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SX104

Week 8

Kinetics III

Forces during walking, running and jumping

Dr. Martine Deighan

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Learning Outcomes

• Explain the direction of the vertical velocity and acceleration of the CoM during the preparation for a vertical jump.

• Know that Impulse = Change in Momentum and use this equation to calculate take-off velocity of a jump.

• Be able to label diagrams of force vs. time during walking, running and jumping.

• Know typical values of BWs for walking, running and jumping.

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Vertical Jump

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Start of Jump

Accel = 0

Velocity = 0

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Downwards Acceleration (speeding-up part of squat)

Accel Velocity

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Upwards Acceleration (slowing down to bottom of squat)

Accel

Velocity

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Upwards Velocity

AccelVelocity

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Take-Off(just before toes leave floor, accel becomes –ve as the

body’s upward motion begins to slow down: accel = -9.81 m.s-1 once airborne)

Accel

Velocity

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Force-Time Graph

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Vertical Jump Velocity

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Impulse = change in momentumUse this known relationship to calc. take-off velocity

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12

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vvm

Ft

vvmFt

mvmvFt

TOvm

Ft

since v1 = start velocity = 0 and v2 = take off velocity

Hence, take-off velocity is ______________by reducing the mass and increasing the Impulse (Ft).

Impulse (Force x time) is the area under the force vs. time graph.

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Impulse on Force-Time Graph

NOTE: the impulse is only considered from the start of the movement, until the end of the movement, i.e. take-off.

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Net Impulse

+ve Area

-ve Area

-ve Area

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Landing Force in Vertical Jump

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Forces in Stride Phases

DriveFlight Landing Mid-stance

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Forces in Walking

Time (seconds)

Forc

e

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-200

-100

0

100

200

300

400

500

600

700

800

900

Fx [N]

Fy [N]

Fz [N]

Walking

Landing

Support

Drive

Contact

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Forces in Jogging

Time (seconds)

For

ce

0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50-400

-200

0

200

400

600

800

1000

1200

1400

1600

1800

Fx [N]

Fy [N]

Fz [N]

Jogging

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Forces in Sprinting

Time (seconds)

Forc

e

0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32 0.34 0.36-200

0

200

400

600

800

1000

1200

1400

1600

Fx [N]

Fy [N]

Fz [N]

Sprinting

Drive

Contact

Take-Off

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Walk vs Run

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Landing style in running

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Walk vs run: braking and propulsive forces

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Horizontal Impulse – how we can tell if someone is speeding up or slowing down

Positive Horizontal Acceleration

Negative Acceleration

Zero Acceleration

Negative impulse from landing phase

Positive impulse from drive phase

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Walk vs run: mediolateral forces

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Reading

• Hall, pg. 395 – 399.

• Hamill and Knutzen, pg. 365 – 368 (the equations on these pages are not part of the learning outcomes for this lecture).