Question

A girl standing on a bridge throws a stone vertically downward with an initial velocity of 15.0 m/s into the river below. If the stone hits the water 2.00 seconds later, what is the height of the bridge above the water?
Dont give the answer, just tell me how to do it pls

Answer 1

Well first, do you know the displacement equation in physics?

Answer 2

yes :)

Answer 3

one of newton's kinematics expressions. delta y = v(initial)x(time) + 1/2at^2; where a = gravity

Answer 4

Almost - it's s = v0 t + (1/2) a t^2. the other guy forgot a t

Answer 5

ninjaedit :)

Answer 6

So
s = v0 t + (1/2) a t^2.

You know t. You know a - that's -9.81 m/s/s, the acceleration of gravity near earth's surface. You know v0 -- that's 15. Now just solve for s.

Answer 7

you guys are amazing :D thank u

Answer 8

Wouldn't v0=-15 (downwards!)

Answer 9

ah yes, sorry. the sign of v0 should be the same as the sign for 'a'.

Answer 10

okay :)

Answer 11

s=10.38?

Answer 12

i did not get that answer

Answer 13

10 does not look right, 2*15m gives 30m.

Answer 14

Oh, you've got a problem with the sign.
Both v0 and a are negative!

Answer 15

s=-15(2)+(1/2)(-9.81)(2^2)
s=-30+(1/2)(-9.81)(4)
s=-30+(1/2)(-39.24)
s=-30+(-19.62)
s=10.38

Answer 16

Check the following sequence:
s=-30+(-19.62)
s=10.38

Answer 17

the very last step is wrong. sign confusion.

Answer 18

ur right. its supposed to be -49.62

Answer 19

that's what i got

Answer 20

but the negative doesnt make sense since its the height above water

Answer 21

that's the distance from the starting point (from the bridge) to the water, with up being positive and down being negative. So the water is 49.62 feet below the bridge.

Answer 22

oh ok :) sorry. my brain is fried today