# How do you find (initial) velocity using conservation of energy?

Without mass; only time, distance, and height is given. For example:

For this lab, the reference level was 100cm above ground therefore the height of the object was 10cm. I determined time and distance and I also found velocity using 2D kinematics.

Now, however, I am required to find velocity using conservation of energy.

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Please elaborate. – ABC Apr 8 '13 at 2:48

## 1 Answer

According to what i understand : case of free fall.

So, Energy initial= $mgh+1/2mv_i^2$

Energy Final = $mg(0)+1/2mv_f^2$

(Taking ground as zero potential reference)

And now you need the relation between $v_f$ and $v_i$ using simple kinematics. Using which you can get $v_f$.

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But still i would recommend using only kinematics. – ABC Apr 8 '13 at 2:57
For this lab, the reference level was 100cm above ground therefore the height of the object was 10cm. I also determined time and distance. I also found velocity using 2D kinematics. Now, however, I am required to find velocity using conservation of energy. Thanks – Matt Apr 8 '13 at 3:14
You don't need time. The distance will help you estimate the change in potential energy. That must equal the change in kinetic energy. That is precisely the physical content in @exploringplanet's equations. The point is that the mass doesn't matter. Whatever the mass, both the (change in) potential energy and (change in) kinetic energy are propotional to it. So you can effectively "cancel" the mass dependence and calculate what happens to the velocity. – Siva Apr 8 '13 at 5:15