1

1Km

0.621m

0.621m

1Km

s

0.278m

60s

1min

60min

1h

1Km

1000m

h

1Km

D. Instantaneous Speed

1) A Speedometer measures instantaneous speed

2) When you use different “speeds”

a) Instantaneous speed tells you about the moment

b) Average speed tells you how long the trip will take

3) Definition of Instantaneous Speed = rate for a given instant = average speed for a short time period where the speed changes little

4) Animation02

II. VelocityA. Velocity = speed in a defined direction

1) 20 mph = speed

2) 20 mph due North = velocity

B. Changes in Velocity

1) Car on a curve: speed stays the

same, velocity is always changing

2) Any change in speed or direction

is a change in velocity

C. Instantaneous Velocity = average velocity for a short time period where the velocity changes little

D. Changes in Velocity are caused by forces

1) Baseball: force of gravity, hand throwing and catching, wind, etc…

2) We will discuss forces more later

III. Vectors = quantities where size and direction are importantA. Definitions

1) Magnitude = size of the vector, a number

2) Direction = orientation of the vector relative to a fixed reference

3) Vectors are written as bold (v) or with a bar on top ( )

4) There are many vector quantities in Physics: velocity, forces, acceleration, momentum, etc… (Speed s is not a vector)

v

5) Vectors are represented by arrows (Velocity is always blue in the book)

B. Math with vectors: Appendix C

1) Specifying magnitude and

direction on a graph

2) Vector Components = any 2 or more vectors that add up to the original vector (usually components on the x and y axes are used)

3) Adding Vectors

a) Plot first vector on a graph

b) Plot the second vector starting at the end (arrowhead) of the first

c) Draw the sum vector from the origin to the last arrowhead

4) Subtracting Vectors

1) Plot first vector on a graph

2) Find what the negative of the second vector is

a) Same magnitude as the positive vector

b) Opposite direction as the positive vector

3) Add the negative of the second vector to the first

a

bc

a + b = c

ab

-b

a

-bc

a – b = c

5) More Examples