m3 - physics 02b - acceleration

8/12/2019 M3 - Physics 02b - Acceleration

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P H Y S I C S

Calculating Acceleration


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What is acceleration?

Accelerationis a change in velocity!


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Introduction to Acceleration (page 25)

Acceleration = Change in velocity / time taken forthe change

Final velocity Initial velocityTime

In Short: a = (v u)

tUnit = m/s2


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Example: The dragster!

Drag races are very popular in the US!

Question 1:

The average dragster car can do 0 to 45 m/s (0 160 km/h)in 1.5 seconds. What is its average acceleration?

Question 2:

The drag car goes so fast that it has to brake usingparachutes. Its speed before braking is 180 m/s. It takes thedragster 15 seconds to stop. What is its acceleration?


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The accelerating dragster

Question 1:

What is known?

u = 0 m/s, v = 45 m/s, t = 1.5 s

The average acceleration of the dragster is?

a = v u = 45 0 m/s = +30 m/s2

t 1.5 s

This is also sometimes called: 3G (3 x 10 m/s2)


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The braking dragster

Question 2:

What is known?

u = 180 m/s,v = 0 m/s!!!,

t = 15 s

The average acceleration of the dragster is?

a = v u = 0 180 m/s =

12 m/s2

t 15 s

This is also sometimes called: -1.2 G (-1.2 x 10 m/s2)


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Positive and negative Acceleration

Acceleration has sizeAND direction. So it is a:VECTOR

Speeding up (going faster) = positive acceleration

Slowing down = negative acceleration We call negative acceleration, retardationor

deceleration. (or braking, slowing down in every dayEnglish)


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Speed and Acceleration are not the same

Zero speed = No motion

Zero acceleration = no changein motion

(but you can still travel at the same high speed)

You hardly feel any

acceleration in the middle

of the flight, when thespeed of the plane is a

constant 930 km/h


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For all moving objects we can make formula?

These formula can help us predict the velocity and position(distance) of an object at any time during motion.

It can also allow us to answer questions like:

How long do I have left before I

Hit that person!

or

How far can I drive away from thisaccident before the cops come here?


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Seriously! Even NASA uses these formula to predictthe velocity, position and acceleration of its satellites!

How long will it be before our satellite reaches Jupiter?


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Formula for horizontal acceleration

From the formula for acceleration we can calculate

the velocity and distance travelled for any movingobject

Velocity (at any time):v = u + a t

v = velocity (at any time), u = initial velocity, t = time

Distance travelled (at any time):

x = u t + a t2

x = horizontal distance travelled (m)


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Formula for vertical acceleration:Falling objects

From the formula for horizontal acceleration

we can find the ones for vertical acceleration!

Velocity:

v = u + g tv = velocity (at any time), u = initial velocity,

t = time (sec), g = acceleration of gravity

Distance travelled:

y = u t + g t2

y= vertical distance travelled (m)


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Motion equation example questions1 and 2

m3 - physics 02b - acceleration

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