1. Motion in a Straight Line
position: a distance in a certain direction from a reference point
reference pt. x
position: 50 m to the East
50 m
50 m
position is a vector quantity;
direction is important
50 m to the South
vector quantity: described by both a magnitude (a size) and a
direction
; example: 50 m to the East
scalar quantity: described by magnitude only

2. ex. A man walks 60 m to the east and then 40 m to the west
starting pt.
60 m x
20 m East
40 m
(a) What distance did the man walk?
100 m
distance: a length traveled; a scalar quantity
; depends on path taken
(b) What is the man’s displacement?
20 m to the East
displacement: a change of position
; independent of path taken
; depends only on difference between initial and final
positions
; vector quantity

3. ex. The highway distance from Indianapolis to Denver is 1712 km. If
the driving time is hr, what is the average speed for the trip?
Average speed: distance traveled in a certain time interval
; scalar d
1712 km
v = =
= v = km/hr t
29.25 hr
Instantaneous speed: speed at a particular instant
velocity: speed in a particular direction
; vector
Average velocity of trip to Denver:

5. ex. The highway distance from Indianapolis to Denver is 1712 km. If
the driving time is hr, what is the average speed for the trip?
Average speed: distance traveled in a certain time interval
; scalar d
1712 km
v = =
= v = km/hr t
29.25 hr
Instantaneous speed: speed at a particular instant
velocity: speed in a particular direction
; vector
Average velocity of trip to Denver:
v = v = km/hr to the West

6. Relative to train: v = 15 m/s 15 m/s
Ball’s velocity:
Tom
Bill
Relative to train: v = 15 m/s
15 m/s
Relative to Earth: v = 15 m/s
Ball’s velocity:
20 m/s
Relative to train:
v = 15 m/s
15 m/s
Relative to Earth:
v = 35 m/s
Ball’s velocity:
20 m/s
15 m/s
Relative to train:
v = 15 m/s
Relative to Earth:
v = 5 m/s

7. Relative to train: v = + 15 m/s + 15 m/s
Ball’s velocity:
Tom
Bill
Relative to train: v = m/s
+ 15 m/s
Relative to Earth: v = m/s
Ball’s velocity:
+ 20 m/s
Relative to train: v = m/s
+ 15 m/s
Relative to Earth: v = + 35 m/s
Ball’s velocity:
+ 20 m/s
- 15 m/s
Relative to train: v = m/s
Relative to Earth: v = + 5 m/s

8. Check this out: Ball’s velocity: + 20 m/s - 20 m/s Relative to train:
v = m/s
Relative to Earth:
v = 0
Check this out:

9. Aristotle: Heavy objects fall faster than light objects
Falling Objects
Aristotle: Heavy objects fall faster
than light objects
Galileo: All objects fall at the same
rate, in a vacuum,
regardless of their mass
Check this out: go to 1:59, watch until 3:40