Combining and rearranging an equation.

[TheTurbonator]

I have to find the speed of which a diver hits the water after jumping from a platform that is 4.7 m in height. It can be assumed that g = 9.8 m s^-1

This is how I've done it:

Since it is assumed, no friction is involved: DELTAEg = Ek

So, mgDELTAh = 1/2mv^2

Which I've rearranged into:
Square root v = gDELTAh
The square root covers the whole of the equation.

Doing the calculation gives me 9.6 m s^-1 which sounds about right but just wanted to make sure.

If I'm right great, but if not, please don't just give me the answer but instead just point me in the right direction. I don't want to get accused of cheating, by the University.

[hairykrishna]

You've lost the 1/2 - should be 2gh.

You've also messed up the square root (although I suspect that may be a typo). You're taking the square root of both sides so v^2 becomes v and the other side is all enclosed in a square root.

[ewenm]

GCSE mechanics: v^2 = u^2 + 2as

u = initial speed = 0
a = acceleration = 9.8
s = distance = 4.7

[hairykrishna]

ewenm said:

GCSE mechanics: v^2 = u^2 + 2as

u = initial speed = 0
a = acceleration = 9.8
s = distance = 4.7

That's effectively what he's derived, or attempted to derive at least.

[ewenm]

hairykrishna said:

That's effectively what he's derived, or attempted to derive at least.

I should hope so too! Same answer whether you approach from conservation of energy or Newtonian mechanics equations (as the two are equivalent).

[anonymous-user]

As a matter of interest, in which course/year/Uni has this question arisen?

Edited by anonymous-user on Monday 22 April 14:34

[TheTurbonator]

hairykrishna said:

You've lost the 1/2 - should be 2gh.

You've also messed up the square root (although I suspect that may be a typo). You're taking the square root of both sides so v^2 becomes v and the other side is all enclosed in a square root.

I cancelled the 1/2 out by multiplying both sides by 2 as:

1/2 mv^2 is the same as mv^2/2.

And cheers, I see what you mean about the square root.

Edit:

Yes sorry I typo'ed the equation I got

What I got is:

v=square root 2(gDELTAh)

Edited by TheTurbonator on Monday 22 April 14:37

[hairykrishna]

TheTurbonator said:

I cancelled the 1/2 out by multiplying both sides by 2 as:

1/2 mv^2 is the same as mv^2/2.

And cheers, I see what you mean about the square root.

Well yes, 1/2mv^2 is just the same as writing (mv^2)/2. I don't see your point.

In your OP the two is missing "Square root v = gDELTAh" - where's the the two gone? It's either a 1/2 on the V side or 2 on the other.

Edited by hairykrishna on Monday 22 April 14:39

[TheTurbonator]

hairykrishna said:

Well yes, 1/2mv^2 is just the same as writing (mv^2)/2. I don't see your point.

In your OP the two is missing "Square root v = gDELTAh" - where's the the two gone? It's either a 2 on the V^2 side or 1/2 on the other.

Yes sorry I typo'ed the equation I got

What I got is:

v=square root 2(gDELTAh)

[hairykrishna]

Ok, makes sense.

Do I remember from another thread that you're starting a physics degree?

[TheTurbonator]

hairykrishna said:

Ok, makes sense.

Do I remember from another thread that you're starting a physics degree?

Yeah just started Thanks for the help, nice feeling when you get something right.

[hairykrishna]

Good luck. I occasionally teach physics undergrads at Birmingham .

It's not clear from the thread whether you genuinely weren't sure about this or if you were just seeking reassurance on an early assignment. If it's the former I'd consider some maths tuition - you'll be struggling soon other wise. I see lots of good students in trouble because their A level maths teaching was crappy.

[TheTurbonator]

hairykrishna said:

Good luck. I occasionally teach physics undergrads at Birmingham .

It's not clear from the thread whether you genuinely weren't sure about this or if you were just seeking reassurance on an early assignment. If it's the former I'd consider some maths tuition - you'll be struggling soon other wise. I see lots of good students in trouble because their A level maths teaching was crappy.

I was just seeking reassurance, everything seemed to fit but I just wanted to make sure. I have actually got a level 1 math module to complete, before I move onto level 2 stuff, which should make sure I'm up to speed, before I tackle the harder stuff.