Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Hard word prob involving cars (HELP ME)

  1. Oct 9, 2005 #1
    Ok, here is the prob.

    You are arguing over a cell phone while trailing an unmarked police cruiser by 25m; both you and the cruiser are traveling at 110km/h. Your argument diverts your attention from the police cruiser for 2.0s (long enough to look at the phone and yell "I wont do that!"). At the beginning of the 2.0 s, the cop begins emergency braking at 5.0 m/s^2. (a) What is the seperation between the two cars when your attention finally returns, assuming it takes you another .4s to realize the danger and start braking. (b) If you too brake at 5.0 m/s^2, what is your speed when you hit the cruiser.

    I have tried this, but we just began learning about this type of problem, and all my answers make no sense. the answers I should get are 15m, and 26.7m/s for a and b respectivly. CAN SOMEONE EXPLAIN TO ME HOW YOU GET THESE?

  2. jcsd
  3. Oct 9, 2005 #2


    User Avatar
    Homework Helper

    So, to make sure I got the problem right:
    There are two cars, A and B. A is behind B by 25 meters, and they both travel at 110 km/h (when t = 0). B starts braking with an acceleration of 5,0 m/s2. (a) How close are the cars at t = 2,0 s? (b) At t = 2,4 s, A starts decelerating 5,0 m/s2. At what speed will A hit B?

    I bolded the t = 2,0 s part, as that is the asked instant (supposing I understood the question correctly).
    For (a), I'll give you another tip: only relative motion counts.
    (b) First, I'd solve the instant when the cars hit.
  4. Oct 9, 2005 #3


    User Avatar
    Homework Helper

    Maybe you've done this already, but it usually helps a lot to draw out a picture of the problem (draw cars as blocks a certain distance apart, label the velocities, etc) to help visualize things.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook