Both kinetics and kinematics are areas of study in physics that deal with the motion of an object, but the difference between them is that only one also addresses the causes of that motion. Together, they help a physicist to understand both the "what" and the "why" questions related to moving things.
TL;DR (Too Long; Didn't Read)
Kinetics is the study of forces that cause motion while kinematics is a mathematical description of motion that doesn't refer to forces.
Other notable differences between kinetics and kinematics include:
- Kinematics doesn't regard the mass of any object in the system to describe its motion, whereas kinetics does.
- Kinematics can be considered a branch of mathematics. Essentially, it deals with applying a set of equations of motion to solve various physics problems.
- Kinetics deals with the laws of motion while kinematics deals with the equations of motion.
The branch of physics known as kinetics deals specifically with analyzing the forces acting on an object (or, when dealing with rotational motion, their analogue: torques). As such, a physicist studying kinetics draws on Newton's laws of motion to study how objects move.
While studying kinetics, a physicist will typically draw force diagrams (also known as free-body diagrams) and use vector math to find values such as net force and the direction of acceleration.
Kinetics falls into the sub-category of classical mechanics known as forces.
The branch of physics known as kinematics deals with using mathematical descriptions – a set of equations of motion – and definitions to explain how real-world objects move about.
Kinematics therefore requires a strong understanding of mathematical concepts such as vectors, scalars and vector addition, as well as physics measurements like velocity, speed, distance, displacement and acceleration.
High school physicists often study kinematics in one dimension (linear motion, like an apple falling from a tree) or in two dimensions (such as projectile motion, like a cannonball leaving the cannon at a diagonal and making an arc in the sky). For motion in two dimensions, they will need to separate a problem into two parts: one for the object's vertical motion and one for its horizontal motion.
Kinematics falls into the sub-category of classical mechanics known as motion.
Example of Kinetics vs. Kinematics
Consider a classic projectile motion situation: a baseball player throwing a ball through the air. A physicist analyzing the ball's motion with kinematics would calculate factors such as final velocity, time in the air and final position.
A kinetics view of the same thing might be to determine how Newton's second law applies to the ball by quantifying the net force on the ball from the player's throw and gravity.
How Kinetics and Kinematics Fit Into Classical Mechanics
Another way to analyze the same scenario of a baseball's motion through the air would be to apply an understanding of conservation of energy in order to explain why the ball eventually stops. This analysis, however, belongs to the sub-category of classical mechanics known as energy.
Altogether, the three main categories of study in classical mechanics are forces, motion and energy.
Why It Matters
Both kinetics and kinematics are important branches of classical mechanics. They allow physicists to understand the nature of motion in different ways and to calculate different values depending on what they are studying.
In this way, the two fields complement each other. Kinematics might answer more of the "what" questions that specifically describe the motion of an object: its velocity, acceleration, position, time and the like.
But without kinetics, physicists wouldn't also be able to answer the "why" questions, such as what caused the object to begin moving in the first place, and why doesn't that motion continue forever? Where does the acceleration pulling a thrown ball back to the Earth come from? To answer all of these questions, a physicist needs equations and a solid understanding of forces.