Hi...

Q) A stone is thrown vertically upwards with a speed of 16m/s from a pointhmetres above the ground. The stone hits the ground 4s later. Find the value ofh.

Using suvat equations, how is this done?

Thanks alot

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- Jun 6th 2007, 01:58 PMApexFalling object
Hi...

Q) A stone is thrown vertically upwards with a speed of 16m/s from a point*h*metres above the ground. The stone hits the ground 4s later. Find the value of*h*.

Using suvat equations, how is this done?

Thanks alot - Jun 6th 2007, 02:19 PMJhevon
the SUVAT equation we need here is:

where is the initial velocity, is the final velocity, is the displacement or position, and is the acceleration.

However, since we did not start on the ground, has to be modified by some constant. that is, we will use

where is the height we are when time, , is zero.

now , and

So,

Now, after 4 seconds, the ball hits the ground, so it's height is zero.

So,

So , since that is what is when - Jun 6th 2007, 03:24 PMApex
Thanks...I kept getting something like 13.06(my way). The solution in the paper returns the same answer as you, but it is done like this:

s=ut + 0.5at^2

and

s= ?

u= 16

v= -

a= -9.8

t= 4

What I don't understand, is why if it's done this way (using these values), it actually gives the value of h, as oppose to the distance*from*h, to where v=0, and then to the ground. i.e., the whole journey of the rock minus the distance h.

Can you please explain how this method works?

Thanks

Apex - Jun 6th 2007, 03:51 PMJhevon
think of it this way. we can use instead of .

Now we know that . however, this equation assumes that the displacement is zero when time is zero. (if you plug in we get 0). here that is not the case. when time is zero, we are at a distance above the ground. so we must have:

Now if we plug in , we get the distance , which is what we want.

Now we simply must find . we know that when , so we simply plug those values in to find

i'm not sure if i answered your question. did i? - Jun 6th 2007, 04:12 PMApex
Oh right, I see what you mean.

Thanks very much for your help:)

br

Apex