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Acceleration is the rate that velocity changes over time.
An object is accelerating if◦ Its speed changes◦ Its direction changes◦ Both its speed and direction change
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Speeding up◦ From red light to green light◦ When you are in a car as it speeds up, your body
goes backward. Slowing down
◦ From green light to red light◦ When you are in a car as it slows down, your body
goes forward.
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The variable for acceleration is “a.”
a = Δv / t
The units for acceleration are m/s2
If the object’s acceleration is negative, the object is probably slowing down.
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A car can get from rest to 27m/s (around 60mph) is 2.5seconds. What was the car’s acceleration?
Given:
Vi = 0
Vf = 27m/s
t = 2.5s
a = (vf – vi) / t
a = (27-0) / 2.5
a = 27 / 2.5
a = 10.8m/s2
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Page 375: #1-5 SHOW YOUR WORK!!
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Before, we used v = d / t, or d = v x t This equation only works if velocity is
constant (not changing) By definition, velocity changes if an object
is accelerating.
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If an object is speeding up,◦ It covers a distance of
V x t plus a little bit. If an object is slowing down,
◦ It covers a distance of V x t minus a little bit.
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Instead of the equation we learned when velocity was constant…
We use the following equation when accelerating,
d = vit + ½ at2
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Example: A runner accelerates at 1.2m/s2 for 30seconds at the beginning of a race. How far does he travel in those 30 seconds?
d = vit + ½ at2
d = 0 + ½ (1.2)(30)2
d = 540m
Given:
a = 1.2m/s2
t = 30s
Vi = 0