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Homework Help: Hard problems, Help Needed

  1. Sep 27, 2010 #1
    Well I am doing my homework and there are a few problems that are so hard that i can't even try to get an answer... Hoping you can give me tips to get started,
    Here they are:

    1) A world-class sprinter accelerates to his maximum speed in 3.9 s. He then maintains this speed for the remainder of a 100m race, finishing with a total time of 8.9 s.

    What is his average acceleration during the first 3.9s?

    2) The engineer of a passenger train traveling at 25.0 m/s sights a freight train whose caboose is 200m ahead on the same track. The freight train is traveling at 15.0m/s in the same direction as the passenger train. The engineer of the passenger train immediately applies the brakes, causing a constant acceleration of -0.100 m/s^2, while the freight train continues with constant speed. Take x=0 at the location of the front of the passenger train when the engineer applies the brakes.

    Where will the colision take place? (i've already determined that there will be one)

    3)two stunt drivers drive directly toward each other. At time t=0 the two cars are a distance D apart, car 1 is at rest, and car 2 is moving to the left with speed v0. Car 1 begins to move at t=0, speeding up with a constant acceleration ax. Car 2 continues to move with a constant velocity.

    At what time do the two cars collide?

    Find the speed of car 1 just before it collides with car 2.

    So, i tryed my best but it seems like it's missing data.. maybe the teacher did a mistake. =S
    Any tips and feedback is appreciated.
  2. jcsd
  3. Sep 27, 2010 #2
    Hi, please can you show your own attempts as per PF guidelines. Once done, I will gladly assist you.

    There is nothing missing from the questions. All are answerable with given data.

  4. Sep 27, 2010 #3
    1 and 3 i didnt even try anything because idk where to start. none of the formulas i got work.

    #2 i found that after 200m the speed of the train is 24.2 m/s with v^2x=vx0^2+2ax(x-x0). Doesnt help much.

    im missing time and postion..how am i supposed do do something without these 2.
  5. Sep 27, 2010 #4
    Well let's start with 2 then seeing as you have a start at it.

    You know the deceleration, initial speed and final speed.

    Using v = u + at, calculate the time it takes to reach 15m/s from 25m/s (after 15m/s no collision can occur).

    Once you have the time, using s = ut + 0.5at^2 you can calculate the distance travelled by the passenger train in the deceleration time.

    Using s = 0.5(u + v)t you can then work out the distance travelled by the freight train (don't forget to add the 200m separation).

    From that you can work out where the collision will occur.

    v = final speed, u = initial speed, a = acceleration, t = time, s = distance
    Last edited: Sep 27, 2010
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