Mechanics unit 2 (this is a continuation of mechanics and biomech unit 1) - deck 7 Flashcards
Why do standardised sets of anthropometric data usually quote the radius of gyration of a body segment rather than the moment of inertia directly ?
Because the radius of gyration is not dependent on the mass of the body segment. This then allows the moment of inertia to be calculated using the radius of gyration and the estimated mass (again using standardised anthropometric data) of the segment
What are the 3 different axis of rotation for body segment ?
- The centre of mass
- The proximal end of the segment
- The distal end of the segment
Why is the radius of gyration quoted about 3 different axes of rotation ?
As the radius of gyration is dependent on the distribution of mass relative to the axis of rotation, it will therefore be different for the 3 different axes of rotation
How is the radius of gyration usually expressed in anthropometric data sets ?
Usually as a ration of the radius of gyration for an axis of rotation to body segment length
e.g. centre of mass: 0.297 for the hand (so this means the radius of gyration is 0.297 of the total body segment length from the centee of mass axis of rotation)
Do the worked example on pg. 38&39 for unit 2 forces
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Do the SAQ 24 pg. 39 forces unit 2
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Define what work is
Work is the product of the applied force and the distance through which it moves
Work is done when a force moves its point of application e.g. a force applied to a block moves the point of application of the force (F), through a distance (s), from a point A to B
State the equation for calculating work
w = Fs
- w = the work done
- F = the applied force
- s = the distance moved
What are the SI units of work ?
Joule - J
What is one joule equivalent to ?
4.1868 calories
Do worked example and SAQ 25 pg. 40 forces unit
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Define what power is
Power is the rate at which work is done e.g, a light bulbs power rating in watts (W) is a measure of the electrical energy it consumes each second.
State the equation for power and the SI units
P = w / t
- P = the power
- w = the work done
- t = the time taken
Si units = watt (W)
Do worked example on pg. 41&42 forces unit 2
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Do SAQ 26 pg. 42 forces unit 2
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Define what energy is
Energy is the property of a system that is the measure of its capacity to do work
What are the 2 forms of energy we need to know about ?
Kinetic and potential energy
Define what kinetic energy is and state what it is equal to
Kinetic energy is the energy possessed by a body by virtue (because of) of its motion, it is equal to the work that would be required to bring the body to rest.
State the equation for calculating the linear (translational) kinetic energy and its SI units
KE = 1/2mv2
- KE = kinetic energy
- m = the mass of the body
- v = the linear velocity of the body
SI units are joule (J)
Do SAQ 27 pg. 42 forces unit 2
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State the equation for calculating the rotary kinetic energy
KE = 1/2ɪω2
- ɪ = the moment of inertia
- ω = the angular velocity
Define what the total kinetic energy of a body is
The total kinetic energy of a body is the sum of its linear (translational) and its rotational kinetic energies
Define what potential energy is
This is the energy possessed by a body by virtue (because of) of its position.
State what gravitational potential energy is equal to
It is equal to the product of a body’s weight x its height
State the equation for calculating gravitational potential energy
PE = Wh = mgh
- W = body’s weight
- h = the body’s height above the ground
- m = body’s mass
- g = the acceleration due to gravity
SI untis are joules (J) again cause its energy
When may something be given a negative potential energy?
If the body goes below the datum position (horizontal baseline) because potential energy is a relative quantity
refer to fig. 44 pg. 43 forces unit 2
Do SAQ 28 pg. 43 forces unit 2
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State the principle of energy conservation
Energy can be redistributed or changed in to another form but can not be created or destroyed
In a closed system what happens to the total energy ?
It remains constant
e.g. consider a free falling body, as it falls it is loosing PE (height decreasing) and it is gaining KE as its velocity increases. The free falling body constitutes a closed system and therefore the sum of the potential (PE) and kinetic (KE) is constant and equal to the total energy in the system:
PE + KE = E
Do worked example and SAQ 29
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