A Closer Look at Impulse Mike Wilson (12 ft dunk)
Horizontal Impulse in Running Time (s) Force (N) + Net Positive Accel. During Contact Positive impulse > negative impulse No Net Acceleration During Contact Positive impulse = negative impulse Net Negative Accel. During Contact Positive impulse < negative impulse _ Impulse is the area under the force-time curve.
Understanding Impulse Horizontal Force (N) Time (s) F = ma +force +acceleration F = ma force - acceleration At each instant in time during a contact, a force acts to produce an acceleration. The Impulse is the net effect of all those instantaneous forces. In other words, it is the average force multiplied by the total time over which the forces have acted. Impulse = Average Force x Time force was applied or Fdt Impulse produces Mass x Change in Velocity or mv
Running Contact During a single running contact, your body undergoes both positive and negative forces that produce positive and negative accelerations. A force acting for a period of time produces an impulse. If the positive and negative impulses cancel each other out (equal areas), then the net impulse is zero and the runner is moving at a constant speed.
Pressure [kPa] 600 400 COP path 300 200 220 Force 150 [N] 900 100 600 300 60 Area 30 [cm2] 10 100 50 kPa Time 200 400 600 [ms]
Force walking 5 km/h jogging 7 km/h running 15 km/h [N] 1500 1000 500 300 220 150 100 60 40 15 kPa walking 5 km/h jogging 7 km/h running 15 km/h Force [N] 1500 1000 500 Time 0.2 0.4 0.6 0.2 0.4 0.2 0.4 [s]
GRF During Walking (Rear) Time from heel strike 6 ms 16 ms 26 ms 36 ms 72 ms 111 ms 200ms
GRF During Walking (Lateral) Time from heel strike 200ms 111 ms 72 ms 36 ms 26 ms 16 ms 8 ms Direction of Walk
Equal and Opposite When a ball hits a bat, or a foot lands on the ground, there are equal and opposite impulses applied to both objects. The force-time curve of one is the inverted force time curve of the other. Peak forces are equal and occur at the same time Ground Force (N) Time (s) Foot
Vertical Impulse: Countermovement jump Fy mg Fy External Forces Time mg Fy = Fy – mg = may
Combining Fy and -mg gives the graph of the sum of the forces in the vertical direction Resultant Forces (Fy) e start Time b a: Bottom of Squat b: Take-Off c: Peak Jump Height d: Touch-Down e: Motionless Finish c d
Acceleration of Center of Mass Time Where is the greatest negative velocity? Where is the greatest positive velocity? At what point is the jumper’s velocity zero?
Fy Time
Counter Movement Jump The following slides depict actual force-time data collected from the force plate in the HK Biomechanics Lab at StFX. Various other variables, also calculated from the force plate data, are plotted on a secondary y-axis to show how they relate to the force-time curve. Graphics of the athlete are also shown at key time points throughout the jump.