Body moment
From ArticleWorld
The term body moment as used in the study of mechanics, is a measure of the turning effect of a force about a certain point in space. Moments are the results of forces acting at a distance from the point in space which is considered during the measurement. The SI unit of body moment is newton-meter (N-m).
The stress which is associated with the body on which the force acts is symmetric.
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Concept of moment in rotational physics
The concept of moment when referring to the study of rotation, essentially points to a physical quantity that represents the magnitude of force that is applied to a rotational system at a certain distance from the axis of rotation. The distance which is involved in such cases is a key element in the operation of machines like the lever, pulley and gear.
The concept of moment is also applied in other simple machines that are capable of generating mechanical advantage.
Mathematical expression of moment of a force
The moment about a point in space that is at a distance of 'd' units away from the application of a force 'F' can be calculated by using the following relation:
- Moment = Magnitude of force × perpendicular distance to the pivot (F × d).
Practical examples of moment of force
Several practical examples of the concept of moment of force can be seen. Two important examples are described in the passages that follow.
See-saw
In order to balance two persons on a see-saw, the weight of the persons at either end has to be equal. If one of the persons moves closer to the fulcrum of the see-saw, the see-saw will tip at the opposite end. This is because the moment of force, which causes the turning effect, depends on the distance of the point of action of the force from the fulcrum.
Ankle moment during standing
The moment of force around the ankle joint of a standing person due to the ground reaction can be estimated. For a typical person of 80 kilograms mass (weight 800N), the moment of force that acts in front of the ankle (about 5 cm away) can be calculated as:
Moment due to the reaction of the ground= Force × moment arm of force
- = 800 × 0.05
- = 40 N-m.
A broad and flat muscle in the calf of the leg called the soleus muscle produces much of the moment which is needed to balance the moment due to the ground reaction and keep the body still.