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Homework Help: Anyone help with my physics HW?

  1. Sep 19, 2011 #1
    1. The problem statement, all variables and given/known data
    I have three questions total (did five already). I don't want the straight-answers but I need help working them through since I want to do them myself. Here's the first one:
    ---To set a speed record in a measured (straight-line) distance d, a race car must be driven first in one direction (in time t1) and then in the opposite direction (in time t2). (a) To eliminate the effects of the wind and obtain the car's speed vc in a windless situation, what number of method should we use: the average of d/t1 and d/t2 (method 1) or should we divide d by the average of t1 and t2 (method 2)? (b) What is the fractional difference in the two methods when a steady wind blows along the car's route and the ratio of the wind speed vw to the car's speed vc is 0.0440?----

    Other question: ----An electric vehicle starts from rest and accelerates at a rate a1 in a straight line until it reaches a speed of v. The vehicle then slows at a constant rate a2 until it stops. (a) How much time elapses from start to stop? (b) How far does the vehicle move from start to stop? Give your answers in terms of the given variables. ---
    2. Relevant equations

    3. The attempt at a solution
    For (a) my guess is method 1. It makes more sense. I don't understand (b), though.
  2. jcsd
  3. Sep 19, 2011 #2
    Woo! Got it! So only need help with the "other question." Thanks.
  4. Sep 19, 2011 #3


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    Gold Member

    For the "other question", why don't you try solving it in 2 parts. The question is looking from "start to stop"--why not consider solving for the "acceleration phase", and then for the "deceleration phase". Then add 'em up.
  5. Sep 19, 2011 #4
    I have an hour left till this is do by e-mail (don't worry it's just homework, not a test or anything). I'll try what you said.
  6. Sep 19, 2011 #5
    I don't get it :(
    Another hint?
  7. Sep 19, 2011 #6
    Δv/Δt - Δ2v/Δ2t?
    No, that's not right.
  8. Sep 19, 2011 #7
    Here's another: "Figure 2-28 shows a red car and a green car that move toward each other. Figure 2-29 is a graph of their motion, showing the positions xg0 = 270 m and xr0 = -35 m at time t = 0. The green car has a constant speed of -20 m/s and the red car begins from rest. What is the acceleration magnitude of the red car? "
  9. Sep 19, 2011 #8
    Solved the 1 I just posted! Yes! Now I just need that dreaded "other question"
  10. Sep 20, 2011 #9
    use the following equation to get the time


    v=final velocity u=initial velocity a=acceleration t = time

    in the first case [itex]a=a_1[/itex] for second case [itex]a=-a_2[/itex]

    in the first case u=0 v=v ; in the second case u=v and v=0, solve for different t's
    and add
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