SEHS terms in Forces, motion and movement

The rate of change of velocity, $a=\frac{(v-u)}{t}$; a vector measured in m s⁻². A negative acceleration, a decrease in velocity, is called deceleration.

The mass of air per unit volume, about 1.2 kg per cubic metre at sea level; it falls as altitude, temperature and humidity rise, reducing drag and lift.

Drag acting on a body moving through air; it opposes the body's motion and increases as the body's speed increases.

The angle between the long axis of a body, such as a discus, javelin or ski jumper, and the direction of the fluid flow relative to it; it determines how much lift and drag are produced.

The angle to the horizontal at which a projectile is released or takes off, also called the angle of projection; it decides how the speed of release is shared between horizontal and vertical motion.

The quantity of rotation possessed by a rotating body, equal to its moment of inertia multiplied by its angular velocity; measured in kg m² s⁻¹.

Movement of a body or body segment along a circular path about an axis of rotation, so that different parts travel different distances; also called rotation.

The angle turned through by a rotating body per unit of time, in a stated direction, measured in rad s⁻¹; linked to linear velocity by $v=\omega r$.

A principle stating that the buoyant force on a body immersed in a fluid equals the weight of the fluid the body displaces.

The area enclosed by the outermost points of contact between a body and the supporting surface, including the area between those points.

A principle stating that the pressure in a moving fluid falls as its speed increases, so a body with faster flow over one side is pushed towards that lower-pressure side.

The study of the forces acting on and within the human body and the effects of these forces on movement, applying the laws of mechanics to sport, exercise and health.

The thin layer of fluid next to the surface of a moving body, slowed by friction with that surface; where it separates from the body decides the size of the wake.

The upward force exerted by a fluid on a body that is partly or wholly immersed in it, equal to the weight of the fluid the body displaces.

The point through which the buoyant force acts, at the centre of the volume of fluid displaced; in a swimmer it usually lies nearer the chest than the centre of mass.

The point at which the whole mass of a body may be considered to be concentrated and through which its weight acts; it moves when body segments move and can lie outside the body.

A number with no units describing how strongly two particular surfaces resist sliding, equal to friction divided by the normal reaction force; the static value (μs) exceeds the dynamic value (μd).

A measure of the elasticity of a collision, the relative velocity of separation divided by the relative velocity of approach, $C_{\text{R}}=\frac{v_{\text{b}}-v_{\text{a}}}{u_{\text{a}}-u_{\text{b}}}$; no units, between 0 and 1.

A brief interaction in which two bodies come into contact and exert equal and opposite forces on each other, changing the momentum of each.

The principle that the angular momentum of a rotating body stays constant unless an external torque acts, so in flight a smaller moment of inertia gives a faster angular velocity.

The principle that the total momentum of colliding bodies is the same after a collision as before it, provided no external resultant force acts on them.

A skill with no obvious beginning or end, in which the end of one movement cycle is the beginning of the next, such as running, swimming or cycling.

The moment within a skill that most determines its outcome, such as the instant of release of a ball, take-off from the ground or contact between a foot, bat or racket and a ball.

A skill with a clear beginning and a clear end, usually performed quickly, such as a basketball free throw, a golf drive or a penalty kick.

The shortest straight-line distance between the start and finish positions of a body, in a stated direction; a vector measured in metres (m).

The total length of the path travelled by a body, regardless of direction; a scalar measured in metres (m).

Travelling closely behind another competitor or vehicle, in their wake, to reduce drag and the energy needed to maintain speed; also called slipstreaming.

The force exerted by a fluid that resists the motion of a body through it, acting opposite to the body's motion relative to the fluid; also called fluid resistance.

The friction between two surfaces that are sliding relative to each other, $F_{\text{f}}={\mu }_{\text{d}}{F}_{\text{N}}$; usually smaller than limiting static friction.

A force whose line of action does not pass through the centre of mass of a body, producing rotation (angular motion) as well as linear motion.

The energy stored in a body that has been stretched, compressed or bent, released as it returns to its original shape, as in a vaulting pole, tendon or trampoline bed.

The recording of the electrical activity produced by muscle fibres when they are activated, usually with electrodes on the skin, to show when a muscle is active and how strongly.

The capacity to do work, measured in joules (J); it can be transformed from one form to another, such as kinetic, potential, chemical or thermal, but not created or destroyed.

A substance, either a liquid or a gas, that can flow and takes the shape of its container; in sport the main fluids are air and water.

The final phase of a discrete skill, in which the body keeps moving after release or contact, ensuring maximum speed at contact and slowing the limbs gradually to reduce peak forces.

A push or pull acting on a body that can change its state of motion or its shape; a vector measured in newtons (N).

A measuring platform in or on the floor that records the ground reaction force applied to it many times per second, used to analyse jumping, landing, running, balance and gait.

The phase of a movement in which the muscles contract to generate the force that moves the body or an object, usually sequencing joint actions from large proximal to small distal segments.

A graph of the force acting on a body against time, usually recorded with a force plate, in which the area under the curve equals the impulse.

Drag caused by the difference between the high pressure in front of a body and the low pressure in the turbulent wake behind it; it depends on shape and frontal area. Also called pressure drag.

A diagram of a single body showing all the external forces acting on it as labelled arrows from their points of application, with lengths roughly proportional to their size.

A force acting parallel to two surfaces in contact that opposes their relative motion, or the tendency of one surface to slide over the other.

The systematic analysis of walking or running, describing the stance and swing phases, joint movements, forces and timing to identify abnormal patterns, guide treatment or improve performance.

A combination of linear and angular motion, as in running or cycling, where rotation of the limbs about the joints produces translation of the whole body.

The energy a body possesses because of its height above a reference level, equal to $mgh$; measured in joules (J).

The force exerted by the ground on a body in contact with it, equal in size and opposite in direction to the force the body applies to the ground.

The vertical distance between the point at which a projectile is released and the level at which it lands; releasing above the landing level lowers the best angle of release.

The product of the average force acting on a body and the time for which it acts, $J=F\mathrm{\Delta }t$; equal to the change in momentum and measured in N s.

The principle that the impulse applied to a body equals its change in momentum ($F\mathrm{\Delta }t=\mathrm{\Delta }p$), so a greater impulse produces a greater change in momentum.

The resistance of a body to a change in its state of motion, whether at rest or moving at constant velocity; directly proportional to the body’s mass.

A small wearable device containing an accelerometer and a gyroscope that measures the acceleration and rotation of the body segment it is attached to, allowing movement analysis outside the laboratory.

The energy a body possesses because of its motion, equal to $\frac{1}{2}m{v}^2$; measured in joules (J).

Smooth flow of a fluid in parallel layers that slide past each other without mixing; it occurs at low speeds and around smooth, streamlined shapes.

The component of the force exerted by a fluid on a moving body that acts perpendicular to the direction of the fluid flow; it can act upwards, downwards or sideways.

An imaginary vertical line passing through the centre of mass towards the ground, showing the line along which weight acts.

Movement in which all parts of a body travel the same distance in the same direction in the same time, along a straight or curved path; also called translation.

The deflection of a spinning object from its path, caused by a pressure difference across it; the force acts perpendicular to the direction of travel, towards the side whose surface moves with the airflow.

The amount of matter in a body, which does not change with location or gravity; a scalar measured in kilograms (kg).

The resistance of a body to a change in its angular motion, depending on its mass and how far that mass is distributed from the axis of rotation; measured in kg m².

The quantity of motion possessed by a moving body, the product of its mass and velocity, $p=mv$; a vector measured in kg m s⁻¹.

A system that tracks the three-dimensional position of the body over time, usually with several infrared cameras following reflective markers on anatomical landmarks, to calculate joint angles and velocities.

The systematic observation, description and evaluation of a movement or skill, usually by dividing it into phases, to understand how it is performed and to identify improvements in performance, safety or health.

The SI unit of force: the force needed to give a mass of 1 kg an acceleration of 1 m s⁻², so 1 N = 1 kg m s⁻².

A law of motion stating that a body stays at rest or moves at constant velocity in a straight line unless an external resultant force acts on it.

A law of motion stating that the acceleration of a body is proportional to the resultant force on it and inversely proportional to its mass, $F=ma=\frac{\mathrm{\Delta }p}{\mathrm{\Delta }t}$.

A law of motion stating that when one body exerts a force on another, the second exerts an equal and opposite force on the first; the two forces act on different bodies.

The contact force exerted by a surface on a body resting or moving on it, acting at right angles to the surface; on level ground it equals weight when there is no vertical acceleration.

The symmetrical curved flight path of a projectile acted on by gravity alone, in which the rise and the fall are mirror images of each other.

The division of a skill into a sequence of functional parts: preparatory, force production, critical instant, and follow-through for discrete skills or recovery for continuous skills.

The rate of doing work or transferring energy, $P=\frac{\mathrm{\Delta }W}{\mathrm{\Delta }t}=Fv$; measured in watts (W), where 1 W = 1 J s⁻¹.

The first phase of a movement, in which the performer gets into the best position to produce force, for example by taking a stance, lowering the centre of mass or moving a limb back to increase its range.

A biomechanical principle stating that a body moves in the direction in which impulse is applied, so force must act in the intended direction of motion, through the centre of mass if no rotation is wanted.

An object or body that has been launched into the air and then moves only under the influence of gravity and air resistance, such as a thrown ball, a javelin or a long jumper after take-off.

The movement of a projectile through the air after release, determined by its velocity and height at release and by the forces of gravity and air resistance acting during flight.

Analysis of movement based on systematic observation and description of its quality, such as body positions, timing and coordination, without precise measurement.

Analysis of movement based on measurement, producing numbers such as joint angles, velocities, forces or muscle activation that can be compared between trials or performers.

The final phase of each cycle of a continuous skill, in which the limbs return to the starting position for the next cycle, often with relaxed muscles to save energy.

The single overall (net) force acting on a body when all the forces acting on it are combined, taking their directions into account; it decides the body’s acceleration.

A quantity that has size (magnitude) only and no direction, such as mass, distance, speed, time or energy.

A skill made of several discrete skills linked together in a set order to form a larger whole, such as a triple jump or a gymnastics floor routine.

The rate at which a body covers distance, equal to distance divided by time ($s=\frac{d}{t}$); a scalar measured in metres per second (m s⁻¹).

The speed of a projectile at the instant it is released or takes off; the most important factor for the horizontal distance a projectile travels.

Rotation of a projectile about an axis through its centre of mass; topspin makes a ball dip, backspin makes it rise and sidespin makes it swerve.

The ability of a body to maintain or regain balance, resisting forces that would disturb its equilibrium; it depends on the height of the centre of mass, base of support, line of gravity and mass.

The friction between two surfaces that are not sliding relative to each other; it matches the applied force up to a maximum (limiting friction) of ${\mu }_{\text{s}}{F}_{\text{N}}$.

Shaping a body, or adopting a body position, so that fluid flows smoothly around it with little separation or turbulence, which reduces form drag.

The sequential use of body segments from the large proximal segments to the small distal ones, each starting as the previous reaches peak speed, to maximise the speed of the end segment or implement.

Drag caused by friction between a fluid and the surface of a moving body; it increases with the surface area in contact with the fluid and with surface roughness. Also called skin friction.

The turning effect of a force about an axis, equal to the force multiplied by the perpendicular distance from the axis to the force’s line of action; measured in N m.

Irregular flow of a fluid in eddies and swirls in which the layers mix; it occurs at higher speeds and behind blunt or rough objects.

A quantity that has both size (magnitude) and direction, such as displacement, velocity, acceleration, force, momentum and impulse; drawn as an arrow.

The rate of change of displacement ($v=\frac{\mathrm{\Delta }s}{\mathrm{\Delta }t}$), describing how fast a body moves and in which direction; a vector measured in m s⁻¹.

The recording of movement on video so that it can be replayed, slowed down, paused frame by frame and measured, either by eye or with software.

The mass of water per unit volume, about 1,000 kg per cubic metre for fresh water and about 1,025 for seawater; it rises with salinity, increasing buoyancy.

Drag caused by the formation of waves when a body moves along or near the surface of water; it rises steeply with speed and is avoided by swimming underwater.

The force of gravity acting on a body’s mass, $F_{\text{g}}=mg$, acting vertically downward from the centre of mass; measured in newtons (N).

The product of a force and the distance its point of application moves in the direction of the force, $W=Fd$; measured in joules (J).