| Chronology | Current Month | Current Thread | Current Date |
| [Year List] [Month List (current year)] | [Date Index] [Thread Index] | [Thread Prev] [Thread Next] | [Date Prev] [Date Next] |
On Mon, 14 Apr 1997, A. R. Marlow wrote:
Scenario: I am sitting peacefully on a bench when the driver in a
parked
car down the street suddenly starts the car and manages 0 to 60 mph in
20
sec. Other than being a little disturbed by the noise, I feel nothing
whatever, even though a quick calculation shows my acceleration
relative to the car to be well in excess of 1 m/s^2.
Where I fear I must be misinterpreting: Are we to say that a force
acts on me because of the acceleration I have relative to the car?
Look at *your* accelerometer. It will tell you that you are accelerating
upward at approximately 9.8 m/s^2 just as it did when the car was parked.
Nothing has changed. As I said repeatedly, it is acceleration **relative
to local inertial (i.e. freely falling) frames** that matters. The only
significant forces acting on you before and after the car started moving
are the support forces from the ground and your bench which, together,
produce approximately 9.8 N of upward force for every kg of your body
mass.
N.B. Since most people apparently can't stand the thought of taking
accelerometer readings at face value, you may want to *interpret* your
accelerometer reading as an artifact of a local gravitational field of
9.8
N/kg downward which brings you into equilibrium with the upward forces of
support.