Re: [Phys-l] check your work (and kinetic energy)

[John Denker <jsd@av8n.com>]

On 01/07/2012 08:08 PM, curtis osterhoudt wrote:
>  how is the ground NOT doing work on the car to slow it down if the brakes are applied?

The book carefully and IMHO correctly defines work by attaching
a string to the object and quantifying the force *on the string*
and the dx of motion *of the string* i.e. the motion of the
external agent that pulls on the string.  (For simplicity let's 
assume the force and the motion are collinear.)

Let's consider the case of a car initially at rest, which then
starts up and drives away.  (This is operationally slightly 
simpler than the braking scenario, and illustrates the same
physics.)

Wrap a string around the tire, on each wheel that drives the
car.

When the car starts up, there is a force on each string, but
the dx is zero.

This is consistent with our notion of energy and energy flow.
When the car starts up, no energy crosses the boundary of
the object (i.e. the car).

Note the contrast:
 ++ There is necessarily momentum flowing across the boundary.
 -- There is ordinarily little if any energy flowing across
  the boundary.  The energy to get the car moving comes
  from *within* the system.  For details, see
      http://www.av8n.com/physics/car-go.htm


If you want to talk about _pseudowork_, that's something
else.  It involves the force applied to the center of mass
and the motion of the center of mass.  Pseudowork is a fine
physics idea, but it's not the same as work.  The chapter 
didn't define pseudowork and the question didn't ask about 
pseudowork.