Is this test question wrong?

1. Mar 11, 2012

burapi

Hi, I think the sample physics problem given by the ADA on their website is wrong. Can anyone confirm? It says: A ball is thrown horizontally at 20 m/s from the top of a building and strikes level ground 50 m from the building. How high is the building?

When I solve this, I get 62m, but the answer is 31m. I assume the discrepancy is the horizontal velocity. I'm assuming it is constant while maybe it should be an average? Can anyone help me confirm this? I thought I had a strong hold on kinematics and I'm worried I'm getting stuck on this problem.

Thank you, anyone!

2. Mar 11, 2012

willem2

The answer on the website is correct, and the horizontal velocity is meant to be constant. It seems you do not use the correct equation.

3. Mar 11, 2012

The time of flight, assuming contant horizontal velocity, is:

t = 50m/(20 m/s) = 2.5s.

Using s = u*t + 0.5 * a * t^2 you get 31 m.

Getting 62m looks like you forgot the 0.5.

4. Mar 11, 2012

Staff: Mentor

My guess is you used the vertical velocity at impact instead of the average vertical velocity...

5. Mar 11, 2012

burapi

Yes, thank you for pointing out I used the wrong equation. I did indeed leave out the 1/2. Ironically I am relieved and can move on!

You guys are so great :)