Homework Help: Physics

Posted by Anonymous on Saturday, March 15, 2008 at 7:42pm.

A ball is thrown from the top edge of a building with initial velocity components of 15m/s(up) vertically and 20m/s horizontally. It strikes ground 140m from the base of the building. What is the height of the building?

I tried using V2^2 = V1^2 + 2ad and it gave me 11.25 which I know is wrong. How would I calculate this?

Physics - Quidditch, Saturday, March 15, 2008 at 10:51pm
The ball lands 140m from the base of the building. The balls initial horizontal velocity is 20m/s.
140m = (20m/s) * t
Solve for t.
Since the time the ball is in flight is now known.
Plug that into the "complete" formula for distance:
d = (1/2)*(g)*(t^2) + v*t + h
Let distance 0 be ground level.
initial vertical velocity is v=15m/s
h is the initial distance above the ground which is the height of the building.
0 = (1/2)*(g)*(t^2) + v*t + h
Solve for h.
Remember that the initial velocity is up and the acceleration of gravity is down (watch the signs)
Physics - ~christina~, Saturday, March 15, 2008 at 11:40pm
I helped you with this question yesterday and gave the exact same info as Quidditch Why didn't you use the info I gave?
- Physics - Anonymous, Sunday, March 16, 2008 at 10:42am
  Thanks! I saw it and used it but I'm not sure if I did it right. I got 140 for the height of the building and I don't think that's right.
  - Physics - Quidditch, Sunday, March 16, 2008 at 6:08pm
    I got something in that range, but I believe your answer has more error than I would expect.
    What did you get for time that the ball was in the air?
    What numbers did you plug into your final equation?
    - Physics - Anonymous, Sunday, March 16, 2008 at 7:02pm
      For time t = d/v
      140/20
      t= 7
      
      0 = 1/2(-10)(7^2)+(15)(7)+h
      -h = -140
      h = 140
      
      We were asked to use 10 for our acceleration.
      - Physics - Quidditch, Sunday, March 16, 2008 at 8:27pm
        You got it!
        Using 10m/(s^2) for gravity, your answer is correct.

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