Kinematics: Ball thrown off of a building

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Homework Help Overview

The problem involves a ball thrown off a building in the +x direction at a speed of 10 m/s, with the goal of determining the height of the building based on the time it takes for the ball to land, which is 2 seconds. The context is kinematics, specifically focusing on projectile motion.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the use of various kinematic equations to find displacement but express confusion regarding the lack of an angle in the problem setup. Some suggest making sketches to visualize the scenario, while others question the geometric relationship between the initial velocity and acceleration.

Discussion Status

The discussion is ongoing, with participants exploring different interpretations of the problem. Some have suggested calculating potential heights using various angles, while others emphasize the need for detailed work to understand the discrepancies in the results. There is no explicit consensus on the correct approach or interpretation yet.

Contextual Notes

Participants note that the problem may be imperfect due to the ambiguity of the angle of projection, as it is only described as being in the +x direction. This has led to varied interpretations of the situation and the potential for multiple correct answers based on different assumptions.

DracoMalfoy
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Homework Statement


A ball is thrown off of a building in the +x direction at 10m/s. If the ball takes 2s to land, how tall is the building?

Homework Equations


  • Vf=Vi(a)(t)
  • Δd=Vi(t)+1/2(a)(t)
  • Vf^2=Vi^2+2(a)(Δd)
  • Δd=1/2(Vf-Vi)(t)

The Attempt at a Solution


  • Vf: 0m/s
  • Vi: 10m/s
  • a: -9.8m/s^2
  • t: 2s
  • Δd: ?
I tried using the second, third and fourth equation to find the displacement but I keep getting 5 or 5.1 or a number a bit lower and none of those are the answers.

the choices are

A. 19.6m
B. 5.1m
C.33.3m
D. 2m
E. 1m
 
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DracoMalfoy said:
  • Vi: 10m/s
  • a: -9.8m/s^2
The equations you quote are for motion in one straight line. What is the geometric relationship between the a and vi quoted above?
 
i have projectile motion equations but the question doesn't come with an angle so I got confused
 
DracoMalfoy said:
is thrown off of a building in the +x direction
Make a sketch

Oh, and: show your work in detail. I can't reconstruct ##\approx## 5 m no matter how hard I try
 
BvU said:
Make a sketch

Oh, and: show your work in detail. I can't reconstruct ##\approx## 5 m no matter how hard I try
i made a hideous sketch lol. I am not sure how to do this when I am not given an angle
 
If it says "in the +x direction", then what is the angle ?

And: please show your work in detail. I can't reconstruct ≈ 5 m no matter how hard I try

Am I repeating myself ?
 
DracoMalfoy said:
i have projectile motion equations but the question doesn't come with an angle so I got confused

Try calculating it with a few selected angles to get the range of possible answers.
Vertically up,
horizontal
Vertically down
The real answer has to lie somewhere in that range.

Note: that investigation will show that the question is imperfect - one answer MUST be wrong, 4 of them COULD be correct, but will also indicate which situation the questioner had in mind when setting this question - and thus the probably answer.
Note also: When they said the ball was thrown in the +x direction, I believe a direction was given - or at least strongly implied..
 
DracoMalfoy said:
i have projectile motion equations but the question doesn't come with an angle so I got confused
The angle is implied.
When it says "x direction" it is assuming the standard use of x, y, z in 3D Earth surface problems: z is vertical, x and y are horizontal.
 

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