# Inital velocity

An object is thrown vertically into the air and reaches a height of 30.0 m. Neglecting air friction, what was the object’s initial velocity?

What would the equation be for this question?

I know that:

Height: 30.0m
Gravity: 9.81m/s

I'm not quite sure what else I need to figure this out? thank! :)

## Answers and Replies

Is this a trick question where its 0m/s?

Galileo
Science Advisor
Homework Helper
The easiest way would be to use conservation of energy:
E = 1/2mv^2+mgh
Alternatively, you could use the equation of motion for the object. You know it has a constant acceleration g downwards.

Thank you! so would it look something like this:

E=1/2 (230kg)(0.02ms) ^2 + (230kg)(9.81m/s)(32m)

0.092 + 77201.6 = 77201 ?

oops i mean

5.43 + 72201.6 = 72207.03

Galileo
Science Advisor
Homework Helper
The power behind conservation of energy is that you can use the equation for E at any point in the objects trajectory and you will always get the same answer.
You haven't been given the mass of the object, but the answer will be independent of the objects mass anyway.

You have two special points in the objects trajectory which are important to your question. Which are those and what is the expression for the energy at those points?

gravity, height and time?

I think using Vf^2=Vi^2 + 2ad would be better..Since the highest point is 30 that means at that point v equals 0.