I don't know what a "Newton force" is, but the metric ("La Systeme
Internationale" or "MKS") unit of force is the Newton, which is one
kilogram meter per second squared.
"Net" force usually means the "total" force, or, the way I like to
say it, the vector sum of all forces on an object. If the sum of all
forces on an object is zero, that means if you consider just one of the
forces on an object, the sum of all of the other forces has to be equal
in magnitude and opposite in direction to the one you are thinking
about.
A good example is the car example you bring up. If a car is parked
on a flat road (like the ones here in Illinois), friction holds it in
place. If there are no external forces pushing horizontally, then the
friction force will vanish. Vertical forces balance out between gravity
and what we call the "normal force", a contact force of the road on the
tires which holds the car up.
Iowa has rolling hills, and other states have genuinely steep
roads in them. If you park a car on a hill, the friction force of the
road on the tires has to balance out the component of gravity which
points down the hill. Consult any mechanics textbook to see this worked
out. You can imagine other forces here, like normal and friction forces
from wheel chocks or a tow-truck chain, but if the sum adds to zero,
each force opposes the sum of the rest.
Tom
(published on 10/22/2007)
