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Question
3. A bus driver forgets to stop at the bus stop. Just as he passes the stop, he begins to decelerate. Five seconds later, the bus is 80 m beyond the stop and has slowed to 10 m/s.
     a. Calculate the speed of the bus as it passed the bus stop.
     b. What is the deceleration of the bus( assumed to be constant.)

Answer
All kinematics problems have 6 variables (Do, Df, Vo, Vf a & t) and 2 equations (Df=1/2*a*t^2+Vo*t+Do and Vf=a*t+Vo). Any time you can identify 4 of the 6 variables you can ALWAYS solve for the other 2!
In this case the variables are: Do=0m (assume this unless the problem indicates otherwise), Df=80m (given), Vo=? (the first answer to the problem), Vf=10m/s (given), a=? (the second answer to the problem) and t=5s (given).
Substituting into the displacement equation:
Df=1/2*a*t^2+Vo*t+Do  becomes  80=1/2*a*5^2+Vo*5+0
While the velocity equation:
Vf=a*t+Vo  becomes  10=a*5+Vo
Solving the velocity equation for the acceleration:
a=(10-Vo)/5
And substituting this into the displacement equation:
80=1/2*[(10-Vo)/5]*5^2+Vo*5
Solving for the initial velocity:
160=50-5*Vo+10*Vo  therefore  Vo=22m/s
Substituting this back into the velocity equation the acceleration becomes:
a=(10-22)/5=-2.4m/s

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James J. Kovalcin

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I am teaching or have taught AP physics B and C [calculus based mechanics & electricity and magnetism] as well as Lab Physics for college bound students. I have a BS in Physics from the University of Pittsburgh and a Master of Arts in Teaching from same. I have been teaching physics for 34 years. I am constantly updating my skills and have a particular interest in modern physics topics.

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