h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Hello,

I remember from my 'O' Level Physics the equation F = ma

Force = mass * acceleration

If a train weighs 1,000 Kg then Mass = 1,000 kg
If the train travels at a constant 100 metres per second then accelartion = 0 metres per second.

1,000 * 0 = 0

Therefore Force = 0

So if I step in front the train it will hit me with zero force and therefore it can't hurt me. This is obviously rubbish, but in light of the equation, why?

The train is being slowed down all the time by the effects of gravity and air resistance and other friction. The train's engines are putting in force to overcome that. That's acceleration.

TRiG.

TRiG, yeah but that's not why the train will kill you if it hits you!

The reason is that the train has kinetic energy ((1/2)mv^2) and a part of that gets transferred to you when it hits you. This energy gets transferred to you via an impulse rather than a force, but an impulse is just like a very large force for a very short period of time.

The force that hits *you* certainly won't be zero

You'd go from 0-100 m/s pdq (ok, slightly less, but about 98m/s or so).

So you'd get hit with a force equivalent to (if you weigh 60kg)

F=ma
60 * 100

6000 erm, Newtons?

In fact, I think you have to divide that by the fraction of the second it took you to get up to speed. If it took 1/10th of a second for you to accelerate up to speed, then it would be 60,000 Newtons.

Which is a lot..

Yes, what he said ^

The force to which you refer in the original post is the force required to keep the train moving at a constant speed (assuming no friction, air resistance etc).

Unless you weigh the same as the train, then it's unlikely you will encounter no force.

B

The force being applied TO THE TRAIN is zero, if it's not accelerating.

As pointed out above - in reality, if you really didn't apply any force to the train, nature would apply its own forces in the form of friction and air resistance, and the train would DEcelerate. So you need to apply a force to counteract that to maintain a constant speed. But the RESULTANT force on the train is zero if its speed is constant.

However... you're not interested in the force on the train. You're interested in the force applying to YOU. You could, if you could be bothered, calculate how much hitting you would slow the train down (not much). You could, if you could be bothered, calculate how far you would compress before you began accelerating (a small but fatal amount). For the sake of simplicity, however, let's assume you weigh 100kg and you accelerate from a standing start to 100mph in 0.1s. That's a fast acceleration, times 100kg. In other words, the force on you would be pretty huge.

Don't play on the railway.

StevieD67


Remember mass does not equal weight.

Something has a mass in kg but has a weight in Newtons as weight is a combination of mass and the force on it(normally gravity).

Something with a mass of 10kg on Earth will have a mass of 10kg on the moon but will weigh a 6th of the weight due to the force of gravity on the moon being a 6th that on Earth.

Hence why one is 'weightless' in space as there is no (appreciable)force acting up on you.

Picky, I know but this is h2g2 and this is the SEx forum!

Hence, you just have to say...'A train has a mass of 1000kg'

Yeah, you see a lot of those space-trains about...

B

"Hence why one is 'weightless' in space as there is no (appreciable)force acting up on you"



There is very much a force acting on you in orbit. It's called gravity. The reason you appear weightless is that you are falling freely in that gravity field. But there's a force acting on you alright.

I think that's where the "appreciable" comes in - to you the experiencer of it.

It's "appreciable", because if I was in the ISS, I'd very much appreciate being able to see the earth every day, instead of seeing it receding rapidly into the distance.

I know but I couldn't "appreciate" or feel that gravity was acting on me in any way directly - only by observing that I'm not shooting off into space.

Big Bad Johnny P, that is what I meant. Ta.

Twiglet

<"Hence why one is 'weightless' in space as there is no (appreciable)force acting up on you"



There is very much a force acting on you in orbit. It's called gravity. The reason you appear weightless is that you are falling freely in that gravity field. But there's a force acting on you alright.>

I didn;t say in orbit I said 'in space' and hence why I put no (appreciable) force...had I put 'no' force I thought someone might pick me up on it!

Hmm.

Thing is, in space, whether in orbit or not, wherever you are, even in interstellar space, you're only "weightless" because you and everything around you are moving at the same speed, falling freely towards whatever the nearest gravity well is. That could be a galaxy a million light years away, or it could be a black hole 1000 miles away (in which case your freefall is going to be quite short). Freefall is independent of the force magnitude.

##Force = mass * acceleration

If a train weighs 1,000 Kg then Mass = 1,000 kg
If the train travels at a constant 100 metres per second then accelartion = 0 metres per second.

1,000 * 0 = 0

Therefore Force = 0##

the moment the train hits, the force is 0, then the train, with a 1000kg mass(and a lot of momentum) accelerates you from a stand still to 100m/s that is where the force comes in, the instant AFTER impact

Twiglet


Correct, it doesn;t totally negate the example of the difference between weight and mass which is what I was trying to do.

Nerdy joke told by the 17year old daughter of my boss at the weekend:

"A Higgs Boson walks into a church. The priest sees him and shouts: 'Hey, we don't want you in here.

'Why not?' asks the Boson,

'Because you call yourself the God Particle and we don't like that,' replies the Father.

'Well, that's as maybe,' retorts the Boson, 'but without me you would have no mass.'

*BOOM*

BOOM! That is going on the List! I LOVE it.

(Side note: is she fit?)

Is she fit?...How could I possibly pass judgement on my boss' daughter?

Your Schrodinger's Hat comment last week went down very well too!

You should be proud of yourself what ever age you are...