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Kinematics: Ball thrown off of a building

  • #1

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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Answers and Replies

  • #2
haruspex
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  • 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?
 
  • #3
i have projectile motion equations but the question doesnt come with an angle so I got confused
 
  • #4
BvU
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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
 
  • #5
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. im not sure how to do this when im not given an angle
 
  • #6
BvU
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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 ?
 
  • #7
PeterO
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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..
 
  • #8
haruspex
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i have projectile motion equations but the question doesnt 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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