Mammoth Memory

Acceleration

Acceleration is a change in velocity. However it can sometimes be easier to think of it as a change in speed. How much an object accelerates depends on how quickly its velocity/speed changes. The formula used to calculate acceleration is:

zzz_acceleration-01-01.jpg

 

`Ac\c\e\l\e\r\ation=((v_2-v_1))/t` 

You may also see this written as

`Ac\c\e\l\e\r\ation=(Deltav)/t` 

          

06 Physics Karting.jpg 

Accelerator pedal to the floor changes speed to stay on time.

 

Example 1

A cyclist increases her speed from 5m/s to 19m/s in 7 seconds, what is her acceleration?

Answer:

`Ac\c\e\l\e\r\ation=(chan\g\e\ \ i\n\ \ velocity)/(time)` 

`Ac\c\e\l\e\r\ation=((19-5))/7=14/7=2\ m//s^2` 

 

 

Example 2

An oil tanker decelerates at a maximum rate of 0.04m/s2. How long to the nearest minute would it take to come to a complete stop if it was initially traveling at 12m/s?

Answer:

`Ac\c\e\l\e\r\ation=(chan\g\e\ \ i\n\ \ velocity)/(time)` 

`-0.04=((0-12))/(time)` 

`time=((0-12))/-0.04=-12/-0.04=300\ \ se\c\o\nds`

`300/60=5\ m\i\n\u\tes`

 

Example 3

A goalkeeper takes a goal kick and the ball travels away from goal, when it reaches an opposing striker at a velocity of 12.4m/s the ball kicked back towards goal by the striker. This results in the ball accelerating at 72m/s2 towards the goal for 0.45 seconds as the striker kicks it. At what velocity does the ball leave the striker’s foot?

Answer:

`Ac\c\e\l\e\r\ation=(chan\g\e\ \ i\n\ \ velocity)/(time)` 

Multiply both sides by time to make change in velocity the subject,

`Ac\c\e\l\e\r\ationtimestime=(chan\g\e\ \ i\n\ \ velocitytimescancel(time))/(cancel(time))` 

`chan\g\e\ \ i\n\ \ velocity=Ac\c\e\l\e\r\ationtimestime` 

 

However there is a trick in this question! The ball is initially traveling away from goal and then accelerates towards goal, this means the acceleration is in the opposite direction and therefore is in a negative direction!

`chan\g\e\ \ i\n\ \ velocity=-72times0.45=-32.4\ m//s` 

`chan\g\e\ \ i\n\ \ velocity=v_2-v_1=v_2-12.4=-32.4\ m//s` 

`v_2=-32.4+12.4=-20\ m//s`

 

NOTE:

The velocity is negative as the direction the ball travels after the striker kicks it is opposite to the direction it was travelling when the goalkeeper kicked it.

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