A boy throws a stone horizontally from the top of a cliff of height h = 50.0 m towards the
ocean below. The stone strikes the water at a distance d = 60.0 m from the base of the cliff.

The time at which the stone lands in the water is 3.19s. Calculate the initial speed of the stone?

I would have said it's zero because the stone is at rest before it leaves the boys hand. But then I found this similar question online and they do a calculation and find that the initial speed isn't zero. Can someone explain what the question is actually asking?

Question

A boy throws a stone horizontally from the top of a cliff of height h = 50.0 m towards the
ocean below. The stone strikes the water at a distance d = 60.0 m from the base of the cliff. 

The time at which the stone lands in the water is 3.19s. Calculate the initial speed of the stone?

I would have said it's zero because the stone is at rest before it leaves the boys hand. But then I found this similar question online and they do a calculation and find that the initial speed isn't zero. Can someone explain what the question is actually asking?

anonymous · Answer

http://www.ux1.eiu.edu/~cfadd/1350/Hmwk/Ch04/Ch4.html Go to the very end of the page and then scroll up to question 4.2 and its answers.

anonymous · Answer

The height doesn't matter if you are only looking for the horizontal part of the initial speed (the speed when it left the boys hand). So it went 60 meters in 3.19 seconds.
Velocity (speed) equals distance divided by time = 60 meters / 3.19 seconds = 18.8 m/s

The problem on the page you posted has a distance of 80 meters, so the answer there is different.

anonymous · Answer

But is that not the speed of the stone when it hit the water?

anonymous · Answer

You asked about the initial speed. The speed in the horizontal direction remains constant until it hits the water. The speed in the vertical direction would accelerate because of gravity. So the total speed would be faster when it hits the water. I'm not sure if this answers your follow-up question.

anonymous · Answer

So how would you calculate the speed of the stone just before it hits the water?

anonymous · Answer

Look at part C of the example you referred to. We already figured out the x component of the velocity. Part C shows how to calculate the y component of the velocity as it hits the ground.

anonymous · Answer

I attached a picture of the velocity right as it hits the water. The diagonal line is the total velocity. The x and y components make up part of a right triangle. So you can figure out the total velocity by the Pythagorian theorem, which is the equation on the drawing.

anonymous · Answer

Ok thanks. Physics is not my strong point at all. :)