Cracking the SAT Physics Subject Test

THE SECOND LAW

Newton’s second law predicts what will happen when a force does act on an
object: The object’s velocity will change and it will accelerate. More precisely, it
says that its acceleration, a, will be directly proportional to the magnitude of the
total—or net—force (Fnet) and inversely proportional to the object’s mass, m.

Fnet = ma

This is the most important equation in mechanics!

The mass of an object is directly related to its weight: The heavier an object is, the
more mass it has. Two identical boxes, one empty and one full, have different
masses. The box that’s full has the greater mass, because it contains more stuff;
more stuff means more mass. Mass is measured in kilograms (kg). (Note: An
object whose mass is 1 kg weighs about 2.2 pounds on the surface of the Earth,
though, as will be discussed later, mass and weight are not the same thing and
should not be confused with each other.) It takes twice as much force to accelerate
a 2 kg object as it takes to accelerate a 1 kg object the same amount. Mass
measures an object’s inertia—its resistance to acceleration.

The Skinny on the Second Law

This law defines force. The second law relates the acceleration an object

of a certain mass experiences when a force is applied to it. The larger the

force on the object, the larger its acceleration. It’s like the difference

between pulling a wagon filled with heavy packages alone and having a

friend help you pull. The wagon pulled by your joint force has a greater

acceleration.

Fnet is the sum of all the forces acting on an object. Beware, there can be

forces acting on an object without causing a net acceleration. This

happens when the forces cancel each other out—that is Fnet = 0 N.

Forces are represented by vectors; they have magnitude and direction. If several
different forces act on an object simultaneously, then the net force, Fnet, is the

vector sum of all these forces. (The phrase resultant force is also used to mean net