Is the ADA's Sample Physics Problem Incorrect?

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Discussion Overview

The discussion revolves around a sample physics problem provided by the ADA, which involves calculating the height of a building from which a ball is thrown horizontally. Participants explore the kinematic equations relevant to the problem and express concerns about the correctness of the problem's stated answer.

Discussion Character

  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant believes the ADA's problem is incorrect, arriving at a height of 62m instead of the stated 31m, and questions the treatment of horizontal velocity.
  • Another participant asserts that the answer on the ADA's website is correct and explains the calculation leading to 31m using the time of flight and kinematic equations.
  • A third participant suggests that the discrepancy may arise from using the vertical velocity at impact rather than the average vertical velocity.
  • A later reply acknowledges the use of the wrong equation and expresses relief upon realizing the mistake.

Areas of Agreement / Disagreement

There is disagreement regarding the initial interpretation of the problem, with one participant questioning the correctness of the ADA's answer while others support it. The discussion reflects a mix of confusion and clarification without reaching a consensus on the initial claim.

Contextual Notes

Participants reference specific kinematic equations and calculations, but there is no resolution of the initial claim about the problem's correctness. The discussion highlights potential misunderstandings in applying the equations.

burapi
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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!
 
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burapi said:
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!

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

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