Describing how an object moves and explaining why it moves that way are genuinely different questions, even though everyday language often blurs them together. Mechanical engineering keeps them formally separate: kinematics describes motion itself — an object's position, velocity, and acceleration over time — without any reference to the forces or masses producing that motion, while kinetics (sometimes called kinetic analysis or dynamics in the narrower sense) analyses the forces and torques that actually cause a given motion to happen.
Kinematics can describe motion with no reference to cause at all
A kinematic description of a mechanism — say, the linkage inside an engine converting rotational motion into linear piston motion — can specify exactly how each part's position, velocity, and acceleration change over time, purely from the mechanism's geometry, without needing any information at all about the forces, masses, or torques actually driving that motion. This is genuinely useful on its own: an engineer designing a mechanism's geometry to achieve a specific desired motion path can work entirely within kinematics, treating the actual forces required to produce that motion as a separate question to be solved afterward, once the geometric motion requirements are settled.
Kinetics adds mass and force back into the picture
Kinetics takes the motion kinematics describes and asks what forces, torques, and masses are actually required to produce it, or conversely, what motion a given set of applied forces will actually produce. This is where Newton's laws of motion come in directly, tying force, mass, and acceleration together. A mechanism that's kinematically well designed — meaning its geometry correctly produces the desired motion path — can still fail on a kinetic level if the forces and torques required to actually drive it through that path at the intended speed turn out to be impractically large, or if the resulting stresses on individual components exceed what the material can safely withstand. Engineers need both analyses for a working design: kinematics to get the intended motion right, and kinetics to confirm that motion can actually be produced and sustained by realistic forces without breaking anything.
What we're still unsure about
The formal distinction between kinematics and kinetics, and the role each plays in mechanical design, is well established and fundamental to mechanical engineering education, resting on rigorously defined classical mechanics. What requires genuine engineering judgement, rather than a fixed formula, is deciding how much kinematic complexity a given mechanism's motion requirements actually justify against the added kinetic cost — more complex motion paths often demand larger forces, more wear, and more expensive components to sustain reliably, and balancing a mechanism's kinematic goals against its practical kinetic constraints is a real design tradeoff engineers work through case by case, not something a single universal rule settles automatically.
This sits inside Dynamics: Kinematics & Kinetics, one of eight topics in Mechanical Engineering, one of four domains in Engineering, one of seventeen subjects the app can quiz you on.