Kinetics

Question 1

What is a moment?

Answer 1

The tendency of a force to cause rotation about a specific axis.

Question 2

What is the equation for a moment?

Answer 2

Moment = Force x Distance

Question 3

What is Newton’s 1st law of angular motion?

Answer 3

A rotating object will continue to do so at a constant angular momentum unless acted on by an external moment

Question 4

What is inertia?

Answer 4

Inertia is an objects tendency to resist change in motion.

Question 5

What is Newton’s 2nd law of angular motion?

Answer 5

An external moment produces an angular accelleration that is proportional to the moment and in the direction of the moment.

Question 6

What is Newton’s 3rd law of angular motion?

Answer 6

For every applied moment there is an equal and opposite reaction moment.

Question 7

What is an eccentric force?

Answer 7

A force whose line of action does not pass through the centre of gravity. They will always cause rotational movement and therefore a moment.

Question 8

What is a couple?

What affects the amount of rotation?

Answer 8

Two equal and opposite parallel forces. The translational effects of the two forces cancel each other out so no translation occurs. Rotation occurs about the body centre of gravity.
The magnitude of the forces and the distance between the lines of action.

Question 9

What is the equation for the moment of a couple?

Answer 9

Moment = F1 x Distance between lines of action

Question 10

What is the resultant moment?

Answer 10

The sum of the moments that have tendency to cause rotation about a given point.

Question 11

How are clockwise and anticlockwise moments defined?

Answer 11

Clockwise - Negative
Anticlockwise - Positive
(It is counter-intuitive)

Question 12

What is an equilibrium?

What is the sum of the moments at equilibrium?

Answer 12

When all parts of a body are at rest or are moving with the same constant velocity.
The sum of the moments is 0.

Question 13

Define the centre of gravity.

Answer 13

The point through which the resultant force of gravity acts.

The point about which all particles of the body are evenly distributed.

Question 14

Describe the suspension method for estimating centre of gravity and discuss pros and cons.

Answer 14

Cardboard cut out.
Suspended from a fixed point.
Draw a vertical line from this point.
Suspend from a second point that is not on this line.
Draw a second line.
The centre of gravity is the point at which the lines meet.
This method is inaccurate for 3D objects and is impractical and time consuming.

Question 15

Describe the reaction board method for estimating centre of gravity and discuss pros and cons.

Answer 15

Employs the theory of equilibrium.
Horizontal board supported by two knife edges.
Read value (R1) on scales, with nobody on the board.
Lie a person supine on the board, feet soles in line with knife edge on wooden block.
Read (R2) value on scales

Question 16

Given that the reaction board is 1.8m in length what is the equation for the point of centre of gravity?

Answer 16

x = 1.8(R2-R1)/W

Question 17

Describe the segmental method for estimating centre of gravity and what are the equations?

Answer 17

The sum of the moments is equal to the moment of the resultant.
This method requires x,y co-ordinates, segment weights and the locations of CoG of segments
MgX = Sum of (migxi)
MgY = Sum of (migyi)

Question 18

How are segment data obtained using cadaver data? Consider the limitations of this method.

Answer 18

Uses regression equations to estimate mass and CoG location.
Masses given relative to total body mass and segments CoG location uses % length from distal end of segment.
These data are generalisations and are not subject specific.
They don’t consider differences between sexes.
They may not be representative of the athletic population or the modern day population.
Don’t consider differences in ethnicity.
There is not much access to cadavers.

Question 19

How are segment data obtained using mathematical modelling? Consider the limitations of this method.

Answer 19

Represent body segments as geometric shapes.
Anthropometric measures used and applied to model.
This method is subject specific.
Geometric shapes are not full representative of the human segments.
This method doesn’t account for differences in density within segments.

Question 20

How are segment data obtained using scanning? Consider the limitations of this method.

Answer 20

Gamma ray scanning.
Allows you to calculate mass per unit surface area.
Prediction equations used.
Expensive and time consuming process.
Dependent on the population the data were taken from.

Question 21

What is moment of Inertia?

What are its determinants?

Answer 21

The resistance of a body to change it’s state of angular motion. It gives a measure of distribution of mass about an axis.
It is determined by mass of the body and the distribution of mass about the centre of gravity.

Question 22

How can moment of inertia be increased?

Answer 22

Greater mass.

Mass concentrated further from the axis of rotation.

Question 23

What is the equation for moment of Inertia?

Answer 23

Moment of inertia = mass x distance from point of rotation^2

Question 24

What are the typical values for the moment of inertia of a human in a straight, piked and tucked position?

Answer 24

Straight - 15.0 kg.m^2
Piked - 6.5 kg.m^2
Tucked - 3.5 kg.m^2

Question 25

What is the parallel axis theorem?

Answer 25

Method used to calculate moment of inertia. | Values for moment of inertia in literature are used to determine whole body moment of inertia.

Question 26

What is the equation for the moment of inertia of a body rotating about a given point (A)?

Answer 26

Ia = Icg + md^2 Ia - Moment of inertia of the body rotating about a Icg - Moment of inertia of a segment about the segment CoG m - Segment mass d - distance between axes of whole body CoG and segment CoG

Question 27

What is the direct measurement technique for determining segmental moment of inertia? Discuss limitations.

Answer 27

Pendulum technique. Suspend segment from fixed point. Allow segment to swing a few degrees from equilibrium. Measure the time take for one oscillation. Sample group is small and not representative of the population. There are different definitions of segment start and end points. It is difficult to measure central segments.

Question 28

What is the ratio and regression technique for determining segmental moment of inertia? Discuss limitations.

Answer 28

Equations based on curve fitting procedures using directly measured segmental data from cadavers. Relate segmental parameters to subjects through simple measurements such as height and body mass. Cadavers have generally no represented young, athletic populations. Subject specific.

Question 29

What is the mathematical modelling technique for determining segmental moment of inertia? Discuss limitations.

Answer 29

Represent body as a series of geometric solids of different shapes. Models can vary in complexity. Size of geometric shapes is obtained from individual anthropometric measurements. Density values come from cadaver data.

Question 30

What is the equation for angular momentum?

Answer 30

Angular momentum = Moment of Inertia x angular velocity

Question 31

What is the equation for determining the angular momentum of a segment about a given axis?

Answer 31

Hs = IsWs/gs + msr^2Wgs/g Hs - Angular momentum of a segment Is - Moment of inertia of a segment about the axis through the segment CoG, gs. Ws/gs - Angular velocity of the segment about the axis through gs. r - Distance between body CoG and segment CoG. Wgs/g - Angular velocity of the segment CoG about the transverse axis through the body CoG.

Question 32

How do you calculate the total angular momentum of the body about the transverse axis through the CoG?

Answer 32

The sum of the angular momenta for all segments.

Question 33

How can athletes use Newron's 1st law in performance?

Answer 33

Divers, gymnasts and trampolinists are able to perform skills mid-air by manipulating their moment of inertia and angular velocity.

Question 34

What impacts angular acceleration?

Answer 34

Directly proportional to the net moment. | Inversely proportional to the moment of inertia of the object.

Question 35

What happens to the angular momentum of a body in flight?

Answer 35

Angular momentum is conserved.

Question 36

Describe the two long jump techniques.

Answer 36

Hang - Generation of angular momentum, extend body, increasing inertia and reducing angular velocity, arms thrown forward, opposite moment applied to legs which move up and forward for landing. Hitch-kick - Generation of angular momentum, cycling of arms and legs to transfer angular momentum, Legs up and forward for landing.