A bus is traveling at 80feet per second. suddenly, the driver appies the brakes. The speed of the braking bus after t seconds is given by f=80-20t^2.

Question

How long will it take the bus to come to a complete stop?  I do not know how to write this out or iam missing something. the answer is 2 sec. but I can not get that. please help me see where I am off. thank you

Answer 1

You need to find when the speed is zero, this would mean the bus comes to a complete stop. Since the speed after braking is given by:

 

f=80-20t²,

 

setting this equal to zero yields:

 

80-20t²=0 Or 80=20t², which leads to 

 

t²=80/20=4, from which it is easy to see that

 

t=2 seconds