# Need help deriving a formula

#### Omar.Castillo

Hey could anyone help me out there to know how you get to the formula of free fall displacement. D= 1/2 x g x t^2? I really have no clue of how to get it using other formulas. I started using the D=Vi x T + 1/2 x A x T^2 but then I got stuck I didnt know which other formula to bring in to cancel the variables I NEED HELP PLEASE :D

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#### Doc Al

Mentor
If you know the formula for uniformly accelerated motion ($D = v_i t + 1/2 a t^2$), then you can get the formula you need by setting the initial speed to zero and the acceleration to g. That gives you $D = 1/2 g t^2$.

#### dextercioby

Homework Helper
It's also important to make the wise choice of the sense of the coordinate axis along the displacement. That is, assuming the $\vec{g}$ field is pointing downwards, choose the vertical axis with the same sense.

Daniel.

#### Dr.Brain

There is no formula here . As Doc Al put it, the initial velocity has to be set to zero for the formula to be applicable , so the formula seems to be forced.It is better to get hold of the three basic kinematic equations rather than learning formulae for each and every situation.

BJ

#### wisredz

Try to draw the velocity-time graph. The area under the curve will give you the displacement.

#### Omar.Castillo

Thanks

Hey thanks to all you guys

I never did know you could set the initial velocity to zero! that solved the entire problem thank you. Now im wondering why I never thought of that well anyways thanks guys

#### punjabi_monster

yes initial velocity can be set to zero if the object stops of an instant, or if it starts from rest.

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