If a body of mass m is in constant motion no acceleration or deceleration occurs ! Acceleration a can be caused by leg muscle force F ! Deceleration can.

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Presentation transcript:

If a body of mass m is in constant motion no acceleration or deceleration occurs ! Acceleration a can be caused by leg muscle force F ! Deceleration can be caused by friction, muscle force or external forces (by running into a wall for example).

Friction occurs between a moving surface and a surface at rest: Friction force: F f  640 N deceleration: a  7.8 m/s 2 N is the normal force!  k is coefficient for kinetic friction: for rubber-concrete:  k  0.8 joints between bones:  k  As smaller the coefficient as less resistance by frictional forces! For a walker of N  W  800 N (m=82kg): Accelerating muscle forces maintain a constant walking speed!

When the body bumps into a solid object (like a wall) rapid deceleration a occurs: The decelerating force F d applied by the wall to the body (or to whatever body part which hits first) causes pressure P d which causes deformation: A is the surface area of the body or body part exposed to the force. Force is only applied over the time period  t until complete stop. Therefore:

To calculate the impact force the time structure of the deceleration process needs to be known. Approximation: treatment of force as a square pulse actual time structure may depend on particular impact

Falls from great height The above equation has to be generalized because of energy transfer arguments! The average force acting on the part of the body which hits the ground is

Body decelerates with average deceleration a from impact velocity v to zero while the center of mass of body moves over a distance a/  CM during the collision

EXAMPLE: Stiffed leg jump on hard ground

Tolerance levels for whole body impact Threshold for survival:  20 mi/h = 36 km/h = 8.9 m/s Effects of impact can be reduced by increasing  t (  h CM ) or by distributing force F over large area to reduce compressing stress. proper landing techniques for parachutes To survive a fall the impact pressure should be:  40 lbs/in 2 = 27.6 N/cm 2 For an impact pressure of 35 N/cm 2  50 % survival chance!

EXAMPLE: Free fall from large heights

hchc h Vf2Vf2 Vt2Vt2 V2V2 free fall actual fall Solution of force equation yields final velocity v!

h/h c e -h/h c V/V t Exponential approach of speed of fall towards the terminal velocity! Terminal velocity represents the state where the forces are in equilibrium!