1. II. Describing Motion Motion Speed & Velocity Acceleration
Ch. 11 Motion
II. Describing Motion
Motion
Speed & Velocity
Acceleration

2. A. Motion Problem: Is your desk moving?
To describe motion completely, we need a frame of reference – a system of objects that are not moving with respect to one another.

3. A. Motion Relative Motion -
Movement in relation to a frame of reference.
Reference point
Motion

4. Quick Write
Explain a time where you experienced confusion about your motion.

5. Measuring Distance Distance – the length of a path between two points.
Units – m (meters)
km (kilometers)
cm (centimeters)

6. Measuring displacement*
07/16/96
Measuring displacement
Displacement is independent of the path taken by the object.
Involves the starting point, ending point, and direction. Vector quantity
Example: A man travels 4 blocks North and then returns 4 blocks South. What is his displacement?
D = 0 blocks
Distance = 8 blocks *

7. v d t B. Speed & Velocity Speed rate of motion
distance traveled per unit time

8. B. Speed & Velocity Instantaneous Speed speed at a given instant
Ex. The speed measured by a police officer using a radar gun
Average Speed
speed over an entire trip __
v = d t

9. B. Speed & Velocity Velocity speed in a given direction
can change even when the speed is constant!

10. Velocity Problems

11. Graphing d/t

12. E. Graphing Motion slope = speed steeper slope = straight line =*
07/16/96
E. Graphing Motion
Distance-Time Graph A B
slope =
steeper slope =
straight line =
flat line =
speed
faster speed
constant speed
no motion *

13. E. Graphing Motion Who started out faster? A (steeper slope)
Distance-Time Graph A B
Who started out faster?
A (steeper slope)
Who had a constant speed? A
Describe B from min.
B stopped moving
Find their average speeds.
A = (2400m) ÷ (30min) A = 80 m/min
B = (1200m) ÷ (30min) B = 40 m/min

14. Acceleration

15. t a C. Acceleration Acceleration the rate of change of velocity
vf - vi t
Acceleration
the rate of change of velocity
change in speed or direction
a: acceleration
vf: final velocity
vi: initial velocity
t: time

16. C. Acceleration Positive acceleration “speeding up”
Negative acceleration
“slowing down”

17. Quick Write
Given an example of something that you have seen in your daily life that is accelerating.
Describe its motion.

18. t a D. Calculations a = (vf - vi) ÷ t t = 3 s
A roller coaster starts down a hill at 10 m/s. Three seconds later, its speed is 32 m/s. What is the roller coaster’s acceleration?
GIVEN:
vi = 10 m/s
t = 3 s
vf = 32 m/s
a = ?
WORK:
a = (vf - vi) ÷ t
a = (32m/s - 10m/s) ÷ (3s)
a = 22 m/s ÷ 3 s
a = 7.3 m/s2 a
vf - vi t

19. t a D. Calculations t = ? t = (vf - vi) ÷ a t = (0m/s-30m/s)÷(-3m/s2)
How long will it take a car traveling 30 m/s to come to a stop if its acceleration is m/s2?
GIVEN:
t = ?
vi = 30 m/s
vf = 0 m/s
a = -3 m/s2
WORK:
t = (vf - vi) ÷ a
t = (0m/s-30m/s)÷(-3m/s2)
t = -30 m/s ÷ -3m/s2
t = 10 s a
vf - vi t

20. E. Graphing Motion
Distance-Time Graph
Acceleration is indicated by a curve on a Distance-Time graph.
Changing slope = changing velocity

21. E. Graphing Motion acceleration slope = +ve = speeds up
Speed-Time Graph
acceleration
+ve = speeds up
-ve = slows down
slope =
straight line =
flat line =
constant accel.
no accel. (constant velocity)

22. E. Graphing Motion Specify the time period when the object was...
Speed-Time Graph
Specify the time period when the object was...
slowing down
5 to 10 seconds
speeding up
0 to 3 seconds
moving at a constant speed
3 to 5 seconds
not moving
0 & 10 seconds