# 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

Fx = 0

and

Fy = 0

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

F = m a

and since this is a vector equation it really means

Fx = m ax

and

Fy = m ay

 Atwoods Machines Homework Return to ToC, Ch 6, Application of Newton's Laws