Question

A rock is thrown horizontally off a 5m high building. It lands 20m away. What is the initial speed of the rock.

I messed up on calculating time, which in turn, messed up the rest of the calculations.

Answer 1

Solution :

Lets first calculate time of flight .

We are given

displacement in vertical direction = y = 5m .

acceleration in vertical direction = g = 9.8 m/s² .

Initial velocity in vertical direction = u_y = 0

Using

putting known values

......................................................

Now lets look at horizontal direction

In horizontal direction there in no acceleration so

displacement in horizontal direction = d = u_xt

given d = 20 m

So

20 = u_x 1

So   .................Answer ...Initial speed of rock