External Forces When Running
According to mechanical physics, a force is an effect that may cause a body to accelerate. Also as stated in Isaac Newton’s second law of motion, force is a vector quantity (has magnitude and direction) that is proportional to the product of the mass of a body and its acceleration.
F = ma : where F is force; m is the mass of the body; and a is the acceleration due to that particular force
When running, there are four important external forces that definitely affect the kinetics of running: drag force, gravity, normal force, and friction.
Due to the interaction with air on Earth, runners experience a resistive force against the airflow. This is called the drag force, or air resistance. The equation for this drag force is given as :
Drag Force = 1/2pvvAD
where p is the density of the fluid (in runner’s case: air); v is the velocity of the runner; A is the cross-sectional area perpendicular to the runner’s velocity; and D is the dimensionless quantity called the drag coefficient.
The drag force is always working against the forward motion of a runner, trying to move them in the negative horizon...
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-Second, hip rotation also helps the runner to have a more natural and smoother run and again reduces the energy required to move the runner’s center of mass.
-Finally, the pelvic rotation decreases the impact at contact with the running path felt by the runner.
Dreyer, Danny and Katherine Dreyer. ChiRunning: A Revolutionary Approach to Effortless, Injury-Free Running. Simon & Schuster. New York, NY. 2004
Dumont, Aaron and Chris Waltham. “Walking.” Physics Teacher. Sept. 1997. Vol. 35 Issue 6, p. 372.
Inman, Verne T., Henry J. Ralston, and Frank Todd. Human Walking. Williams & Wilkins. Baltimore, MD. 1981
Watkins, James. An Introduction to Mechanics of Human Movement. MTP Press Limited. Lancaster, England. 1983.
Zatsiorsky, Vladimir M. Kinetics of Human Motion. Sheridan Books. Champaign, IL. 2002.
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