Any car any model

[Oldboy913]

Can someone tell me?
If you have a car travelling at 40 miles per hour (relative to the road surface) which part of the car is travelling at 80 miles per hour?

[jenky]

If you followed a single point on the surface of one of the tyres, it would be travelling at more than 40mph, relative to the center of the tyre, wouldn't it....?

[squarebear]

I suppose the top of the tyres would be moving at twice the speed of the car.

The very bottom of the tyre would be stationary as it is stuck to the road. The center would be going the same speed as the car so I would imagine the top of the tyre would be going twice as fast.

[Oldboy913]

Well done Squarebear I'll give you A+

[ACtheTROLL]

or is it all relative(ity)???

so then (still on the front tyre) ... the lower rear quadrant is travelling slower than the car

aaaaaagghhhhh ..... let me offff

[plowter]

Sorry squarebear, you'll need to run that explanation past me again.

At a point in time the bottom of the tyre is stationary, the centre is moving forward at 40mph and the top is moving forward at 80mph.

So after one hour the bottom of the tyre remains where it was, the centre has moved forward by 40 miles and the top is now eighty miles away. Kaboom.

Doesn't the diameter of the wheels come into this? I hate physics.

[squarebear]

lol that's not quite what I mean.

The top of the tyre is changing all the time. Imagine it rotating as the car moves along, the very top of the tyre will be moving much faster than the rest of it.

The point where the bottom of the tyre meets the road (the grip) is always stationary, otherwise the car would skid all over.

Does that make sense?

[plowter]

Thanks squarebear, think you knew I was jesting.

I'm terrible at physics. What I don't understand is how you know that the outer part of the wheel moves at exactly twice the speed of the hub on any vehicle.

What happens if the wheel is double the size?

[squarebear]

It could be the size of a dustbin lid and would still be twice as fast.

The centre of a wheel always moves at the same speed as the car 40 mph, otherwise it would drop off the axle. The bottom of the tyre is always gripping the road and moving at 0 mph.

If the bottom is 0 and the middle is 40 then the top must be 80, no matter how big the wheel is. I think....

(I'm confusing myself now lol)

[plowter]

Right. That's the physics I don't get.

If the bottom is stationary, the centre is moving at 40mph and the top at 80mph then surely that only applies if the wheel is revolving round the point of contact on the road?

My thinking is that the centre is staionary and that the outer edge of the tyre (top and bottom) both rotate at the same speed because they're the same distance from the centre. How fast the outer edge goes depends on how far they are from the centre.

Think this is where pies and and cicumradidiameters come in. Can I sue my old school because I can't remember this stuff?

[squarebear]

Imagine the car wasn't moving but was hoisted up while it reached 40mph.

At this point, the centre of the wheel would be 0mph, the bottom would be -40mph (40mph in reverse) and the top would be 40mph as the wheel spins round in mid-air.

However, when the car is moving at 40mph, you have to add 40 on to each of the figures, so the bottom becomes 0mph (-40+40), the centre (the hub) becomes 40mph (0+40) and the top becomes 80mph (40+40).

Maybe we should just catch the bus instead :-)

[plowter]

That makes much more sense now.

Other issues I can't quite get my head round include :
- why don't you suffocate if you stick your head out of a car window at 60mph?
- if you jump into the air while travelling inside a train compartment why don't you smack into the rear wall at 100mph?
- if a plane is carrying a load of birds, is it lighter if all the birds inside fly at the same time?

I've got a headache from all that thinking you've done. Cheers mate.

[squarebear]

lol me too. I think I'm going to lie down for a bit.

Catch you later bud.

[Oldboy913]

Thanks for your input, Its more obvious if you look at a tank track there you can see that each link on the top run has to go twice as fast to get back to the front again to do its job again.

[ACtheTROLL]

so if the car crashed in a forest with no-one to hear it ... would the cat die?