IB Syllabus focus: 'Linear and angular motion can be analysed using Newton's laws. Motion is described using speed, velocity and acceleration, and resultant motion depends on the sum of acting forces.'
This section introduces the core language used to describe motion in sport, linking straight-line and rotational movement to Newton's laws and to the effects of balanced and unbalanced forces.
Forms of motion
In biomechanics, linear motion describes movement in which the body or object travels from one place to another without rotating as a whole.
Linear motion is movement in a straight or curved path in which all parts of a body or object move the same distance in the same direction at the same time.
Practice Questions
FAQ
Average velocity describes displacement over an entire time period, such as a full sprint or lap.
Instantaneous velocity describes velocity at one exact moment, which is more useful when analyzing phases like takeoff, landing, or a change of direction.
Motion is always described relative to something else, called a frame of reference.
For example, a cyclist may be stationary relative to the bike seat but moving rapidly relative to the road. In biomechanics, choosing the correct reference point helps avoid confusing descriptions.
In angular motion, points farther from the axis of rotation travel a greater distance in the same amount of time.
That is why the tip can have a greater linear speed than the hand. A simple way to express this is $v=r\omega$, where larger $r$ gives larger $v$ if $\omega$ stays the same.
Displacement depends only on the start and finish positions, not the total path traveled.
So a player who runs around the court and returns to the exact starting point can have high speed and cover a large distance, but still have zero displacement and therefore zero average velocity.
The hub is useful for describing the bike's forward travel because it follows the overall linear motion of the bicycle.
The rim is useful for describing angular motion because it rotates around the axle. Looking at different points on the same object can reveal different aspects of motion very clearly.
