1. Distance, Displacement, Speed, and Velocity

2. Questions for Consideration
What is frame of reference?
What is distance?
How is displacement different from distance?
What is speed?
What is velocity?
What are scalar and vector quantities?

3. Frame of Reference
Coordinate frame within which to measure position, motion, or other properties of an object.
OR...
Observational frame tied to the motion of an observer.
In Newtonian physics, all motion must be defined in terms of a reference frame.
Relative Motion

4. Reference Frame
Is the speed of the ball different relative to the pitcher, the truck driver, and the jet pilot? Why or why not?

5. Distance Distance (d) – how far an object travels.
Does not depend on direction.
Imagine an ant crawling along a ruler.
What distance did the ant travel?
d = 3 cm cm 1 2 3 4 5 6 7 8 9 10

6. Distance Distance does not depend on direction.
Here’s our intrepid ant explorer again.
Now what distance did the ant travel?
d = 3 cm
Does his direction change the answer? cm 1 2 3 4 5 6 7 8 9 10

7. Distance Distance does not depend on direction.
Let’s follow the ant again.
What distance did the ant walk this time?
d = 7 cm cm 1 2 3 4 5 6 7 8 9 10

8. Displacement
Displacement (x) – difference between an object’s final position and its starting position.
Does depend on direction.
Displacement = final position – initial position
x = xfinal – xinitial
In order to define displacement, we need directions.
Examples of directions:
+ and –
N, S, E, W
Angles

9. Displacement vs. Distance
Example of distance:
The ant walked 3 cm.
Example of displacement:
The ant walked 3 cm EAST.
An object’s distance traveled and its displacement aren’t always the same!

10. Displacement
Let’s revisit our ant, and this time we’ll find his displacement.
Distance: 3 cm
Displacement: +3 cm
The positive gives the ant a direction! cm 1 2 3 4 5 6 7 8 9 10 + -

11. Displacement Find the ant’s displacement again. - + Distance: 3 cm
Remember, displacement has direction!
Distance: 3 cm
Displacement: -3 cm cm 1 2 3 4 5 6 7 8 9 10 + -

12. Displacement Find the distance and displacement of the ant. - +
Distance: 7 cm
Displacement: +3 cm cm 1 2 3 4 5 6 7 8 9 10 + -

13. Displacement vs. Distance
An athlete runs around a track that is 100 meters long three times, then stops.
What is the athlete’s distance and displacement?
Distance = 300 m
Displacement = 0 m
Why?

14. Speed Speed (s) – Rate at which an object is moving.
speed = distance / time
s = d/t
Like distance, speed does not depend on direction.

15. Speed A car drives 100 meters in 5 seconds.
What is the car’s average speed?
s = d/t
s = (100 m) / (5 s) = 20 m/s
100 m
5 s
4 s
1 s
2 s
3 s

16. Speed
A rocket is traveling at 10 km/s. How long does it take the rocket to travel 30 km?

17. Speed
A racecar is traveling at 85.0 m/s. How far does the car travel in 30.0 s?

18. Pulling It All Together
Back to our ant explorer!
Distance traveled: 7 cm
Displacement: +3 cm
Average speed: (7 cm) / (5 s) = 1.4 cm/s
Average velocity: (+3 cm) / (5 s) = +0.6 cm/s cm 1 2 3 4 5 6 7 8 9 10 + -
5 s
4 s
1 s
2 s
3 s

19. Scalar and Vector Quantities
Scalar Quantity – has magnitude but not direction.
Distance and speed are scalar quantities.
Vector Quantity – has magnitude and direction.
Displacement and velocity are vector quantities.

20. Average vs. Instantaneous Speed
During a typical trip to school, your car will undergo a series of changes in its speed. If you were to inspect the speedometer readings at regular intervals, you would notice that it changes often. The speedometer of a car reveals information about the instantaneous speed of your car. It shows your speed at a particular instant in time.

21. Average speed
Average speed is a measure of the distance traveled in a given period of time; it is sometimes referred to as the distance per time ratio. Suppose that during your trip to school, you traveled a distance of 5 miles and the trip lasted 0.2 hours (12 minutes).
On the average, your car was moving with a speed of 25 miles per hour. During your trip, there may have been times that you were stopped and other times that your speedometer was reading 50 miles per hour. Yet, on average, you were moving with a speed of 25 miles per hour.

22. Video