IB Syllabus focus: 'Applications of Newton's laws include stability factors, summing joint forces, impulse and linear momentum. Stability depends on centre of mass, support base, line of gravity and mass.'
This subtopic applies Newton’s laws to human movement by explaining why performers stay balanced, how forces combine at joints, and how force applied over time changes linear momentum.
Stability in Human Movement
What stability means
In biomechanics, stability describes how well a body resists being disturbed. A performer is more stable when a larger unbalanced force is needed to move them out of position or make them lose balance.
Stability: The ability of a body to resist a change in equilibrium or to regain balance after being disturbed.
Practice Questions
FAQ
Carrying equipment can shift the body’s center of mass upward, downward, forward, backward, or sideways depending on where the load is placed.
For example:
a backpack moves the center of mass backward
a barbell overhead moves it upward
a heavy object held to one side moves it laterally
This changes where the line of gravity falls relative to the base of support, so the performer may need to widen the stance or lean to stay balanced.
Muscles usually attach very close to joints, which gives them a poor mechanical position for producing movement. To rotate a limb or hold a load, the muscle may need to produce a force much greater than the external load.
That means:
the visible load may look small
the internal muscle force may still be very large
the resulting compressive force at the joint can be high
This is why slow, controlled lifts can still place substantial stress on joints.
Average force is the force value used when impulse is calculated over a time interval.
Peak force is the highest force reached at any instant during that interval.
A technique change can reduce peak force without greatly changing total impulse. For example, softer landings often spread the force over more time, making the force-time curve lower and broader.
This matters because tissues are often damaged by very high peaks, even when total momentum change is the same.
The arms are body segments with mass, so moving them changes the body’s overall mass distribution.
This can:
shift the center of mass
move the line of gravity
help keep that line within the base of support
That is why people naturally swing or extend the arms when slipping, landing, or walking on a narrow surface. The arm movement is a rapid way to help restore balance without moving the feet first.
Coaches usually estimate it by observing where the body’s mass appears to fall relative to the feet or other contact points.
Simple methods include:
side-view or front-view video
freeze-frame analysis at key positions
visual alignment from the head and trunk down toward the ground
A rough plumb-line approach can also be used on video by drawing a vertical line downward from the estimated center of mass. It is not exact, but it is often good enough to judge whether a performer is balanced or likely to topple.
