Newton's laws
Summary
Newton's laws are three rules of classical mechanics: velocity stays constant unless a net force acts (first), Fnet = ma (second), and forces come in equal, opposite pairs on different objects (third). Most testable: acceleration depends on the net force, not on any single force.
Detail
Meaning: First law (inertia): an object's velocity stays constant, whether at rest or moving in a straight line at steady speed, unless a nonzero net force acts on it. Second law: the net force sets the acceleration, which points the same way as the net force and is smaller for a larger mass. Third law: if A exerts a force on B, then B exerts a force of equal magnitude and opposite direction on A.
Equation: Fnet = ma, where Fnet is the vector sum of all forces on the object (N, with 1 N = 1 kg·m/s²), m is mass (kg), and a is acceleration (m/s²). Third law: FAB = −FBA, with both forces in N.
On the MCAT: Expect free-body diagrams for blocks on inclines, pulleys, connected masses, elevators (apparent weight) and circular motion, where the net force toward the center is mv²/r. Equilibrium problems set Fnet = 0, which means constant velocity, not necessarily rest. You may need to resolve forces into components and predict how acceleration changes when mass or net force changes.
Common confusion: Third-law pairs do not cancel because they act on different objects. The normal force on a book resting on a table is not the third-law partner of its weight. The partner of the weight is the book's gravitational pull on Earth. Also, constant velocity needs no net force, so motion alone does not imply a net force.
Sources
- Physics (Giancoli)
- University Physics (Young and Freedman)
- Fundamentals of Physics (Halliday, Resnick and Walker)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.