## Summary

We have established a pattern of identifying all the forces, making a clear "free body diagram", and applying Newton's Second Law,

F= maFor bodies at rest -- we say they are in equilibrium -- we know the acceleration

. It is zero.If

a = 0then we know

F = 0While we may write simply

F, remember this is thenet force-- the vector sum of all the forces acting on the mass m.Remember, tho', force is a vector. So saying

F = 0really means

F_{x}= 0and

F_{y}= 0Remember, it is really the

net forcewe are using in Newton's Second Law, so these really mean

F = 0Since this is a

vector equation, it really means

F_{x}= 0

and

F_{y}= 0Of course, an object does not need to be at rest to apply Newton's Second Law,

Whenever we apply

F= ma, remember the forceFis avectorand the forceFis thenet force-- the vector sum of all the forces -- acting on the particular mass m.So while we may write or say

F= mathis really means

F= maand since this is a vector equation it really means

F_{x}= m a_{x}and

F_{y}= m a_{y}

Return to ToC, Ch 6, Application of Newton's Laws(c) Doug Davis, 2005; all rights reserved