Draw a proper free-body diagram.
Resolve vectors into their components.
For each axis, set up an expression for F (^) net , and set it equal to ma .
Solve your system of equations.
Note the marked similarity of this method to that discussed in the chapter on equilibrium.
Following these steps will get you majority credit on an AP free-response problem even if you do not
ultimately get the correct answer. In fact, even if you only get through the first one or two steps, it is likely
that you will still get some credit.
Only Net Force Equals ma
THIS IS REALLY IMPORTANT. Only F (^) net can be set equal to ma . You cannot set any old force equal
to ma . For example, let’s say that you have a block of mass m sitting on a table. The force of gravity, mg ,
acts down on the block. But that does not mean that you can say, “F = mg , so the acceleration of the block
is g , or about 10 m/s^2 .” Because we know that the block isn’t falling! Instead, we know that the table
exerts a normal force on the block that is equal in magnitude but opposite in direction to the force exerted
by gravity. So the NET force acting on the block is 0. You can say “F (^) net = 0, so the block is not
accelerating.”
A Simple Example
A block of mass m = 2 kg is pushed along a frictionless surface. The force pushing the block has a
magnitude of 5 N and is directed at θ = 30° below the horizontal. What is the block’s acceleration?We follow our four-step process. First, draw a proper free-body diagram.