Help with physics

[midzt]

I was wondering how I would find the force from the speed and the mass. I think the speed would be constant.

This is in the situation of a crash test. Where the car is 780kg and it is impacting at 12m/s.

Thanks for any help, its for my AS coursework.

[silent ben]

Kenetic energy is

0.5 x mass x (velocity squared)

So that gives you joules transfered to the wall. I'm missing a very important equation, ut i cant remember what it is

[fondie]

The force of what?

Assuming no friction or other losses the force required to propel the car is 0.

If you are meaning the force of the wall on the car or vice versa when it impacts, then you can't work that out without more information.

[RoGuE|SaBeR]

Only thing you can work out with those values is momentum where momentum is the product of mass and speed.

momentum = 780 x 12 = 9360 kgm/s

[midzt]

The force of what?

Assuming no friction or other losses the force required to propel the car is 0.

If you are meaning the force of the wall on the car or vice versa when it impacts, then you can't work that out without more information.

What other information would I need?

[GreenPiggy]

Presumably you are after the force the car hits the wall with. To find that you have to know how long it takes for the car to crumple and come to a stop from the moment of collision.
You would than use the initial speed of the car to find out the acceleration and use Force = mass x acceleration to work out the force.

[Spud1]

Force = mass x acceleration

you have the mass, and can work out a

a=change in v/ change in time

meh just written that and realised its what greenPiggy said

[fondie]

What other information would I need?

As GreenPiggy said, if you have the total time taken until the car came to rest, you will be able to work out the mean force on the car.

However, as the force exerted constantly changes throughout the impact, if, for example, you wished to know the maximum force exerted between car and wall you would require the maximum rate of decceleration during the impact. I don't know the scope of your question though.

[mark22]

Basically the equation you need is force is proportional to rate of change of momentum.

Or

F.t = m(u1-u2)

So as said already you need the time to bring the car to a stop and I suppose there's no damage to the wall as it's an As question.

(I'm not going to go in to the real equation behind PrivatePyles answer, it's basically right though )

How I miss my physics degree. (don't do it)

[midzt]

Thanks for the help everyone!

How I miss my physics degree. (don't do it)

I'm thinkin of doing automotive engineering

[Twigman]

Physics is cool
Mechanics is also cool.

Ft is impulse I think they call it. Impulse = change in momentum.

[mark22]

You are correct.

And yes physics is cool, but fourier transforms and solid state physics of semiconductors and all the other stuff now in my brain is really uneccesary.

[Twigman]

correct about the coolness of proper science? (biology is not a science...go away)
or about the impulse thing?
or both?

Fav lectures atm: Enigneering Physics

[GreenPiggy]

Fourier transforms are definitely the devil's work. Ditto for statistical & thermal physics
And i like the subject enough to teach it.

[mark22]

Correct about both.

Oh yeah all that thermal physics and statistical stuff was a bit of a chore. Not that bad if you can get your head around it though, long enough to pass that course anyway.

I liked astrophysics as a course. Lots of cool slides and professor was on that space program on tv with the guy from jurassic park and thought he was a bit special.

Only thing I wish I'd gone on to do for no other reason than understanding it is general relativity. Special relativity wasn't the great subject I was hoping it to be.

[Lee H]

wahooooooooooooooooooooooooooosh

You know what that was ? My brain going on vacation as soon as the words "relative" and funny formulae appeared..

If all you Sci-fi fans wanna imagine my face at the moment after reading this thread just imagine a scene out of Stargate SG-1 where Sams discussing generators in geeky talk