On 04/10/2010 23:07, Dave wrote:
On 04/10/2010 22:11, dennis@home wrote:


"Rick Cavallaro" wrote in message
...
On Oct 4, 12:28 pm, "dennis@home" wrote:

have you worked out why it takes four times as long to stop a car
that is
going twice as fast yet.
You can use either formula to find out as the constant doesn't matter.

Asked an answered. You claimed it takes 4 times as long to stop a car
that's going 20 mph vs. one that's going 10 mph. My answer: no it
doesn't. A normal car will take twice as long.

And there we have the proof you don't understand energy.

For anyone that cares..

the brakes use friction to turn kinetic energy to heat
they have a maximum rate at which they can do this..
so as a car travelling twice as fast has four times the energy it takes
four times as long to dissipate the heat in the brakes.
So it takes four times as long to stop.
It really is that simple.. rick doesn't have a clue, which is why he
can't answer the energy questions.

This brings to mind a conversation with my real ale pub landlord about
the chevrons that mark some motorways that declare 'keep 2 chevrons
apart'. To my mind, they are too close together, but he maintains that
any modern car will stop within that distance as all the cars are
slowing at the same rate.

He's right, they're not the full stopping distance apart, and they won't
work if something suddenly stops faster than it normally can.