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motion in one dimension



A rock is thrown downward from the top of a 40.0-m-tall tower with an initial speed of 12 m/s. Assuming negligible air resistance, what is the speed of the rock just before hitting the ground?

i asked this question somewhere else, but i still don't understand how to do it,

the person told me to "Make assumptions like negligible air resistance, starting velocity is zero.

Combining and rearranging the equations:"

Ek = 1/2mv^2

Eg = mg(delta)h

but i do not know where to put what in.

the only thing i did was i draw the diagram, i have no idea what else to do.

can you please explain how to do this.

thank you

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You can do it one of two ways, the first with kinematics, the second by conservation of energy. I would assume no air resistance to make things simpler.

If you choose the kinematics path, the following resources may help:

Free Fall Video

Free Fall Tutorial

If you choose energy, check these out:

Conservation of Energy Video

Conservation of Energy Tutorial

Good luck!

(If you go with Kinematics, and choose down as the positive direction:



a=9.8 m/s^2

Solve for v_f using vf^2=vi^2+2ad)

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