Kinesiology 3341A/B Lecture Notes - Lecture 10: Deadlift, Cerebral Palsy, Kinetic Energy
13 Dec 2019
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Chapter 9 Biomechanics: Work, Power, and Energy
Work
-Work= force x distance
-*The distance needs to be in the direction of the force
-How far does the centre of mass move?
-Work is expressed in joules, force in newtons, and distance in meters
-When lifting a box off the ground, the acceleration is changing over the distance the
box is moved (F=ma) — therefore, the force is changing
-If the force is constant=
force x displacement=
area underneath the box
-If the force is changing=
draw a similar shape and
calculate the area
-The units for work are newton-meters… but that is the unit for a moment of force —
Work is a force applied through a distance, therefore, it is expressed in Joules (kgm/
s2m or kgm2/s2)
When Force and Displacement Are in Different Directions
-We need to put the force and displacement in the same direction
-Find out the horizontal component of the force (if the distance is occurring horizontally)
Rotational Work
-Force moves with the rotating
object — when rotation happens
through the arc, the force will still
rotate with it, causing distance and
force to still travel in the same
direction, even though the direction
is rotating
*Use angle theta in radians= needs
to be unit-less
*F x r= moment
*Linear Work= F x d
*Angular Work= M x theta (in radians)
-Applying a moment through an angle
Muscles and Work
-Positive Work: concentric contraction — the muscle is active and the displacement
and force are acting in the same direction
-Generating energy (ie. lifting a weight up)
-Negative Work: eccentric contraction — the muscle is active and lengthening (the
displacement and force are in opposite directions)
-Absorbing energy (ie. lowering a weight)
-No Work: isometric contraction — the muscle is active but no movement (wasting
energy)
*Don’t assume that the
moment is always positive
Power
-Power: the rate of doing work
-Power= work/time
-Power= force x distance/time (which is velocity)
-Power= force x velocity
-Example: clean and jerk
Instantaneous vs. Average
-Average Power= ^work/^time
Document Summary
*the distance needs to be in the direction of the force. Work is expressed in joules, force in newtons, and distance in meters. When lifting a box off the ground, the acceleration is changing over the distance the box is moved (f=ma) therefore, the force is changing. If the force is constant= force x displacement= area underneath the box. If the force is changing= draw a similar shape and calculate the area. The units for work are newton-meters but that is the unit for a moment of force . Work is a force applied through a distance, therefore, it is expressed in joules (kgm/ s2m or kgm2/s2) When force and displacement are in different directions. We need to put the force and displacement in the same direction. Find out the horizontal component of the force (if the distance is occurring horizontally)
