# Help With finding initial vertical speed

## Homework Statement

A Ball is thrown upwards. What is its initial vertical speed? The acceleration of gravity is 9.8 m/s2 and maximum height is 7.2 m ,. Neglect air resistance.

## Homework Equations

Vfinal squared = velocity initial squared + 2g times change in y

## The Attempt at a Solution

velocity final squared = velocity initial squared + 2 (-9.8)(7.2)
VF^2= VI^2 + -141.12
what now?/

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Doc Al
Mentor
What's the final velocity? (When it reaches the max height.)

the final velocity is not given. all that is given is v0, max height is 7.2 meters, and that the acceleration of gravity is 9.8 m/s/s/.

is the finaly velocity 0?

Doc Al
Mentor
is the finaly velocity 0?
Of course. • 1 person
ohh ok... so would i use the same formula i used above to find the initial vertical speed?

what equation should i use to calculate the initial vertical speed?

Doc Al
Mentor
ohh ok... so would i use the same formula i used above to find the initial vertical speed?
Just use that equation you started with and solve for the initial speed.

how do i set it to be that velocity initial squared is on the left side of the equal sign, and velocity final squared is on on the right side?

Doc Al
Mentor
how do i set it to be that velocity initial squared is on the left side of the equal sign, and velocity final squared is on on the right side?
The final velocity is zero, so your equation becomes:

0 = Vi2 - 141.12

So how to get Vi2 alone? How can you get rid of that - 141.12?

you add 141.12 to the 0 and then find square root of 141.12?

Doc Al
Mentor
you add 141.12 to the 0 and then find square root of 141.12?
Sure! Basic algebra.

so the initial vertical speed would be 11.8793939239 m/s ?

Doc Al
Mentor
so the initial vertical speed would be 11.8793939239 m/s ?
Yes. But please round off to a reasonable number of significant figures. (2 or 3 is plenty.)

• 1 person
ok, i have one more question i need help with....