1Center of Gravity• When gravity acts on a body, every particle of which it is composed is attracted toward the earth. The resultant force is the body’s weight.• Through which point does this resultant force act?CENTER OF GRAVITY (CG)• Definitions:– Theoretical point at which all of the body’s weight is considered to be concentrated – Point about which a body will balance• It is not necessarily the point about which there are equal amounts of weight. Rather, it is a “point” about which these weights are “balanced”.• Example: teeter-totter CG location is dependent on weight distribution of body.– CG location is dependent on the weight and the distribution of this weight within the body.2• Human body:– Is the CG of the human body always in the same place?– In the anatomical position, the CG is near the waist.• Females: 53-56% of standing height• Males: 54-57% of standing height– The CG does NOT have to lie within the physical matter of the body:• tire, basketball, football helmet– In humans, the CG may also fall outside body’s physical matter:• (e.g., high jumper, pole vaulter) Long Jump• Within which physical boundaries must the CG lie?3Linear Kinetics•Force– Effect that one body has on another– A push or a pull applied to an object– That required to change the state of motion of an object (i.e., that which causes acceleration)Acceleration (Effect) Force (Cause)Characteristics of Force• Force is a vector quantity:– Magnitude & direction• A third, unique characteristic:– The point of application (especially important relative to the determination of moments or torques)• Therefore, to completely understand the influence of a net force, we must have knowledge of all three characteristics4Newton’s Laws of Motion• First Law: Inertia– a body will continue in its state of motion unless acted upon by a net force. • Second Law: ΣF = ma – Acceleration is proportional to the net force acting on a body. • Third Law: Action-reaction– For every force there is an equal and opposite force. •Example:– What is the net force acting on the gymnast if she maintains this static position?– Draw and label each of the forces acting on the gymnast in this situation.– Identify each of the reaction forces.5Linear Kinetics Exercise• Draw and label all of the verticalforces which are acting on this athlete.• Express these forces in terms of Newton’s 2nd Law.• Under what circumstances will his body (CM) experience a positive acceleration? • Under what circumstances will his body (CM) experience a negative acceleration? • Under what circumstances will his body (CM) experience zero acceleration?• Draw the acceleration-time curves of the CM during the following activities:– From a standing position, suddenly squat down and hold this positionacceleration0timeacceleration0time– From a squatted position, suddenly stand back up and hold this upright position.6How well do you know it? (can you do it backwards?)timeacceleration0timevelocity0displacement0timetimeacceleration0timevelocity0displacement0timeSquat Down Stand UpSummary• Positive Acceleration– increasing velocity in the upward direction– decreasing velocity in the downward direction– changing directions from downward to upward • Negative Acceleration– decreasing velocity in the upward direction– increasing velocity in the downward direction– changing directions from upward to downward•Zero Acceleration– constant velocity– peak velocities7Pressure• Force applied to an object is rarely applied at a single point. Rather, it is distributed over an area. Pressure = Force/AreaPressure = “force per cross-sectional area”Examples• Lying down vs. standing• High heels vs. tennis shoes• Rock in your shoe• Atmospheric pressure• Catching a baseball with a mitt vs. barehanded• What are differences in Force?• What are the differences in surface or contact area?• What are the resulting effects on