AccelerationThe last topic in Physics!

Reminder If you see this on a

POSITION-TIME graph:

It means that the object has a CHANGE in VELOCITY

If you see this on a POSITION-TIME graph:

It means that the object is moving with UNIFORM MOTION

Reminder

A CHANGE in VELOCITY means that the object has either:

Changed speed (faster or slower)

Changed direction

Both

Acceleration

Acceleration is the rate of change in velocity

Acceleration refers to the change in velocity over a period of time

Think about it… if velocity is the change in displacement over a period of time, what do you think the formula for acceleration is?

AccelerationQ: What is the symbol for acceleration?

A: a

Q: What is the formula for acceleration?

A:

Q: What are the units of measurement for acceleration?

A: m/s2

Acceleration and Newton

Sir Isaac Newton (1643-1727) came up with several key formulas that help us understand the world around us. His second law applies to what we’re learning now:

In simple terms, this formula tells us that FORCE IS ACCELERATION… so acceleration is just a push or pull on an object that causes it to change the velocity it’s moving at

Force = a push or a

pull

Reminder

First, establish which direction is positive and which direction is negative:

West East

Left --------------------------Right

South ----------------------------North

Down Up

Acceleration = Force

Acceleration = force (a push or a pull)… so to figure out if the acceleration is + or – just think of what direction you would have to push someone to change their velocity

Pushing Demonstrations!

1. Stand up and push your chairs in. Make sure the aisles are clear

2. Grab a partner. Look them in the eye and say, “I trust you.”

3. Listen to Ms. T for directions

Example A

Velocity Direction Δv = vf - vi

Acceleration

+ 2 m/s to + 6 m/s 

---------------------------------------------------------------

vi vf

+ 6 – 2 = + 4 m/s ++

Example B

Velocity Direction Δv = vf - vi

Acceleration

+ 8 m/s to + 1.5 m/s 

-------------------------------------------------------------

vi vf

+ 1.5 – 8 = - 6.5 m/s -+

Example C

Velocity Direction Δv = vf - vi

Acceleration

- 7 m/s to -3 m/s 

-------------------------------------------------------------

vf vi

_ -7 – (-3) = - 4 m/s __

Example D

Velocity Direction Δv = vf - vi

Acceleration

0 m/s to -9 m/s 

-------------------------------------------------------------

vf vi

_ 0 – (-9) = +9 m/s +_

Acceleration Word Problems

A skater goes from a standstill to a speed of 6.7 m/s in 12 seconds.  What is the acceleration of the skater?

a = vf – vi

tf – ti

a = 6.7 12 12

a = 6.7 – 0 12 - 0

a = 0.6 m/s2

Acceleration Word Problems

During a race, a sprinter increases from 5.0 m/s to 7.5 m/s over a period of 1.25s.  What is the sprinter’s average acceleration during this period?

a = vf – vi

tf – ti

a = 2.5 1.25

a = 7.5 – 5 1.25 - 0

a = 2 m/s2

Acceleration Word Problems

You are hiking down a mountain at -3.5 m/s when you start getting chased by zombies. You speed up to -9.7 m/s really quickly (like, in 5 seconds). What is your acceleration?

a = vf – vi

tf – ti

a = -6.2 5

a = -9.7 – (- 3.5) 5 - 0

a = -1.24 m/s2

Acceleration Word Problems

  

Δv = a x Δt

Δv = 0.457 x 36

Δv = 16.452 m/s

vf = Δv + vi

vf = 16.452 + 0

vf = 16.452 m/s

A baby sitter pushing a stroller starts from rest and accelerates at a rate of  0.457m/s2.  What is the velocity of the stroller after it has traveled for 36 seconds?