Mechanics unit 2 (this is a continuation of mechanics and biomech unit 1) - deck 7

Question 1

Why do standardised sets of anthropometric data usually quote the radius of gyration of a body segment rather than the moment of inertia directly ?

Answer 1

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

Question 2

What are the 3 different axis of rotation for body segment ?

Answer 2

1. The centre of mass
2. The proximal end of the segment
3. The distal end of the segment

Question 3

Why is the radius of gyration quoted about 3 different axes of rotation ?

Answer 3

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

Question 4

How is the radius of gyration usually expressed in anthropometric data sets ?

Answer 4

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)

Question 5

Do the worked example on pg. 38&39 for unit 2 forces

Answer 5

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Question 6

Do the SAQ 24 pg. 39 forces unit 2

Answer 6

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Question 7

Define what work is

Answer 7

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

Question 8

State the equation for calculating work

Answer 8

w = Fs

• w = the work done
• F = the applied force
• s = the distance moved

Question 9

What are the SI units of work ?

Answer 9

Joule - J

Question 10

What is one joule equivalent to ?

Answer 10

4.1868 calories

Question 11

Do worked example and SAQ 25 pg. 40 forces unit

Answer 11

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Question 12

Define what power is

Answer 12

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.

Question 13

State the equation for power and the SI units

Answer 13

P = w / t

• P = the power
• w = the work done
• t = the time taken

Si units = watt (W)

Question 14

Do worked example on pg. 41&42 forces unit 2

Answer 14

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Question 15

Do SAQ 26 pg. 42 forces unit 2

Answer 15

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Question 16

Define what energy is

Answer 16

Energy is the property of a system that is the measure of its capacity to do work

Question 17

What are the 2 forms of energy we need to know about ?

Answer 17

Kinetic and potential energy

Question 18

Define what kinetic energy is and state what it is equal to

Answer 18

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.

Question 19

State the equation for calculating the linear (translational) kinetic energy and its SI units

Answer 19

KE = 1/2mv²

• KE = kinetic energy
• m = the mass of the body
• v = the linear velocity of the body

SI units are joule (J)

Question 20

Do SAQ 27 pg. 42 forces unit 2

Answer 20

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Question 21

State the equation for calculating the rotary kinetic energy

Answer 21

KE = 1/2ɪω²

• ɪ = the moment of inertia
• ω = the angular velocity

Question 22

Define what the total kinetic energy of a body is

Answer 22

The total kinetic energy of a body is the sum of its linear (translational) and its rotational kinetic energies

Question 23

Define what potential energy is

Answer 23

This is the energy possessed by a body by virtue (because of) of its position.

Question 24

State what gravitational potential energy is equal to

Answer 24

It is equal to the product of a body’s weight x its height

Question 25

State the equation for calculating gravitational potential energy

Answer 25

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

Question 26

When may something be given a negative potential energy?

Answer 26

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

Question 27

Do SAQ 28 pg. 43 forces unit 2

Answer 27

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Question 28

State the principle of energy conservation

Answer 28

Energy can be redistributed or changed in to another form but can not be created or destroyed

Question 29

In a closed system what happens to the total energy ?

Answer 29

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

Question 30

Do worked example and SAQ 29

Answer 30

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