# Kinematics: Ball thrown off of a building

• DracoMalfoy
In summary: So when it says "x direction" it means "horizontal".but if the question doesn't say anything about the angle, how can you assume it?It's not so much assuming as interpreting. It seems clear that the problem setter intended that the ball was thrown horizontally, and therefore that the problem is about the vertical motion of the ball.

## 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.