Question

A rock dropped from ad cliff hits the ground 2.5 seconds later. Another rock thrown horizontally from the same cliff lands 20 m from the bad of the cliff. What was the thrown rock's initial speed?

Answer 1

The horizontal and vertical motions of the rock are independent.
The second rock is thrown horizontally - thus its falling down vertically will be equivalent to the first rock. Since in both cases, the rocks have Zero initial horizontal velocity when released.
Right?

Answer 2

Yes exactly sir

Answer 3

Thus both the rocks will hot the ground in the SAME amount of time. Right?

Answer 4

Yes sir that is true

Answer 5

The second rock will thus hit the ground after 2.5 seconds.

Answer 6

Yes since they will fall at the same time right?

Answer 7

Right. So now can we find the horizontal speed of the second rock? It falls 20 mts away from the cliff edge. In 2.5 seconds.

Answer 8

But sir how would you find the length of the cliff?

Answer 9

Don't you need that?

Answer 10

We dont need to know that right? We KNOW that:
1. In 2.5 seconds the second rock travelled 20 mts in the horizontal direction;
2. The force of Gravity acts only in the vertical direction - there is NO FORCE in the horizontal direction.

Answer 11

Oh ok sir so then you would just substitute 20 for d if you were using d=vt?

Answer 12

Yes. Exactly. That will give you "v" - the horizontal velocity.

Answer 13

Thank you so much sir

Answer 14

Welcome!
So v=8 m/s - just to confirm.

Answer 15

Yes sir

Answer 16

Thank you sir