Kinetics and Kinematics in Sports

The same physical laws that govern all objects in the universe also govern the human body. In mechanics, movement is studied as either static (objects in equilibrium) or dynamic (objects in motion), and dynamics is subdivided into kinematics and kinetics.

Kinematics — describing motion

Kinematics is the geometry of motion. It analyses motion in terms of time, displacement, velocity (speed) and acceleration, whether the motion is in a straight (linear) line or a rotary (angular) direction — but it does so without any reference to the forces that cause the motion. In other words, kinematics describes the how of movement: how far the body's segments and joints travel and how fast.

The key kinematic quantities a Class 11 student should know are:

• Distance — the total path length travelled (a scalar).
• Displacement — the straight-line change in position, with direction (a vector).
• Speed — distance covered per unit time (a scalar).
• Velocity — displacement per unit time, i.e. speed in a stated direction (a vector).
• Acceleration — the rate of change of velocity.

Motion itself may be linear (a skater gliding), angular (a gymnast turning about a bar) or general (a runner moving forward while the limbs rotate). Because motion must be measured to be described accurately, measurement is central to kinematics.

Kinetics — the forces that cause motion

Kinetics is the action of forces in producing or changing motion. It considers the forces that cause bodies to move — a force being a push or a pull — and includes the influence of gravity, friction and torque. All levers in the human body are pull-type machines, yet in performing a skill the effort can become a 'push', as when the hand pushes the shot. Importantly, kinetics includes Newton's three Laws of Motion.

Newton's three laws of motion in sport

• First Law (Inertia): a body stays at rest, or in uniform motion in a straight line, until an external force acts on it — a sprinter in the blocks must overcome inertia to start.
• Second Law (Acceleration): the acceleration of a body is proportional to the force applied, in the direction of that force (F = m x a) — a shot-putter applies maximum force for maximum acceleration.
• Third Law (Action-Reaction): for every action there is an equal and opposite reaction — a swimmer pushes the water backward and is driven forward.

Levers in the body

Because kinetics is about force, it is also where the body's levers are discussed. A lever is a rigid bar turning about a fixed point: in the body a bone is the lever, a joint is the fulcrum, and a muscle supplies the effort to move the load. Human levers produce force to overcome resistance, and that is work being done.

How kinetics and kinematics differ

The two branches answer different questions — kinematics describes motion, kinetics explains its cause: