1. Acceleration

2. Warm- up Define velocity Give an example
(If you have not done so – hand in your lab)

3. A recap of Motion Motion can be in: Uniform motion A straight line
Circle/curved path
Back and forth motion
Uniform motion
The simplest kind of motion
Along a straight line
Feels smooth
No change in velocity

4. What if motion is not constant?
Un-uniform motion
Along a curved or up and down path
Feels bumpy
Ex: roller coasters, stop and go traffic, walk/jog/walk
Change in velocity
This is acceleration!!
The rate* at which an object’s velocity changes
*rate = a change over time

5. Three types of Acceleration
Positive acceleration
Increase in velocity
Speeding up
Negative acceleration
Decrease in velocity
Slowing down
Change in direction
without a change in speed

6. Is there a change in velocity?
You are riding your bike at 9 km/hr. Ten minutes later, your speed is 6 km/hr You ride your bike around the block at a constant speed of 11 km/hr You ride your bike in a straight line at a constant speed of 10 km/hr You are stopped at a red light and then it turns green

7. Doing the math To find acceleration: A = ∆V ∆ t
∆ is the greek letter delta and means “the change in”
To find the change in velocity:
final velocity of the object (Vf) minus initial velocity of the object (Vi)
To find the change in time:
final time of the object’s motion (tf) minus initial time of the object (ti)

8. What’s the unit?
Acceleration is a unit of speed divided by a unit of time
Ex: m/s/s
Can also be written as m/s2
Ex: km/hr/s
Ex: km/hr/hr
Can also be written as km/hr2
What does this unit mean?
It tells you how much velocity is changing per unit of time
Ex: 5 m/s2
Means that velocity is increasing (because it’s a positive #) by 5 m/s every second that the object travels

9. What does it mean?
Ex 2: if a car is accelerating at a rate of 10 m/s/s then its velocity is increasing by 10 m/s every second it is traveling
1s = 10 m/s
2s = 20 m/s
4s = 40 m/s
3s = 30 m/s

10. Some words to look out for
Zero words
Words that either mean the objects starts at rest (not in motion) or stops its motion
Stop
At rest
At a standstill

11. Let’s calculate
A jet starts at rest at the end of a runway and reaches a speed of 80 m/s in 20s. What is its acceleration?
Write what is given:
Vi = 0 m/s (it started at rest)
Vf = 80 m/s
∆t = 20 s
Solve for acceleration:

12. Let’s calculate…again
A skateboarder is moving in a straight line at a speed of 3 m/s and comes to a stop in 2 seconds. What is his acceleration?
Write what is given:
Vi = 3 m/s
Vf = 0 m/s (came to a stop)
∆t = 2 s
Solve for acceleration:

13. Velocity – Time Graphs
Velocity-Time graphs show the change of velocity over an elapsed time
AKA Speed-Time graphs
Remember that speed does NOT take into account direction
Time is always the independent variable
Velocity is always the dependent variable

14. The slope of a Velocity-Time graph is equal to acceleration
Slope = rise/run
Slope = change in velocity / time
a = Δv / t
The steeper the slope, the faster the acceleration
Remember acceleration can be speeding up, slowing down, or sharp turns
A positive slope is speeding up and moving forward
A negative slope is EITHER slowing down OR moving backwards
A zero slope means that the velocity is NOT changing, meaning that the object is moving at the same speed in the same direction

17. Check Your Understanding
Which person(s) could be slowing down?
Person C. They have a negative slope; they could be moving backwards too (there is not enough info on the graph to tell).

18. Check Your Understanding
Which person(s) could be slowing down?
Person C. They have a negative slope; they could be moving backwards too (there is not enough info on the graph to tell).