Subject: Does Newton's Second Law apply to all forces?

I am having this discussion with a friend who says that F=m*a only applies 
to accelerated bodies, hence force occurs. He feels that there are all 
sorts of forces out there which do not involve acceleration. I say that if 
it is a force, mass is accelerated, just as the formula states. He further 
insists that formulas like work, power or torque are all uniquely 
different and F=m*a does not apply.  I tell him that its a NATURAL LAW and 
applies to all forces.  Furthermore, any formula that describes some sort 
force must prove to Newton's Laws. In addition to this minor controversey, 
as forces go I assume that F=m*a also applies to the three forces gravity, 
electoweak force and strong nuclear. I know that in cases of relativity 
Einstein states that Newton is wrong about space and time being absolute, 
but isn't old Issac pretty much correct on everything else?


Thank you for your quick response and assessment that all natural forces must
follow Newton's Second Law. As for what example that better explains my
friends viewpoint, please read the following.

First my friend insists that the force used in Newton's Second Law is
peculiar only to bodies at acceleration, and that this formula does not apply
to any other type of force. He believes that forces can exist without
acceleration. He further states that if an applied force does not move an
object, there is no acceleration observed.  I agree that if an object does
not move from any directional force applied it certainly does not accelerate
in that direction. That's because the applied force was not strong enough.
Nevertheless that force still needs to be addressed. He says it's just a
force, there is no acceleration in that force. When asked what makes up this
force, if doesn't show an acceleration, he proclaims that its Work!

To further illustrate, if an applied force of 100 ft-lbs./sec/sec is used to
pick up a 200-lb. object, there is no acceleration of that object because the
force is less than the object's weight.  Nevertheless there still is a force
being applied. If the object was 50 lbs., then it would move at an acceleration 
of 2 ft/sec/sec.  All we did here was change the mass.

Another way to look at this same example is as a summary of forces.  F1 is
100 ft-lbs./sec/sec in direction Y and F2 is 200 ft-lbs./sec/sec in direction
 -Y (or a body at rest). When each opposing force is added together, the net
force is in favor of F2, hence the object's position remains unchanged. To be
more specific F2 has a greater force than F1.

In each case there is an applied force, even though the object did not move.
This force as Newton's Second Law translates, is a product of a mass and at
acceleration. I believe my friend would argue with me that you couldn't use
F2 in a summary force example. What do you think?

Larry Spinello

Response:

Re: Does Newton's Second Law apply to all forces?
I am having this discussion with a friend who says that F=m*a only applies 
to accelerated bodies, hence force occurs. He feels that there are all 
sorts of forces out there which do not involve acceleration. I say that if 
it is a force, mass is accelerated, just as the formula states. He further 
insists that formulas like work, power or torque are all uniquely 
different and F=m*a does not apply.  I tell him that its a NATURAL LAW and 
applies to all forces.  Furthermore, any formula that describes some sort 
force must prove to Newton's Laws. In addition to this minor controversey, 
as forces go I assume that F=m*a also applies to the three forces gravity, 
electoweak force and strong nuclear. I know that in cases of relativity 
Einstein states that Newton is wrong about space and time being absolute, 
but isn't old Issac pretty much correct on everything else?


_______________________________________________________________
Yes, Sir Isaac's ideas are correct, except, as you
said, when it comes to relativistic situations.

Can you come up with a specific example of what your
friend is trying to point out?  It seems that, for 
instance, when you are standing on the ground you
are not accelerated.  True that you do not actually
obtain a velocity, but that's because there is an
opposing force to the gravity (the ground holds you
up!), so the *net* force is zero.  If the ground
disappeared you would accelerate.

	John Link, Admin
 MadSci Network
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Re: Does Newton's Second Law apply to all forces?