"Tauno Voipio" wrote in message
...
Matt Whiting wrote:
Robert M. Gary wrote:

Remember that F=ma. There is no force without acceleration. There is no
acceleration without force.

OK, Einstein, please explain where the acceleration is when two equal
forces are opposing each other diametrically.


The point is that there is *no* net forcce then.

Right. Matt is drawing the distinction between force(s) and net force.

Consider two objects sitting motionless on your floor. The first is being
pushed in opposite directions with the same amount of force. The second is
just sitting there.

In both cases, there is no net force and no acceleration. In the first case,
however, there are actual forces (even though there is no *net* force). In
other words, in the first case, the sum of the *absolute values* of the
forces upon the object is nonzero, even though the sum of the forces is
zero. In the second case, even the sum of the absolute values of the forces
is zero.

But in both cases, there is not only zero net acceleration, but also zero
accelerations. That is, there are no actual accelerations taking place--no
actual changes of the object's velocity (that's what acceleration is)--whose
absolute values have a nonzero sum. (Although 5-5=0, it would not be correct
to say that every motionless object has both a +5 and a -5 acceleration; we
have to distinguish the numbers in our equations from the actual physical
events they designate.)

Thus, in the first case, there are actual forces upon the object, but no
actual accelerations of the object (that is, no actual changes of the
object's velocity). Robert's claim that force "is really acceleration" isn't
correct. Forces cause (or prevent) accelerations, but it's important to
distinguish a cause from its effect.

This might all be pedantic in the context of aviation, except that (as
pointed out earlier) it's actually important for pilots to understand the
difference between Va (which limits acceleration) and Vno (which limits
force) in order to understand why Va changes with the plane's weight,
whereas Vno doesn't.

--Gary