Hetkkiirus

Hetkkiirus is the speed of an object at a specific moment in time. In physics, it represents the magnitude of the instantaneous velocity at that moment and is a scalar quantity, meaning it has only size, not direction. The term derives from Estonian where hetk means moment and kiirus means speed.

Mathematically, if the object's position is given by a vector r(t), the instantaneous speed is the magnitude of the velocity: hetkkiirus = |dr/dt|. The velocity itself is the vector dr/dt, which has both speed and direction. Thus, the same trajectory can have a constant hetkkiirus even as velocity changes due to a changing direction.

Hetkkiirus is defined as the limit of the average speed over an increasingly small time interval: v(t) = lim Δt→0 [s(t+Δt) − s(t)]/Δt, where s(t) is the path length. In one dimension, hetkkiirus reduces to the absolute value of the time derivative of position, |dx/dt|. In three dimensions, it is sqrt[(dx/dt)² + (dy/dt)² + (dz/dt)²].

Practical notes: hetkkiirus depends on the chosen frame of reference; different observers may measure different instantaneous speeds. It can be measured with devices such as speedometers, which estimate instantaneous speed from wheel rotation or other sensors, though wheel slip and measurement errors can affect accuracy.