Acceleration & Deceleration

Acceleration & Deceleration

Year 10

​What we will learn about...

​- definitions

​- how to calculate

​- worked examples

​Acceleration

​Acceleration

• ​Acceleration is a vector quantity - the rate at which an object changes velocity

​Acceleration

• ​Acceleration is a vector quantity - the rate at which an object changes velocity

• ​the bigger the acceleration, the faster the speed or direction of an object increases

• ​if velocity or speed is changing, so is acceleration

​Example:

​Example:

• ​They both are moving off after stopping

• ​Car A is increasing their speed by 1 m/s, Car B by 2 m/s

• ​Car B has a greater acceleration - because it is changing velocity at a faster rate

​How do we calculate acceleration or deceleration?

• ​If speed is increasing - positive

• ​If speed is decreasing - negative (deceleration)

​How do we calculate acceleration or deceleration?

• ​If speed is increasing - positive

• ​If speed is decreasing - negative (deceleration)

NOTE: sometimes questions will say speed and sometimes they will say velocity - if you are being asked to calculate acceleration, use what the question has provided you!

​Question example:

• ​A drag racing car reaches a speed of 140 m/s in 4.8 seconds from a stop start. Calculate the average acceleration of the car; ​

• ​a = change in speed/time taken

• ​a = (140-0)/4.8

• ​a = 29.2 m/s²

​Another example:

• ​The same car has to slow down before it reaches the end of the track. Calculate its acceleration as it comes to rest after 5.4 seconds from a speed of 140m/s:

• ​a = change in speed/time taken

• ​a = (0-140)/5.4

• ​a = -25.9 m/s²

• ​This is deceleration

​Speed/Velocity vs Time Graphs

• ​Very similar to distance/displacement vs time graphs

• ​We can use this to directly determine acceleration status

Multiple Choice

Which of the following is a unit for acceleration?

• km/s

• m/s²

• mi/hr

• ft

Multiple Choice

In each of the following cases, determine where the car has no acceleration ?

• A car shortly after a stoplight turns green.

• A car approaching a red light

• A car with the cruise control set at 80 km/h

• A car turning a curve at a constant speed

Multiple Choice

A roller coasters accelerates from an initial velocity of 6.0 m/s to a final velocity of 70 m/s over 4 seconds.  What's the acceleration?

• 24 m/s²

• 18 m/s²

• 16 m/s²

• 16 m/s

Multiple Choice

A dog runs with an initial speed of 7.5 m/s on a waxed floor.  It slides to a stop in 15 seconds.  What is the acceleration?

• -7.5 m/s

• -7.5 m/s²

• -0.5 m/s²

• 7.5 mi//hr