> And not because, oh, I don't know, it's 100x smaller mass has 
> significantly lower inertia to overcome in the first place?

That's only important once you get up to a high enough speed that you can't 
overcome friction (about 10-20 mph in road cars, 60 mph in fast cars, up to 
100 mph in racing cars).  Before that coefficient of friction is the only 
thing that matters.

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scott wrote:
>> And not because, oh, I don't know, it's 100x smaller mass has 
>> significantly lower inertia to overcome in the first place?
> 
> That's only important once you get up to a high enough speed that you 
> can't overcome friction (about 10-20 mph in road cars, 60 mph in fast 
> cars, up to 100 mph in racing cars).  Before that coefficient of 
> friction is the only thing that matters.

My point being that a smaller mass requires a smaller force to 
accelerate it, and thus less traction.

A cheetah's legs don't need to generate 500 BPH to get it from 0 to 60 
in 4 seconds. Because a cheetah doesn't weigh a quarter of a ton...

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> My point being that a smaller mass requires a smaller force to accelerate 
> it, and thus less traction.

Actually exactly the same traction.  If you accelerate an object at 10 m/s^2 
then it will need a coeffcient of friction of about 1.0 no matter what the 
mass is.  This is the point I was trying to make.

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scott wrote:
>> My point being that a smaller mass requires a smaller force to 
>> accelerate it, and thus less traction.
> 
> Actually exactly the same traction.  If you accelerate an object at 10 
> m/s^2 then it will need a coeffcient of friction of about 1.0 no matter 
> what the mass is.  This is the point I was trying to make.

So a force of 1N and a force of 1,000N both require the same amount of 
traction?

How does *that* work?!

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> So a force of 1N and a force of 1,000N both require the same amount of 
> traction?
>
> How does *that* work?!

If the 1000 N force is on an object weighing 1000 kg, and the 1 N force is 
on an object weighting 1 kg, then yes, they both require the same amount of 
friction and will result in the same acceleration.

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scott wrote:
>> So a force of 1N and a force of 1,000N both require the same amount of 
>> traction?
>>
>> How does *that* work?!
> 
> If the 1000 N force is on an object weighing 1000 kg, and the 1 N force 
> is on an object weighting 1 kg, then yes, they both require the same 
> amount of friction and will result in the same acceleration.

This must truly be the definition of "counter-intuitive". I cannot begin 
to imagine how this can be true. It is surely self-evident that applying 
a large force requires better grip than does applying a small force.

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>> If the 1000 N force is on an object weighing 1000 kg, and the 1 N force 
>> is on an object weighting 1 kg, then yes, they both require the same 
>> amount of friction and will result in the same acceleration.
>
> This must truly be the definition of "counter-intuitive". I cannot begin 
> to imagine how this can be true. It is surely self-evident that applying a 
> large force requires better grip than does applying a small force.

Not if the contact force is proportionally higher.

Press your thumb and finger together very lightly and see how much sideways 
force you need to slide them across each other.  Now press them together 
much harder and you'll find you need a much bigger sideways force to 
overcome the friction.

It's the same with a 1000 kg car and a 10 kg animal, because the car is 
pushing down on the road 100 times more than the animal it can generate 100 
times more forward force before slipping.

http://en.wikipedia.org/wiki/Friction

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>>> If the 1000 N force is on an object weighing 1000 kg, and the 1 N 
>>> force is on an object weighting 1 kg, then yes, they both require the 
>>> same amount of friction and will result in the same acceleration.
>>
>> This must truly be the definition of "counter-intuitive". I cannot 
>> begin to imagine how this can be true. It is surely self-evident that 
>> applying a large force requires better grip than does applying a small 
>> force.
> 
> Not if the contact force is proportionally higher.

I had assumed that the "amount of friction" already takes into account 
the contact force, along with the surface roughness, deformability and 
all the other things that affect friction.

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Stephen wrote:
> 
> Do you call that an argument? LOL

No, I call that a partly definition.

> From my memory of mechanics at school, hp was more an advertising unit ;)
> Bloody farmers couldn’t tell an erg from an egg :-)

Hp can be misleading, since it can mean lots of different hp's. It can
be either SAE or DIN and from the crankshaft or from the tires etc.

-Aero

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scott wrote:
>> So a force of 1N and a force of 1,000N both require the same amount of 
>> traction?
>>
>> How does *that* work?!
> 
> If the 1000 N force is on an object weighing 1000 kg, and the 1 N force 
> is on an object weighting 1 kg, then yes, they both require the same 
> amount of friction and will result in the same acceleration.

In other news, both feathers and hammers fall at the same speed in airless 
conditions.

-- 
Darren New, San Diego CA, USA (PST)
   Forget "focus follows mouse." When do
   I get "focus follows gaze"?

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