Question for the resident boffins.....

**marty** · 28th February 2006, 11:42

Originally Posted by Skyryder

No ones talking about cornering. If you travel in a straight line, lateral momentum is going to alter the gravity forces on the object. Lets look at this from another angle.

Say you are the passanger in a car travelling at 50kph and you drop a lead weight of 50 kg's out of the window onto the road. Now lets say from the point frome where the weight left your hand to the point of where it landed on the road is 50 meters.

Now lets say the second time you did this you were travelling twice the speed 100kph and you dropped the 50kg weight out of the window. All things being the same as the first experiment the distance travelled would be double. Same weight same gravity forces double the speed and double the distance.

Conversely the opposite would happen. Travel at 50kph drop a weight of 100kg and that would be 50% shorter than the first experiment or thereabouts. In all cases both weight and graity forces are the same but the differential in distance is governed by speed.

The same applies to mass when it is at velocity. Gravity is unable to exert the same amount of force to a moving object as it can to a stationary object and as such if the weight can be measured it will 'appear' to be lighter..

Its why rockets fly and planes glide. One uses wings the other sheer speed to keep it in the air. When the speed slows the rocket falls to the ground.

Skyryder

mm, gravity is constant at sea level, and always acts vertically down. it doesn't matter if the item is moving or not. a rocket needs some upward thrust component, or small wings (faster it goes the smaller the wings it needs). show me a non-winged, non vectored thrust cylindrical rocket that flies horizonatally please. and anyway, when a plane slows, it will also fall to the ground. it doesn't get any lighter or heavier just cause it's flying. it still weighs the same - it's just that the vertical component of lift is greater than the opposing vertical component of weight. and on your 50kg vs 100kg lead block analagy, apparantly fat guys will freefall out of a plane faster than skinny guys.

you need to start backing up some of your comments with researched facts.

**MisterD** · 28th February 2006, 11:51

Originally Posted by marty

mm, gravity is constant at sea level, and always acts vertically down.

Rather the direction of gravity defines "down"

Originally Posted by marty

it doesn't matter if the item is moving or not. a rocket needs some upward thrust component, or small wings (faster it goes the smaller the wings it needs). show me a non-winged, non vectored thrust cylindrical rocket that flies horizonatally please.

You're contradicting yourself Marty, the faster it goes the smaller wings it needs....to the point where if it goes fast enough it doesn't need them at all.

It would be difficult to do, but in theory you could travel fast enough that the acceleration due to gravity would just maintain altitude. In effect a very low orbit. I think this was calculated earlier in the thread.

**scumdog** · 28th February 2006, 12:09

Originally Posted by MisterD

It would be difficult to do, but in theory you could travel fast enough that the acceleration due to gravity would just maintain altitude. In effect a very low orbit. I think this was calculated earlier in the thread.

It would require some form of upwards thrust to counter-act the pull of gravity.

Even a bullet drops as soon as it leaves the barrel (If the barrel is horizontal)

**Jamezo** · 28th February 2006, 12:11

Originally Posted by MisterD

Rather the direction of gravity defines "down"

You're contradicting yourself Marty, the faster it goes the smaller wings it needs....to the point where if it goes fast enough it doesn't need them at all.

It would be difficult to do, but in theory you could travel fast enough that the acceleration due to gravity would just maintain altitude. In effect a very low orbit. I think this was calculated earlier in the thread.

The 'wing size/air speed' relationship is a matter of aerodynamics, most certainly different to a hypothetical atmospheric orbit. marty's comments hold perfectly true.

As for the fat/skinny guys skydiving, this phenomenon is explained by the nature of air resistance: The rate at which a falling object reaches its terminal velocity is related to the ratio of the weight force (gravity) to the drag force (air resistance). At low speeds, the fat person has a higher net downwards force, and a very similar drag force to the skinny person, giving them a better ratio, and a slight edge on the initial acceleration.

As terminal velocity is approached, the overriding factor will be the proportionality of weight force to the drag coefficient. I'm not an expert on human aerodynamics, so I have little idea how body shapes of given weights correlate to drag coefficients.

**MSTRS:Me** · 28th February 2006, 12:31

Originally Posted by scumdog

Ah, but how do you KNOW that your accelerating and not just keeping the same speed and the revs go up to compensate for the 'reduction in tyre diameter??

The wear on the trailing edge of the tread pattern on the rear tyre tells me I am

Originally Posted by scumdog

And by your theory if I stop braking while going around corners I would run off the INSIDE of the bend???

If you can manage a corner without braking (scary thought, I know) then you will have your answer

**scumdog** · 28th February 2006, 12:36

O.K. is it engine torque that makes it easier to go around left hand bends than right hand bends??

And MSTRS, do you know the feathering occured when you went around bends or just when you hit the straight again?? After all the feathering that was not in the centre of the tyre would have occured as the bike was going slower on the smaller diameter shoulder part of the tyre.

**TwoSeven** · 28th February 2006, 12:36

Originally Posted by Jamezo

If you were referring to 'does a motorcycle (or indeed any wheeled contrivance) weigh less as it travels rapidly around the curvature of the earth', then, yes, the answer is yes.

Ok, just for kicks, I shall figure out exactly how substantial this effect might be (the answer, which is rather obvious, is 'not very')

Let us compare the accelerations of the normal gravity force, and the 'driving around the earth very fast' acceleration:

A(G) = 9.81 ms-2, fairly standard

A(D) = v^2 / r, bog standard rotational motion formula

We can take the radius as 6,378m, the equitorial radius of the Earth.

Let us take the speed, for example, as 50ms^-1, or 180km/h.

So the acceleration towards the centre of the earth caused by a motorcycle travelling over the curvature of the earth at sea level and 180km/h can be calculated as:

I dont believe you have used the right formula for your statements that went with it. But then I am not familiar really with that formula.

My question is that you stated that the motorcycle is accelerating towards the centre of the earth, but in your formula you give the value of 'r' as being the same. For something to accelerate towards the centre of something 'r' must get smaller.

As I understood newtons law, the wieght can only change if the distance from the centre of the earth changes, or another gravitational body pulls on it. So maybe it might weigh less on a full moon or something

**MisterD** · 28th February 2006, 13:10

Originally Posted by Jamezo

The 'wing size/air speed' relationship is a matter of aerodynamics, most certainly different to a hypothetical atmospheric orbit. marty's comments hold perfectly true.

Thought experiment: Remove gravity, then get a rocket and fire it horizontally. It would travel at a tangent to the earth's surface and therefore appear to climb. Now reinstate gravity, the balance between it's pull towards the earth and the rockets velocity determines whether the curved path the rocket follows hits the earth, runs parallel to it (orbit) or spirals outwards.

**MSTRS:Me** · 28th February 2006, 14:20

Originally Posted by scumdog

O.K. is it engine torque that makes it easier to go around left hand bends than right hand bends??

Not torque - tis the camber of the road (serious answers from me dooooo happen)

Originally Posted by scumdog

And MSTRS, do you know the feathering occured when you went around bends or just when you hit the straight again?? After all the feathering that was not in the centre of the tyre would have occured as the bike was going slower on the smaller diameter shoulder part of the tyre.

Feathering is towards the edge so has to happen when leaning over, acceleration has to be happening because the wear is on the trailing edge. Heavy braking will cause the leading edge to wear (on the rear tyre and assuming you don't lockup/lowside)).

**marty** · 28th February 2006, 15:10

i give up.

**TwoSeven** · 28th February 2006, 16:47

Originally Posted by Hitcher

Does the light in the fridge go out when you shut the door?

You'd have to write a formula to prove which door you are shutting and if it is connected to the fridge. For example, if I shut the front door to the house, the status of the fridge light does not change.

Also, inertia on the fridge door [assuming the requirement is narrowed to determine which door is being talked about] would be important. It could possibly bounce back open again, in which case the state of light offedness would be very short.

**thealmightytaco** · 28th February 2006, 16:52

You're all nuts. Centripetal force acts equally up down everywhere for a fully enclosed rotating object like a wheel so nothin' gonna happen there, and if anyone here can notice the say...

160kg*0.000392ms^2= 63 gram!!

...63 gram change in the weight of their bike when doing 180k around the surface of the earth, I'd be very surprised.

**Virago** · 28th February 2006, 17:50

Originally Posted by Hitcher

Does the light in the fridge go out when you shut the door?

(Muffled shout) - YES!

**Ixion** · 28th February 2006, 17:54

Define "out". Do you have proof that it was ever "not out". Define "fridge" Define "door". Define "shut" . Do you have proof that the door was ever "not shut". If the "light" "goes out", where does it go to.

EDIT: That damned cat isn't in there is it? Dead or alive.