Summary
We have established a pattern of identifying all the forces, making a clear "free body diagram", and applying Newton's Second Law,
F = m a
For bodies at rest -- we say they are in equilibrium -- we know the acceleration. It is zero. If
a = 0
then we know
F = 0
While we may write simply F, remember this is the net force -- the vector sum of all the forces acting on the mass m.
Remember, tho', force is a vector. So saying
F = 0
really means
Fx = 0
and
Fy = 0
Remember, it is really the net force we are using in Newton's Second Law, so these really mean
F = 0
Since this is a vector equation, it really means
and
Of course, an object does not need to be at rest to apply Newton's Second Law,
Whenever we apply F = m a, remember the force F is a vector and the force F is the net force -- the vector sum of all the forces -- acting on the particular mass m.
So while we may write or say
F = m a
this really means
and since this is a vector equation it really means
and
Return to ToC, Ch 6, Application of Newton's Laws (c) Doug Davis, 2005; all rights reserved