1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Need help on projectile motion by 10:00 tonight (~ 2 hrs 10 mins from now)

  1. Nov 13, 2007 #1
    1. The problem statement, all variables and given/known data

    A rescue plane wants to drop supplies to isolated mountain climbers on a rocky ridge 235 m below. The plane is traveling horizontally with a speed of 280 km/h (77.8 m/s).

    a) How far in advance of the recipients (horizontal distance) must the goods be dropped (Fig. 3 41a)? Already figured this out and got it right: 538.8 m

    b) Suppose, instead, that the plane releases the supplies a horizontal distance of 425 m in advance of the mountain climbers. What vertical velocity (up or down) should the supplies be given so that they arrive precisely at the climbers' position (Fig. 3 41b)?
    Magnitude: ???

    2. Relevant equations

    3. The attempt at a solution
    Part A work:

    x= 538.8 m
    Part B work

    t=1.5 s

    tfinal= 6.93s - 1.5s
    tfinal = 5.5s

    vy0~= 70 m/s

    I think I did this problem right, but the computer keeps saying that it's wrong. Is this a rounding error? I have tried 70 m/s, but it said that was wrong so I think that I might have to round to 69.7 m/s instead of 70 m/s since the computer is picky sometimes. I only have one chance left to get it right. :P

    If this isn't a rounding error, what have I done wrong? Any help is appreciated, thanks.
  2. jcsd
  3. Nov 13, 2007 #2
    your values for y and y0 need to be switched. because 0 is not your initial height, 235 is. also, if you designated up as positive and down as negative, your generic equation should be y=y0-(vy0)t-0.5gt^2. try that.
  4. Nov 13, 2007 #3
    Wow, that was obvious. Thank you, it worked. :)
  5. Nov 13, 2007 #4
    you're welcome =) glad I could help.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook