Netwon's second Law
Summary
Newton's second law says the net force on an object equals its mass times its acceleration (Fnet = ma). Acceleration points along the net force and is smaller for larger mass. Testable point: zero net force means zero acceleration (constant velocity), not zero motion.
Detail
Meaning: The vector sum of all forces on an object sets its acceleration. The acceleration points in the same direction as the net force and is inversely proportional to mass, so doubling the net force doubles a, and doubling the mass halves it. If the net force is zero, the object is at rest or moves at constant velocity. The law applies separately to each axis, which is why you resolve forces into components.
Equation: Fnet = ma, where Fnet is the net force in newtons (N = kg·m/s²), m is mass in kg, and a is acceleration in m/s². The more general form is Fnet = Δp/Δt, where p = mv is momentum in kg·m/s and t is time in s.
On the MCAT: Expect a free-body diagram, then a sum of forces along each axis to solve for a or an unknown force. Typical setups are blocks on inclines with friction, pulleys, elevators (apparent weight), and circular motion, where the net inward force equals mv²/r. Graphs of a versus Fnet are lines with slope 1/m.
Common confusion: Students think a net force is needed to keep an object moving, or that velocity points along the net force. Net force determines acceleration, not velocity. A crate sliding at constant speed has zero net force because the push balances friction. Also, weight (mg) is one force in the sum, not the net force.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- University Physics (Young and Freedman)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.