Projectile motion ball thrown down vertically

  • #1

Homework Statement


A ball is thrown vertically downward with a velocity of 30 ft from the top of a building.

Find

a.)What it's speed will be after falling 2 seconds
b.)How far it will fall in 2 seconds
c.)What will be its velocity after 30 ft
d.)If the ball was released at 120 ft above the ground, in how many seconds will it strike the ground.


Homework Equations



For a I used the equation Vy = Vyo - gt
For b I used the equation ΔY=Vyo(t) - 1/2gt^2
For c I used the equation ΔY=Vyo(t) - 1/2gt^2 in order to find t, or the time it takes to move 30 ft and I got .72 s. Then I put that into the first equation of Vy=Vyo - gt.
For d I used ΔY=Vyo(t) - 1/2gt^2.

The Attempt at a Solution



For a I got -94 ft/s.
For b I got -124 ft/s.
For c I got 53.04 ft/s.
For d I got 2.74 seconds

I have a few questions. First, can someone check if these answers are correct? My professor did not provide an answer sheet for the exam review. Also I was only able to get a proper answer for d if I made the 120 ft negative. If the ball was released at 120 ft above the ground wouldn't it be positive? Also, c gave me some trouble, if it is incorrect, could someone please provide an in-depth explanation? Thank you so much.
 

Answers and Replies

  • #2
265
0
I think your answers for (a) and (b) are correct.

For (c), it should be -53.04ft/s.
Instead of working out the time first, you can just use 1 equation :
V2 = V02 - 2gΔy

For (d), you release the ball at the origin of your coordinate system, so the ground is at y=-120ft.
From (b) you know the ball falls 124ft in 2s, so to fall 120ft will be just under 2s, so your answer is not correct here.
 
  • #3
35,134
11,372
You should be careful with the units - "30ft" is not a velocity, and "-124ft/s" is not a length.
 
  • #4
I think your answers for (a) and (b) are correct.

For (c), it should be -53.04ft/s.
Instead of working out the time first, you can just use 1 equation :
V2 = V02 - 2gΔy

For (d), you release the ball at the origin of your coordinate system, so the ground is at y=-120ft.
From (b) you know the ball falls 124ft in 2s, so to fall 120ft will be just under 2s, so your answer is not correct here.
Well the question for d did not specify an initial velocity, it just said released, so I think it's right.

You should be careful with the units - "30ft" is not a velocity, and "-124ft/s" is not a length.
Oh right, I better not make that mistake on the exam haha.
 
  • #5
265
0
For part (d), yes you're correct.
I didn't notice it started from rest.
 
  • #6
For part (d), yes you're correct.
I didn't notice it started from rest.
Cool, thank you very much for your assistance.
 

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