Unit 10 - Acceleration

Unit 10 - Acceleration

By S Harris

RECALL

• speed is the rate of change of motion (distance over a set amount of time)

• velocity includes the speed and the direction of an object

Acceleration occurs/happens when an object changes its speed, direction, or both.

• So acceleration happens when velocity changes

Velocity and Acceleration

Question:

• when you think of the word acceleration what comes to mind?​

What qualifies as acceleration?

​

Answer:

• 3 ways that an object can accelerate include:

  • speeding up

  • slowi​ng down (also called decelerating)

  • changing direction

What qualifies as acceleration?

​

​Acceleration has a direction:

• if an object is speeding up, acceleration is in the same direction as velocity

• if an object is slowing down, acceleration is in the opposite direction of velocity​

Direction of Acceleration

​

​These graphs are used when an object travels in a straight line and does NOT change direction.

• ​line A = acceleration (speeding up)

• line B = moving at a constant velocity (no change in speed)​

• line C = decelerating (slowing down)​

Speed-Time Graphs and Acceleration

​

​A

​B

​C

Graphing Types of Acceleration

Multiple Choice

Which graph shows Flash running at a constant velocity, stopping to help a civilian, and then quickly accelerating (increasing in speed) to help Iris?

Ticker tape diagrams (or oil drop diagrams) can be used to tell the story of an object's motion​. We can determine if the object is:

• moving fast

• slow

• has constant velocity

• increasing in velocity

all based on the spacing

of the dots

Describing Motion with Diagrams

​

​Acceleration = Δv = final - initial

​

​Example Problem:

• a skateboarder has an initial velocity of 3 m/s west and then comes to a stop (meaning a velocity of 0 m/s) in 2 s. What is the skateboarder's acceleration? (hint: stopped means a velocity of 0 m/s)

  • ​Δv = final - initial = _____ - _____= _____

  • t = _____

  • a= ______ ➗ ______ = ______ m/s²

Calculating Acceleration​

​A

​t

​​Δv

t

t

​

When calculating acceleration if you have a:

• negative sign or value = acceleration is in the opposite direction of velocity

• no sign (positive) or value = acceleration is in the same direction as velocity​

  ​

Calculating Acceleration​

​A

​t

​Δv

Multiple Choice

Was the direction of the skateboarder's acceleration east or west?​ (hint: look at the answer to the question we did 2 slides ago. the direction of velocity was west.)

• East (slowing down)

• West (speeding up)

Multiple Choice

An airplane starts at rest and then accelerates down a runway for 20 s. At the end of the runway, its velocity is 80 m/s north. What is its acceleration? (hint: at rest means that velocity initially is 0 m/s)

• 0.25 m/s² north

• -0.25 m/s² south

• 4 m/s² north

• -4 m/s² north