kinematics
Summary
Kinematics describes motion using displacement, velocity, acceleration and time, without considering the forces that cause it. Most testable: the standard equations, such as v = v₀ + at, apply when acceleration is constant, as in free fall or projectile motion.
Detail
Meaning: Kinematics is the description of motion using displacement, velocity, acceleration and time, without asking what forces cause it. Displacement, velocity and acceleration are vectors, so direction and sign matter.
Equation: v = v₀ + at; Δx = v₀t + ½at²; v² = v₀² + 2aΔx, valid when acceleration is constant. Here v and v₀ are final and initial velocity (m/s), a is acceleration (m/s²), t is time (s), and Δx is displacement (m).
On the MCAT: Passages describe a dropped object, a thrown ball or a braking car and ask for a time, distance or final speed. Near Earth's surface, free-fall acceleration is g = 9.8 m/s² downward. On graphs, the slope of position versus time is velocity, the slope of velocity versus time is acceleration, and the area under velocity versus time is displacement. With negligible air resistance, a projectile's horizontal velocity is constant while its vertical motion has a = −g, and the two components share the same time.
Common confusion: Zero velocity does not mean zero acceleration. At the top of a thrown ball's path, vertical velocity is zero but acceleration is still g downward. The equations do not apply if acceleration changes.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- University Physics (Young, Freedman)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.