1. Velocity time graphs

2. Goals for today Discuss V-T graphs Homework
In the books p proms 2-7, 10-11,
14-16, 22-28
-- Graph handout
Test for honors is Wednesday
CP is Thursday

3. Differences between V-t graphs and P-T graphs
The vertical #’s tell how fast, not location
V-t does not indicate start position
Direction is indicated by position on graph, not slope
Acceleration is indicated differently on each type of graph

4. Horizontal lines on a V-T graph
Velocity (m/s)
Time (s)

5. Using the motion detector
Can you mimic the graph
Using the motion detector

6. Tilted lines on V-T graphs
V (m/s)
T (m/s)

7. What do tilted lines mean on a V-T graph?
The object is changing its velocity
Acceleration
How it accelerates is determined by
Location of graph (Above, Below x-axis)
Type of tilt

8. Acceleration is… A change in the velocity of the object
Tilted lines on a V-T graph indicate acceleration
Acceleration can be calculated by finding the slope value of these lines

9. Tilted lines on V-T graphs
V (m/s)
T (m/s)

10. What kind of motion is indicated on this graph?
V (m/s)
T (s)

11. What would curves mean on a V-T graph
Curves would indicate non-constant acceleration
Think roller coaster rides, when you get pushed against the harness or back into the seat
The ‘Jerk’

12. Identify motion for each part of the V-T graph
V (m/s)
T (m/s)

13. Identify motion for each part of the V-T graph
V (m/s)
T (m/s)

14. Create a V-t graph that…
Section 1: Shows constant motion of (-3) m/s for 5 seconds.
Section 2: Shows constant acceleration (slowing down in the negative direction)
Section 3: Shows no movement for 5 seconds
Section 4: Shows constant acceleration (speeding up in the positive direction)

15. Answer

16. Homework For all graphs: Find the direction
Constant Velocity or Non-Constant Velocity
Fast, Average or Slow
Faster, Slower (Change in Velocity)

17. Finding overall displacement from V-T graph
How far does an object travel if it goes
3 m/s for 10 seconds? V 3 t 10

18. Acceleration
Anytime the velocity of an object changes, the object accelerates
Getting Faster
Getting Slower
If the motion of object changes direction going the same speed

19. Velocity changes if…
Speeding up
Slowing down

20. Velocity changes if…
The direction of the motion changes, but the speed does not
This type of acceleration is call centripetal acceleration

21. Equation for average acceleration
a = (vf - vi) / t
vf = final velocity
vi = initial velocity
t = elapsed time
a = acceleration

22. Ex #1
A flowerpot falls off a second story windowsill. The flowerpot start from rest and hits the sidewalk 1.5 s later with a velocity of 14.7m/s. Find the average acceleration.

23. What information is given? Identify the information.
Is the time
14.7 m/s
is the final velocity
Starts from rest
Means that the initial velocity is zero (0)
“at rest” always means that vi = o

24. Answer #1 a = (14.7 – 0) / 1.5 = 9.8 What are the units?
(14.7 m/s – 0 m/s) 1.5 s = (m/s) / s
m / s2

25. Ex #2
While driving at an average velocity of 15.6 m/s down the road, a driver slams on brakes to avoid hitting a squirrel. The car stops completely in 4.2 seconds. What is the acceleration of the car?

26. What you want to solve for
Givens
15.6 m/s = vi
4.2 s = t
0 = vf
What you want to solve for
acceleration

27. Solution
a = (0 – 15.6) / 4.2 = (-3.7) m/s2

28. Questions on homework in book
Pg 324
Pg 328

29. Mr. Baker’s Monster problem
On a dark and scary night, Mr. Baker was walking down the street at m/s. He heard a horrible sound of hundreds of shuffling feet behind him and increased his speed to a new m/s. What was the amount of time (in minutes) needed to increase his speed, if his rate of acceleration was 1.9 m/s2 ?

30. Answer Given: vi = 5.234 m/s a = 1.9 m/s2 vf = 78.985 m/s
Want: time (in minutes)
a = (vf – vi) / t
1.9 = ( ) / t