1. 9.1 – Describing Acceleration

2. Changes in Velocity
Since velocity is a vector quantity, a change in magnitude (speed), direction or both can be considered a change in velocity.
Just like with displacement, change in velocity is calculated by subtracting the final velocity from the initial velocity.

3. Positive Change in Velocity
Example: You ride your bicycle at 9 m/s [N] and then speed up to 15 m/s [N]. What is the change in velocity?

4. Positive Change in Velocity
The initial velocity was in the positive direction (north) and the change in velocity was also positive. Since the initial velocity and change in velocity were in the same direction, the object speeds up.

5. Negative Change in Velocity
Example: You ride your bike at 9m/s [N] and slow down to 4m/s [N]. What is the change in velocity?

6. Negative Change in Velocity
The initial velocity was in the positive direction (north), but the change in velocity was negative. Since the initial velocity and the change of were in opposite directions, the object slows down.

7. Constant Velocity
Constant velocity means the velocity stays the same. Since velocity is a vector quantity, both the magnitude and direction must stay the same.
An object with constant velocity is said to have uniform motion.
A Position-Time graph for an object with uniform motion (constant velocity) would form a straight line.

9. Non-Uniform Motion
Non-Uniform motion indicates that an object is not travelling at constant velocity.
In other words, the velocity is changing. Since velocity is a vector quantity, a change in magnitude (speed), direction or both is considered non-uniform motion.
The rate (how quickly) at which an object changes its velocity is called acceleration.

10. Non-Uniform Motion
Non-uniform motion has a curved line on a Position-Time graph.

11. 9.2 – Calculating Acceleration

12. Calculating Acceleration
Acceleration is defined as rate an object changes its velocity.
Since acceleration describes both the magnitude and direction, it is considered a vector quantity.

13. Positive Acceleration
Example: A long distance runner sees the finish line and increases her velocity to from 6m/s [E] to 12m/s [E] in 3 seconds. Calculate the acceleration.

14. Positive Acceleration
The initial velocity was in the positive direction (east) and the acceleration was also positive. Since the initial velocity and change in velocity were in the same direction, the object speeds up.

15. Negative Acceleration
Example: A car brakes as it approaches a stop sign. It slows down from 50km/hr [N] to rest in in 10 seconds. Calculate the acceleration in m/s2.

16. Negative Acceleration
The initial velocity was in the positive direction (north), but the change in velocity was negative. Since the initial velocity and the change of were in opposite directions, the object slows down.
Negative acceleration is sometimes referred to as deceleration.

17. Velocity-Time Graph
A velocity-time graph is used to illustrate the relationship between position and time.
Time is the independent variable and is labeled on the horizontal or x-axis.
Velocity is the dependent variable and is labeled on the vertical or y-axis.
Try to Plot the Data on graph provided.

18. Velocity (m/s [N])
Time (s) 10 5 15 20 25 30

19. What information can we get from a Velocity-Time graph?
Velocity – by reading the corresponding y-value for a given x-value (time).
Acceleration – by calculating the slope for a given time interval.
Displacement – by calculating the area under the curve for a given time interval.

20. Acceleration Due to Gravity
All objects near the surface of Earth will be attracted and pulled downward due to the force of gravity.
Near the surface of Earth, the acceleration due to gravity is -9.8m/s2

21. Practice
An object is dropped and reaches a downward velocity of 30m/s just before it hits the ground. How long did it fall for?
An object is released and falls for 3 seconds. What is its final velocity?

22. Demo
If a piece of paper and a rock are released from the same height, which will hit the ground first?
Although both object experience the same acceleration due to gravity, an upward force air resistance opposes the downward motion. The more air resistance an object experiences, the slower it will fall.