Physics question.

[tony1851]

Nothing earth-shattering, but just a quick question for anyone knowledgeable in physics:
If you are on a bus moving at - say - 30 mph and you get up off your seat to walk to the front of the bus, does your walking action slow the bus down to a degree; or conversley does it take a fraction more engine power to keep at the same speed?
Thanks.

 

[EFLImpudence]

I suppose it must to a miniscule amount.

Like when Top Gear calculated the increased fuel consumption if you had a moustache.

 

[Nozzle]

Think of the problem in terms of energy rather than forces -

For you to slow down the engine you need to make the engine do more work. That would only happen if you were somehow attached to something outside the bus, or if you suddenly got heavier.

My answer is no.

Though what I'm struggling to come to a conclusion on is..

Imagine a large military weapon on a train wagon being pulled by a locomotive. The artilery piece fires and the recoil is going to cause an opposite action on the locomotive. Is it the same problem, but on a larger magnitude, as you stepping forward? This might be more a problem involving "impulse" rather than speed/kinetic energy.

What if you are on a tread-mill on the bus, and it's doing 30mph. Depending on which way the treadmill is pointing - are you aid or loading the engine? I think no.

Nozzle

 

[Deleted member 63160]

Do the plane on a conveyor belt one. It's gotta be due again and follows on nicely

 

[rsgaz]

And when you stop walking, the tiny amount of energy you took from the bus originally will be 'given back'.

 

[rsgaz]

I worked it out for you...

If the bus is 10 tonnes, at 30 mph, it's kinetic energy is 899301 Joules.

A person weighing 70kg, walking at 5mph has a kinetic energy of 175 Joules. So you have 'taken' 175 Joules from the bus, by moving forward.

So, the bus now only has 899126 Joules, which equates to 29.997 mph! I don't think the other people on board would notice!!

 

[Safety First]

does your walking action slow the bus down to a degree;
Thanks.

Yes, Newtons 3rd law. Your traction on the floor forces your mass forward, and the floor/bus backwards- forces of both being equal. It did seem odd at first given that all the mass (bus+occupants) remains constant and you are not affecting the drag (assuming you are not on an open topped bus with an affro). But then pulling large containers of water/loose materials reminds me that when a hauled mass 'decides' to change direction, you feel it.

or conversley does it take a fraction more engine power to keep at the same speed?
Thanks.

Yes again; they are the same question and answer, just moved to consideration of energy sources.

It might be easier to imagine an exaggerated version; If you were pulling a (broken down?) bus of 100 people forward at 3 mph and they all ran to one end and then back to the opposite, when and how would your towing experience change?

I am wrong almost all the time

 

[EddieM]

@op. Yes to both.

 

[KenGMac]

The person or persons on the bus are tiny in mass compared to the 13 tonne bus, yes one person or several people running forward or back then stopping abruptly will have a tiny effect one way or another.

But if you consider forces outwith the envelope of the bus?

The old [code name] Warthog, the American tank busting aircraft had a very rapid firing Gatling gun type of forward firing.

The early versions had problems when the gun was used, the aircraft started rapidly to stall and fall out of the sky.

In this instance we have a very heavy aircraft, travelling not too fast by modern attack jet standards lets say 500 MPH but when the gun fired as above a stall.

The Americans then placed a cap on the number of rounds [in there thousands per Minuit] that the gun would fire no matter if the pilot had his finger on the trigger in the release position.

Point is that something leaving the aircraft having been propelled the recoil [or a person stopping or starting[ will have an effect in proportion to the mass of what is being moved, a person or several thousand rounds of large caliber ammunition.

It is down to the total mass being moved, either by the bus or the person on the bus.

All mass and movement must have an effect, no matter how small or large.

Ken

 

[Deleted member 63160]

Yes, Newtons 3rd law. Your traction on the floor forces your mass forward, and the floor/bus backwards- forces of both being equal. It did seem odd at first given that all the mass (bus+occupants) remains constant and you are not affecting the drag (assuming you are not on an open topped bus with an affro). But then pulling large containers of water/loose materials reminds me that when a hauled mass 'decides' to change direction, you feel it.






Yes again; they are the same question and answer, just moved to consideration of energy sources.

It might be easier to imagine an exaggerated version; If you were pulling a (broken down?) bus of 100 people forward at 3 mph and they all ran to one end and then back to the opposite, when and how would your towing experience change?

I am wrong almost all the time

Did this in o level physics. Conservation of momentum I think it was called. M1V1 =M2V2 blah blah. Our question was to do with a recoil on a pistol I recall.

 

[Safety First]

The old [code name] Warthog, the American tank busting aircraft had a very rapid firing Gatling gun type of forward firing.

The early versions had problems when the gun was used, the aircraft started rapidly to stall and fall out of the sky.

Ken

This is a myth. The closest it got to reality was the volume of barrel gases entering the Warthog engine caused problems, but I don't think anything fell out of the sky through firing, unless it was a target.

 

[Safety First]

Oh...., and this has something to do with it all, in a way...

 

[Nozzle]

I worked it out for you...

If the bus is 10 tonnes, at 30 mph, it's kinetic energy is 899301 Joules.

A person weighing 70kg, walking at 5mph has a kinetic energy of 175 Joules. So you have 'taken' 175 Joules from the bus, by moving forward.

So, the bus now only has 899126 Joules, which equates to 29.997 mph! I don't think the other people on board would notice!!

I don't buy it. When you move yourself, either along 'static' ground or along the floor of a bus - the energy to do so comes from your food. I write static in quotes as of course the earths surface is moving all the time too. The bus has given you kinetic energy to get you to 30mph, and you're doing the extra leg work to make that a brisk +5mph. The overall kinetic energy of the bus varies, with all the people moving about and their jaws doing 10 to the dozen is not extra energy from the engine, it is from the stored chemical energy within each individual. None of which is given back to the engine when they stop walking/shut the hell up.

Nozzle

 

[EFLImpudence]

Nevertheless, 'tis true.

 

[rsgaz]

the energy to do so comes from your food.

I realise that, I didn't mean you literally use energy from the bus, but you have taken momentum from it, because you are now in motion relative to the bus and you have mass, so it's motion does now have less Joules. The whole system still has the same energy. And then when you stop, your momentum is given back to the bus, and again, the whole system still has the same energy, 899301 Joules, which came from dead dinosaurs of course!!

I have massively over-simplified the physics for it so we could see a ball-park figure of the speed change for the OP.

If a bus hit a person in the road, and the person became stuck to the front of the bus, do you agree that the 'bus+person' is now doing slightly less than 30mph? Assuming no braking at all, etc.