Work, Energy and Power
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34 Terms
1st law of thermodynamics
conservation of energy, cannot be created or destroyed
2nd law of thermodynamics
when energy is transferred from one form to another, some is wasted (entropy)
Fenn’s method
TME = PE + KE + RKE
rotational kinetic energy =
½Iw²
energy unit
kgm²s^-2
work done by a 1N force that moves an object through 1m in the direction of that force
1 J
work
force x distance, in the direction of the force
Fdcos(theta)
work
work done by a system on the outside world is
positive
what contraction has negative work done on it
eccentric
why is there no mechanical work done for isometric contractions
d=0
power =
force x velocity, in direction of force
Energy / time (J/s)
power
power
rate at which work is done
Elftmann’s method for power
Pj = Mj (w1 - w2)
joint moment x joint angular velocity =
power
what happens when the velocity and moment are in the same direction
positive power (concentric)
which work accounts for interaction forces and energy transfer between joints
compensated
the modulus of the sum of energies
compensated
the sum of the modulus of energies
non-compensated
what does non-compensated work not account for
interaction between segments
calculates work at each joint independently
non-compensated
muscle indeterminacy
more muscles across a joint than number of joints
passive energy transfer
joints must have equal and opposite forces and velocity in same direction
in javelin, is the power from the arm or body passive transfer
body
hidden power
power that is produced but cannot be measured
examples of hidden work
breathing, antagonistic muscle activity, moving muscles relative to skeleton
more accurate, reflects true energy cost, suited to complex whole-body movements e.g. running
advantages and uses of compensated work
advantages of non-compensated work
simpler to compute, useful for isolated joint analysis, understanding of joint demands helps injury prevention and rehabilitatiom
disadvantages of compensated work
complex, requires detailed motion-capture and force data, less useful for individual joint contributions
over or underestimates work done as ignores energy transfer between joints, does not represent net mechanical cost of movement
disadvantages of non-compensated work
which method uses work is integral of power and power = joint moment x angular velocity
non-compensated
which method works out the total mechanical energy cost of movement and how it is shared across joints
compensated
understands where and how power is generated at joint level
non-compensated