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I think you all need to get out more

oyster wrote:

mrjon wrote:If a cargo plane can carry the weight of 1000 pigeons, can it carry 2000 pigeons provided you keep 1000 of them flying around the cabin?

Ahh, now I did that one in a test! The mass (weight) of the birds is always in the truck. If the birds all took off within the truck, their downward force via the wings is still being supported by the truck. Result, truck's load is constant.

If "flying" is applying a downward force then how come formula 1 cars are said to have "downforce" when their wings are upside down?

Also, let's say that I was in the container, holding a brick. Then I launched the brick. While the brick is in "flight" will its weight be registered on the scales?

Not all aerial manoeuvres are upwards.

oyster wrote:While on a trip, I wondered about the velocity of a departing bullet from a moving vehicle. Does the bullet gain the forward speed of the vehicle if fired forward, thus making it faster than if fired from a stationary position? Similarly, does the bullet loose some speed if fired from the back of the vehicle in the opposite direction to travel? I believe there is some relative time/place issues, but I do not know enough to develop the maths. Does any of this really matter? No. But I have to keep the grey matter churning.

if two vectors have the same (or opposite) direction, then the answer is trivial, as HerrFlick states: you add or subtract them.
But what if they are at an angle to each other? This is a bit trickier.
I made a paint sketch of this:
https://scontent-bru.xx.fbcdn.net/hphot ... e=559B427F

A boat crosses a river. Boat speed is 3 m/s (red arrow). The current speed is 4 m/s (green arrow). So what is the resulting speed and what direction does the boat have? It's illustrated with the grey arrow in my sketch. The speed is going to be 5 m/s

In physics, everything that has a magnitude and direction is a vector. Velocity, acceleration, force, torque etc.

it's quite interesting to apply vectors to a moving wheel for instance.

If a tree falls in a forest and nobody is there to see it does anyone give a s**t?

A butterfly can stop a train...

Butterfly flys towards train travelling in the opposite direction. Collides with train windscreen so decelerates rapidly and, obviously, what's left of it begins to travel in the same direction as the train. It reverses direction, therefore must have stopped, relative to the ground, at some point in time. The insect is attached to the train from the point of impact. Therefore, for a split second, the train has also stopped

Mike

Boxermed69 wrote:A butterfly can stop a train...

Butterfly flys towards train travelling in the opposite direction. Collides with train windscreen so decelerates rapidly and, obviously, what's left of it begins to travel in the same direction as the train. It reverses direction, therefore must have stopped, relative to the ground, at some point in time. The insect is attached to the train from the point of impact. Therefore, for a split second, the train has also stopped

Mike

Ok then. I've thought of one of that ilk much more relevant.

Piston in yer boxer going up at a hell of a rate of knots towards tdc. Gets there and then reverses direction at a rate of knots. So at some point, allegedly, it's speed must have been zero. So it is stopped completely.

By a similar logic to your butterfly thang, Since the piston is attached to the crankshaft the crankshaft must have stopped frozen for an instant. But then the crankshaft is moving the bike, so the bike must have stopped dead. You are sat on the bike.....

Should I open another beer or call it quits?

Corvus wrote:

Boxermed69 wrote:A butterfly can stop a train...

Butterfly flys towards train travelling in the opposite direction. Collides with train windscreen so decelerates rapidly and, obviously, what's left of it begins to travel in the same direction as the train. It reverses direction, therefore must have stopped, relative to the ground, at some point in time. The insect is attached to the train from the point of impact. Therefore, for a split second, the train has also stopped

Mike

Ok then. I've thought of one of that ilk much more relevant.

Piston in yer boxer going up at a hell of a rate of knots towards tdc. Gets there and then reverses direction at a rate of knots. So at some point, allegedly, it's speed must have been zero. So it is stopped completely.

By a similar logic to your butterfly thang, Since the piston is attached to the crankshaft the crankshaft must have stopped frozen for an instant. But then the crankshaft is moving the bike, so the bike must have stopped dead. You are sat on the bike.....

Should I open another beer or call it quits?

Does it follow then that everything is a series of stopped situations all relative to time?

Corvus wrote: By a similar logic to your butterfly thang, Since the piston is attached to the crankshaft the crankshaft must have stopped frozen for an instant. But then the crankshaft is moving the bike, so the bike must have stopped dead. You are sat on the bike.....

Should I open another beer or call it quits?

Your logic is flawed. While the linear path of the pistons reverse directions at TDC. and can be said to be stationary for and instant, the rotation of the crankshaft will continue unaffected.


Just to be sure I am now going to perform a small experiment by wheeling the bike out in this beautiful sunshine and riding it until it applies a smile vector of the greatest magnitude.!!

Merecat wrote:

Corvus wrote: By a similar logic to your butterfly thang, Since the piston is attached to the crankshaft the crankshaft must have stopped frozen for an instant. But then the crankshaft is moving the bike, so the bike must have stopped dead. You are sat on the bike.....

Should I open another beer or call it quits?

Your logic is flawed. While the linear path of the pistons reverse directions at TDC. and can be said to be stationary for and instant, the rotation of the crankshaft will continue unaffected.


Just to be sure I am now going to perform a small experiment by wheeling the bike out in this beautiful sunshine and riding it until it applies a smile vector of the greatest magnitude.!!

Hee Hee, good man, doing some field testing. It's a tough job but.....

Yes I'm sure it is flawed, but it was never really meant to be serious. No more serious than the idea of a butterfly stopping a train anyway.

But having said that, to quote something you said above " stationary for an instant". Ok, how small is an instant? It does seem "obvious" that if something is traveling in one direction and then travels in the opposite then it must be at zero speed for some time. With respect to a crank, conrod and piston surely that implies that there must be a sector of the circle which equates to zero speed for the piston. If there's time involved there must be a sector, surely? No matter how small the time value, the sector has to be there. But zero speed for the piston only happens when all the points line up, with a line that has no width (by definition of what a line is), so no sector of the circle. Therefore no time can be involved. If there isn't any time involved, for a sector to be described, then the piston can't actually stop. Conundrum or what.

Merecat wrote:
......... the rotation of the crankshaft will continue unaffected.

...

Not entirely unaffected. I refer you to the phenomenon of inertial torque.

On a rolling wheel, in the point of contact with tarmac, the wheel has 0 speed.
The forward moving vector, and the rotational vector cancel each other out.

Tapio wrote:On a rolling wheel, in the point of contact with tarmac, the wheel has 0 speed.
The forward moving vector, and the rotational vector cancel each other out.

I realise you mean in a theoretical sense. In real life, given that tyres rely on friction and therefore there must be a small (although sometimes large!) amount of slip between the tyre and road, the linear (horizontal) speed of the centre of the rear wheel won't actually be what it theoretically should be for the circumferential speed of the tyre at rolling radius. Does this affect the zero value of the vector at road contact?

To put it another way, in a burn out situation (as in warming up tyres before a sprint) is the vector zero?

Tapio wrote:On a rolling wheel, in the point of contact with tarmac, the wheel has 0 speed.
The forward moving vector, and the rotational vector cancel each other out.

We can try to apply this principle to a tyre on the road but it fails on lots of levels. Deformation and slip spoil it. A billiard ball on a flat measuring table is very close to perfection, but even that will have tiny anomalies.

The principle seems to hinge on a straight line and a point. Perfect geometry. But yet again I seem to reach the same conundrum. Because a line, by definition, has zero width then what portion of the circle has a zero vector value in contact with the perfectly horizontal surface? Zero portion! So all of the circle must experience some vector value, no matter how minuscule!

This could drive you nuts. No wonder the ancient Greeks wandered around in their dressing gowns all day!