Maths / Physics Masters of OCUK

[dfarrall]

I have a question thats really puzzling me,

A chav falls from a roof and receives injuries which (a doctor states) are consistent with an impact at 30 m/s.

What comparison would the doctor use as the basis for the claim that the impact occurred at 30m/s?

I have literally no idea how to work this out, obviously terminal velocity would be more than this so i doubt its that.

If the chav did hit the ground at 30m/s, how far had he fallen?

Obviously the equation is v^2 = u^2 +2as but im not sure how to use it to find out as you dont seem to have all the needed variables?

(V = Final Velocity, U = Initial velocity, A = Acceleration, S = Displacement

Any explanations very welcome as i'v been puzzled by this for weeks now.

 

[RomanNose]

I have a question thats really puzzling me,


I have literally no idea how to work this out, obviously terminal velocity would be more than this so i doubt its that.


Obviously the equation is v^2 = u^2 +2as but im not sure how to use it to find out as you dont seem to have all the needed variables?

(V = Final Velocity, U = Initial velocity, A = Acceleration, S = Displacement

Any explanations very welcome as i'v been puzzled by this for weeks now.

You will need to know the aerodynamic profile of the chav.
It's more complex than this.

 

[hp01jpc]

For the second part:

u = 0 (presuming he started off stationary)
v = 30 (his velocity on impact)
a = 9.81ms-2
s = ?

I think

 

[SpeedFreak]

v^2=u^2+2as
v=30
u=0
a=9.81

 

[hp01jpc]

That first bit is strange. What comparison would the doctor use? Well common sense would dictate that he would be comparing the injuries to other impacts at a similar velocity! But this is not exactly mathematical/physics reasoning so I'm not sure what the question is getting at.

 

[dfarrall]

Were does the acceleration variable come from? Is it a standard figure or something you worked out

That first bit is strange. What comparison would the doctor use? Well common sense would dictate that he would be comparing the injuries to other impacts at a similar velocity! But this is not exactly mathematical/physics reasoning so I'm not sure what the question is getting at.

My thoughts exactly.

 

[oldbag]

I feel really thick.

 

[whitecrook]

gravity

 

[Judgeneo]

standard for earths gravity at low altitude

 

[dfarrall]

Oh god of course, cant believe i hadn't worked that one out sooner.

Im going to say the first one is probably not scientific (unless somebody has a better idea!) so thanks OCUK (Y)

 

[memyselfandi]

At the top the chav's energy is purely potential
E=mgh
At the bottom the chav's energy is purely kinetic
E=0.5mv^2

hence:
v^2=2gh

If you know the high the chav fell from you know the velocity and vice versa.

(Note very rough as doesn't take into account wind resistance etc but you get the idea)

 

[Vonhelmet]

Acceleration due to gravity. Roughly 9.81ms^-2 at or near ground level.

 

[Vonhelmet]

At the top the chav's energy is purely potential
E=mgh
At the bottom the chav's energy is purely kinetic
E=0.5mv^2

hence:
v^2=2gh

If you know the high the chav fell from you know the velocity.

(Note very rough as doesn't take into account wind resistance etc but you get the idea)

And v^2 = 2gh is the same as v^2 = 2as where a = g and s = h.

The circle is complete.

 

[SpeedFreak]

That first bit is strange. What comparison would the doctor use? Well common sense would dictate that he would be comparing the injuries to other impacts at a similar velocity! But this is not exactly mathematical/physics reasoning so I'm not sure what the question is getting at.

The height would determine the injuries

Chav fell from 46 meters btw so in this case I'm thinking dead as being the condition.

 

[Judgeneo]

Tempted to write out the answer using Hamiltonian mechanics, then realised I couldn't be bothered.

 

[dfarrall]

Tempted to write out the answer using Hamiltonian mechanics, then realised I couldn't be bothered.

Just googled that and im still not sure what it is..

 

[Judgeneo]

Just googled that and im still not sure what it is..

Its very complicated for simple situations like this, but much more efficient that standard approaches for complicated situations

 

[Temi_D]

Am I the only one that started laughing at: "A chav falls from a roof..."?

 

[Macky]

gravity

+1

 

[dfarrall]

Am I the only one that started laughing at: "A chav falls from a roof..."?

I edited it to make it slightly more interesting, wasnt the actual question