Centre of Gravity

1. Biomechanics Centre of Gravity Craig Harris

2. Lesson Objectives Define centre of gravity / centre of mass Experience how centre of gravity is altered due to movement, and the affect that can have on performance. Define stability and realise its importance in the sporting environment. To introduce the methods of calculating COG

3. Centre of Gravity Centre of gravity can be defined as the point in a body or system around which its mass or weight is evenly distributed or balanced and through which the force of gravity acts. Centre of mass is the point in a body at which the entire mass may be assumed to be concentrated. For our purposes they are the same thing!!

4. Centre of Gravity The centre of gravity can be considered an imaginary point in space It is not a physical entity, we cannot touch or see it. Nor can the centre of gravity be fixed or permanently marked on an object. Why??

5. Centre of Gravity The centre of gravity is a useful concept for analysis of human movement When using Newton’s Laws of motion, it is the COG of a body whose motions are ruled by the laws. Newtons 3 laws Therefore we should be able to locate or at least estimate the location of the COG of an object

6. COG of the human body The COG of the human body is determined by the location of its limbs at any given time. When Standing in the anatomical position, the height of your centre of gravity is :- 55% of your standing height for women 57% of your standing height for men. ANY SUGGESTIONS AS TO WHY???

7. COG of the human body Consider standing in the anatomical position, what effect on the COG would the performance of the following movements have?

8. Where is the COG for these athletes?

9. Consider the following

10. “Hang In the Air” Athletes such as basketball players, gymnasts, figure skaters, rugby players etc. Appear to hang in the air. During such jumps, they appear suspended in the air, instead of rising and falling in a parabolic path as a projectile should Can we attempt to explain why?

11. “Hang In the Air” During the jump, the jumpers head stays at the same level, but the COG follows a parabolic path. Due to the raising and lowering of the arms and legs

12. Centre of Gravity and Stability The mechanical definition of Stability is “ the capacity of an object to return to equilibrium or to its original position after having been displaced.” In many sports stability is important . Name 3 sports in which the performers do not want to be moved from a certain position Name 3 sports athletes need a stable base in which to move from a certain position

13. Factors affecting Stability Size of the base of support The horizontal distance from the edge of the support and the COG Height of the COG Mass of the body Direction of the base in relation to the force

14. Calculating Centre of Gravity There are two methods that we can use to calculate a persons COG. They are :- The Reaction board method The segmental method

15. The reaction board method The reaction board method is easily applied to static position i.e. The anatomical position, both above hands above head, etc. Involves a subject laying upon a board, with one end standing on a weighing scales. Only the height of a person COG can be calculated.

16. The segmental method This method involves the calculation of a bodies centre of gravity from a photograph of human movement. The human body is broken down into segments (limbs). Centre of masses are calculated for each segment. Is a more versatile method as it can be applied to more dynamic situation

17. To Conclude We can now define centre of gravity / centre of mass We can now display and understand of how centre of gravity is altered due to movement, and the affect that can have on performance. We can define stability and realise its importance in the sporting environment. Been introduced to the methods of calculating COG