Height of a building with a Projectile

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Homework Help Overview

The problem involves determining the height of a building from which a ball is launched at an angle of 38° with an initial speed of 16 m/s. The ball impacts the ground after 45 seconds, and the gravitational acceleration is given as 9.80 m/s². There is a significant discrepancy in the expected height, with an answer of 9480 m mentioned.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the initial calculations for vertical velocity and the application of kinematic equations. There are questions about the accuracy of the original poster's calculations and the validity of the given answer. Some participants suggest verifying the time of flight and the initial conditions.

Discussion Status

The discussion is ongoing, with participants providing feedback on calculations and questioning the plausibility of the height derived. There is a recognition of potential errors in the original problem setup or assumptions, but no consensus has been reached on a corrected approach.

Contextual Notes

Participants note the unusual height of 9480 m and express skepticism regarding the problem's parameters, suggesting that the values may not be realistic for a typical building scenario. There is also a mention of air resistance potentially affecting the outcome, indicating that assumptions about the environment may need to be reconsidered.

guma671
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Homework Statement



A ball is launched at an angle of 38° at 16m/s off of the top of a building, the ball finally hits the ground after 45seconds. What is the height of the building? Gravity is normal 9.80m.s squared. The answer to the problem is 9480m.

Homework Equations



I just needed a starting point. I wasn't sure really where to go with the problem.

The Attempt at a Solution



First i tried to find Vy by 16.0m/s * sin(38°) and got a value of 4.741m/s. I then plugged it into the equation Δy=VyiΔt+1/2gΔtsquared but the answer came out to around 10,000.
 
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Why don't you show your complete calculations?
 
what do you mean?
 
oh well with the answer i got around 10,000 it was just Y=(16m/s)(45s)+1/2(-9.8)(45)squared which came out to -9202.5
 
Always good to check things.
1. You know sin(30°) = 0.5 so you should expect Vy > 8 m/s !
2. If even that leads to an improbable answer, check that you read the OP correctly. Perhaps it says 4.5 seconds ?
 
Posts crossed. I am a slow typer. Now you use Vo, not Vy. Not that it matters much on a 10000 m scale. But it does on a 100 m scale.
 
Come again. What building is 9480 m high ?
 
If the 9480 m is in the answer book, you may have to deal with a case of sloppy re-use of old exercises. They change a number but don't bother to check if it's sensible. In this case that is double painful: the 9480 is nonsense and on top of that, after 1 second, the vertical component of the velocity is 0. In the 44 seconds after that it would go -430 m/s, way over the speed of sound. If the building is on Earth (9.8 m/s^2), the air restance can't be ignored anymore, even for a lead ball.
 

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