Helllllp! I was only peripherally paying attention to Tina Fanetti's
question for help on the University Physics problem but did read Larry
Woolf's data and his politically correct problem. I decided to put the
problem on a test late last night. I e-mailed Larry for help but haven't
heard back yet and am getting desperate and hoping one of you will grace me
with some assistance. Here is the e-mail I sent him:
Hi,
I need help because I seem to be having a brain fart. I just put on my test
for my advanced physics students a problem based on the one you cited, I just
changed things to meters though. Here is the problem:
Headlights illuminate the road up to 48.78 m (which is 160 feet) in front of
you. You are driving along and see a stop sign. What is the fastest speed
you can drive and still stop safely at night? Your reaction time is divided
into a perception time ("I need to brake") and a movement reaction time
(movement of the foot). For alert drivers, the average reaction time is 1.6
second. For drivers encountering an unexpected obstacle around a blind curve
or not really focusing on the road, an average reaction time is closer to 2.5
seconds. Calculate the fastest speed for both an alert driver and unfocused
driver. The average deceleration of a car is 5.18 m/s^2.
OK, I cannot seem to get it. I know that for the reaction time part the car
is going constant velocity and covers a distant dependent on the velocity.
Then the car brakes and ends with a final velocity of zero. But all I know
is the total distance the car is allowed to go, the acceleration, the final
velocity. The initial velocity for the braking part is the velocity for the
reaction time part and the distance traveled braking is the 48.78m - distance
traveled during the reaction time part. I seem to have too many unknowns.
So, how do you solve it?
It's my bad for blindly putting it on the test last night and not working it
out before the class sat down to take the test, but as I quickly typed it in
after a crosscountry meet, I just assumed I (and most students) would have no
problem. Can you give us a hand here so that I can set myself and students
straight? Thank you so much!
--
Julie Hilsenteger
Physics Teacher
Centennial High School
3505 SE 182nd
Gresham, OR 97030
503-661-7612
julie_hilsenteger@centennial.k12.or.us