1. A Closer Look at Impulse
Mike Wilson (12 ft dunk)

2. 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.

3. 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 mv

4. 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.

5. 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]

6. 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]

7. GRF During Walking (Rear)
Time from heel strike
6 ms
16 ms
26 ms
36 ms
72 ms
111 ms
200ms

8. GRF During Walking (Lateral)
Time from heel strike
200ms
111 ms
72 ms
36 ms
26 ms
16 ms
8 ms
Direction of Walk

9. 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

10. Vertical Impulse: Countermovement jumpFy mg Fy
External Forces
Time mg
Fy = Fy – mg = may

11. 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

12. 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?

13. Fy
Time

14. 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.