Fizzicks: a question inspired by the TT

It's the change in *your* speed (accelerating from near stationary to near the bike's speed in a fraction of a second) that will hurt a bit.

Of course the bike will also decelerate as it hits you (conservation of momentum).

Since your change in speed is very rapid and your mass not zero, the force acting on you will be large and painful.

NT

Cursitor Doom was thinking very hard :

force = mass x (velocity / time) = (mass x velocity) / time = momentum / time

F = dp/dt = m*dv/dt = ma

The thing which hits you will change its speed. Even if it's a train it will do - possibly by an undetectable amount, but it will do. Therefore there is a force, which will be the thing which hurts you.

The acceleration you need to worry about is yours - you will go from 0 to significant proportion of the bikes speed, over a very short distance... The distance basically taken up in the deformation of the hard heavy bike, and the squashy thing it hits (you can make a fair guestimate on which will get most bent out of shape!)

So lets say you do that acceleration over a 10cm distance... Lets guess your terminal speed (no pun intended!) is 60 m/sec, we just need to work out the acceleration.

We know v^2 = u^2 + 2as, so we get

So if your mass is 90kg, that would mean the bike will need to push you with a force of 90 x 18000 = 1620 kN to get you up to speed in that 10cm space.

About as much force as being placed under a 160 tonne weight - its going to be messy!

Yerwot? I don't understand the question. Remember energy is never destroyed just changed, so I'm afraid you would be very dead. Brian

The bike speed *will* change, by the law of conservation of momentum...

This is implied in the use of the word 'hit' which implies contact.

Furthermore, it will be an inelastic collision, which means that some fraction of the kinetic energy of the bike & rider will be used to *break* things in addition to speeding them up.

#Paul

Noel Coward was a charmer. As a writer he was brahma. Velvet, jackets and pyjamas, had a gay divorce and other dramas.

There ain't half been some clever bastards (Lucky bleeders, lucky bleeders) There ain't half been some clever bas-tards.

Van Gough did some eyeball pleasers. He must have been a pencil squeezer. He didn't do the Mona Lisa, That was some Italian geezer.

There ain't half been some clever bastards (Lucky bleeders, lucky bleeders) There ain't half been some clever bas-tards.

Einstein can't be classed as witless. He claimed atoms were the littlest. When you did a bit of splitting-em-ness Frighten everybody shitless

There ain't half been some clever bastards. Probably got help from their mum (who had help from her mum). There ain't half been some clever bastards. Now that we've had some, let's hope that there's lots more to come.

There ain't half been some clever bastards (Lucky bleeders, lucky bleeders) There ain't half been some clever bastards.

- Ian Dury

... applied to 1-3 small areas of your body.

NT

Correct. Well, strictly speaking you will change its velocity. In the case of the bike, you are likely to change its direction slightly.

There's the same force on you *and* the bike.

How do these recoil-less guns work, then? They appear to defy the laws of fizzicks as I misunderstand them.

From what I have read some vent gases to the rear, with the vented gases counterbalancing the recoil, while others use movement of a piston by the gases to do the same.

SteveW

Something goes in the opposite direction to the bullet. If you can't see it it'll be a gas.

Most are rocket launcher style tubes... so the force for accelerating the projectile is not coupled to the "gun"

enough gas is ejected backwards to counteract the bullet going forward

FTFY

Pictures or it doesn't brmmmm happen!

Heat will be generated too from an impact.

heat is random kinetic energy of molecules..