Alice drops a rock down from a 40 meter tall building (valice, i = 0) at the same time that bob (lying on the ground) throws a rock straight upwards from the ground with vbob, i = 20 m/s. at what height do the rocks pass each other?

Rocks will pass each other at 20.4m.

Explanation:

Alice drops the rock from (y is in the vertical axis) at .

Bob throws the rock from at .

is gravity's acceleration.

Position equation for alice's rock:

Position equation for Bob's rock:

We we'll first find the time at which the rocks meet. For this, we will equalize Bob's and Alice's equations:

⇒ ⇒

So, we can take and just put it in any of the two laws of motion to see at what height the rocks meet. We will take Alice's equation (using g=9.8m/s):

Rocks will pass each other at 20.4m.

the answer would be 32*f