Linear and projectile motion?

  • #1

Homework Statement


A ball is thrown downward from a tower 65.0 m tall. If the ball takes 1.30 seconds to hit the ground, what was its initial velocity?

The answer my teacher provides is 43.6 m/s, but I am having a problem reaching that answer.

Homework Equations


Possibly Vi = Vf - aT? This is the equation I am trying to use, but the final velocity is not given, I don't think.


The Attempt at a Solution


First, I tried Vi = 0 + 9.8 x 1.3, but obviously, it doesn't work. So whether there is information missing or I am using the wrong formula, I don't know. Any help would be appreciated!
 

Answers and Replies

  • #2
PeterO
Homework Helper
2,435
62

Homework Statement


A ball is thrown downward from a tower 65.0 m tall. If the ball takes 1.30 seconds to hit the ground, what was its initial velocity?

The answer my teacher provides is 43.6 m/s, but I am having a problem reaching that answer.

Homework Equations


Possibly Vi = Vf - aT? This is the equation I am trying to use, but the final velocity is not given, I don't think.


The Attempt at a Solution


First, I tried Vi = 0 + 9.8 x 1.3, but obviously, it doesn't work. So whether there is information missing or I am using the wrong formula, I don't know. Any help would be appreciated!

The values you have are:

x - displacement - 65
a - acceleration due to gravity - 9.8
t - time - 1.3

All motion and acceleration is down, so I have made all values positive

You want vi

so you need the motion formula with out the final velocity. That is

x = vi.t + 0.5.a.t2

sub values into that and see how you go.
 
  • #3
Thank you! That seem to have worked. (: I knew there was something wrong with the formula I was using.
 
  • #4
PeterO
Homework Helper
2,435
62
Thank you! That seem to have worked. (: I knew there was something wrong with the formula I was using.

You should always identify those values you know, and the value(s) you want, to help in deciding which formula to use.
 

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