SOLUTION: Suppose you’re driving 90.0 kilometers an hour and suddenly see red flashing lights in the rearview mirror. “Great,” you think, and you pull over, taking 20.0 seconds to come

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Question 1160717: Suppose you’re driving 90.0 kilometers an hour and suddenly see red flashing lights in the rearview mirror. “Great,” you think, and you pull over, taking 20.0 seconds to come to a stop. What was your average acceleration, in m/sec2?
Found 2 solutions by ikleyn, jim_thompson5910:
Answer by ikleyn(52778) (Show Source):
You can put this solution on YOUR website!
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The speed is V = 90 km/h = 90000 m/h = m/s = 25 m/s. The acceleration is a = = = m/s^2 = 1.25 m/s^2 ANSWER

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It is deceleration, in this case, instead of acceleration.

Solved, answered, calculated, explained and completed.

Answer by jim_thompson5910(35256) (Show Source):
You can put this solution on YOUR website!

1 minute = 60 seconds
60 minutes = 3600 seconds (multiply both sides by 60)
1 hour = 3600 seconds

To convert from 90 km/hr to km/sec, we divide by 3600
This is the same as multiplying by (1/3600)
90 km/hr = (90 km/hr)*(1 hr/3600 sec)
90 km/hr = (90)*(1/3600) km/sec
90 km/hr = 0.025 km/sec

To convert from km/sec to m/sec, we multiply by 1000 since there are 1000 meters in 1 km
0.025 km/sec = (0.025 km/sec)*(1000 m/1 km)
0.025 km/sec = (0.025*1000) m/sec
0.025 km/sec = 25 m/sec

To wrap things up so far,
90 km/hr = 25 m/sec

Vi = initial velocity = 25 m/sec
Vf = final velocity = 0 m/sec
t = change in time = 20 seconds

a = acceleration
a = (Vf-Vi)/t
a = (0-25)/20
a = -25/20
a = -1.25
Negative acceleration indicates the object is slowing down. A counter force of the brakes is being applied opposite of the car's initial direction.

Average acceleration is -1.25 m/sec^2
Every second, the car slows down by 1.25 m/sec.

Side note: 1.25 m/sec = 4.5 km/hr