How Do You Calculate Initial Velocity in Projectile Motion?

[killjoy2019]

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!

 

[PeterO]

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 v_i

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

x = v_i.t + 0.5.a.t²

sub values into that and see how you go.

 

[killjoy2019]

Thank you! That seem to have worked. (: I knew there was something wrong with the formula I was using.

 

[PeterO]

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.