# Physics calculation

Crowley
Posers and Puzzles 27 Sep '05 14:01
1. Crowley
Not Aleister
27 Sep '05 14:012 edits
Hi guys, I need to do a calculation, but I can't find anything on the web to help me out.

2. PBE6
Bananarama
27 Sep '05 17:501 edit
Originally posted by Crowley
Hi guys, I need to do a calculation, but I can't find anything on the web to help me out.

Sounds like you need a few equations:

1. Conservation of momentum:

m1*v1 + m2*v2 = m1*u1 + m2*u2

Where: m1, m2 are the masses of the objects; v1, v2 are the velocities before the collision; and u1, u2 are the velocities after the collision; and

2. Momentum/energy relation for an elastic collision:

0.5*m*(v^2) = dp/dt

Where: m is the mass of the object, v is the velocity, and dp/dt is the change in momentum over time.

What is the question exactly?
3. AThousandYoung
All My Soldiers...
27 Sep '05 20:231 edit
Originally posted by Crowley
Hi guys, I need to do a calculation, but I can't find anything on the web to help me out.

OK. It was accident.
A motorbike hit a pedestrian, I want to know how fast the bastard was going.

I have the weight of the pedestrian and the bike and I have the displacement of the pedestrian.

What formula can I use?

The kinds of collision problems that are dealt with in low level physics are perfectly inelastic and elastic collisions. Your example is neither; it's an inelastic collision, but not perfectly inelastic. You need to know that kinetic energy is not conserved. However unlike a perfectly inelastic collision, the two bodies do not stick together and act like one body.

What you do know is that momentum is conserved. So, if you determine what the momentum of the pedestrian was, and you know the mass of the motorcycle and the rider including gas in the tank, you can figure out the velocity of the motorcycle. How would you do that though?

If you can determine the distance the person skidded on the concrete and the coefficient of friction between person and concrete, I think you can determine how fast the person was going when they hit.

It's a pretty complex problem and more information is needed to solve it.
4. Ragnorak