Center of Gravity/MassCenter of Gravity/Mass

Center of Gravity (CG): the point Center of Gravity (CG): the point around which a body’s weight is around which a body’s weight is equally balanced or distributed in all equally balanced or distributed in all directions directions

Center of mass (CM): the point Center of mass (CM): the point around which a body’s mass is around which a body’s mass is equally balanced or distributed in all equally balanced or distributed in all directionsdirections

Or, the point about which the sum of moments = 0Or, the point about which the sum of moments = 0

NOTE:

• If the body is homogenous in density and symmetrical, CG lies at the center of a body

• CG not necessarily within the boundaries of a body

Can the CG fall outside the human body?

Why is the center of gravity of interest in the study of human biomechanics?

It serves as an index of total body motion

Path of the center of gravity of a runner.

The body responds to external forces as though all mass were concentrated at the CG; this is consequently the point at which the weight vector is shown to act in a free body diagram

Determination of the Center of Gravity: The Determination of the Center of Gravity: The reaction board techniquereaction board technique

WrbBW

F1F2

d

distance to CG of reaction board

d = distance to CG

Ascale

Lrb

For equilibrium, moments For equilibrium, moments about A = 0about A = 0

FF22(L) – W(L) – Wrbrb(L(Lrbrb) – BW(d) = 0) – BW(d) = 0

Given: L = 198 cmGiven: L = 198 cm

WWrbrb = 218.1 N = 218.1 N

BW = 712 NBW = 712 N

FF22 = 476 N = 476 N

LLrbrb = 99 cm = 99 cm

d = ???d = ???

L

476 (198) – 218.1(99) – 712(d) = 0

476 (198) – 218.1(99) = 712(d)

d = (476 (198) – 218.1(99) )/712 = 102 cm

If body height is 176 cm: d = (102/176) x 100 = 58% of height

Stability and BalanceStability and Balance Balance – ability to control equilibriumBalance – ability to control equilibrium

Stability – resistance to the disruption of Stability – resistance to the disruption of equilibriumequilibrium

Factors affecting stability:Factors affecting stability:

• Size: more mass, more inertia, more resistance to Size: more mass, more inertia, more resistance to changechange

• Relation of CG to base of support – if the line of the CG Relation of CG to base of support – if the line of the CG falls within the base of support, stability resultsfalls within the base of support, stability results

• Height of CG: Higher the CG, lower the stabilityHeight of CG: Higher the CG, lower the stability

Base of support: Area bound by the outermost regions of contact between a body and support surface(s)

4. horizontally positioning the center of 4. horizontally positioning the center of gravity near the edge of the base of gravity near the edge of the base of support on the side of the external support on the side of the external forceforce

5. Lowering the center of gravity5. Lowering the center of gravity