Linear Kinetics

View the links at the bottom of the page to see supplemental material and additional explanations

Concepts

• Force
• Inertia
• Mass
• Center of Mass
• Linear Momentum
• Linear Impulse

Newton's Laws

Systems

• Define a system
• Define the external forces made on the system
• Define the internal forces made on the system
• Draw a free-body diagram
  1. Sketch the system
  2. Draw the reference frame
  3. Draw the forces at their point of application
     1. Weight at the CM
     2. External forces at their points of application
     3. Fluid forces
     4. Magnetic forces
• Compute the acceleration of the system CM
  1. Sum the forces in the X, Y and Z directions
  2. Divide each sum of force component by the mass of the system
  3. Answer is the acceleration of the system

How do biomechanist's get sum of force information?

1. Videotape or film a movement skill
2. Digitize the video/film images to get coordinate data of body landmarks
3. Use the landmark data to generate displacement versus time curves for each landmark and in each direction
4. Use the displacement versus time data together with known data about the locations of segment CMs to compute displacement versus time curves for the segment CMs
5. Use the segment CM displacement data together with known data about segment masses to compute displacement versus time curves for the body CM
6. Take the derivative of the displacement versus time curves to get velocities of any landmark or CM
7. Take the derivative of the velocity versus time curves to get acceleration of any landmark or CM
8. Once we know the acceleration of the CM of either the body or an individual segment, the sum of forces (net force) made on the segment can be obtained as the product of the CM acceleration and the mass of the body or segment.
9. Knowing this and using the information in the free-body diagram, it may be possible to determine the magnitudes and directions of individual forces exerted upon a body
10. At times, a force plate is used to directly measure the magnitude of an external force applied to the body.

Example/Review

Generate a vertical displacement versus time graph for a person performing a countermovement vertical jump.

• Draw a free-body diagram
• Create a velocity versus time graph for the movement
• Create an acceleration versus time graph for the movement
• Create a net force versus time graph for the movement
• Examine the forces indicated in your free-body diagram
  • Create a weight versus time graph
  • Using the graphs created above and your equation of sum of forces in the vertical direction, create a ground reaction force versus time graph

Friction

• Frictional Forces
• Normal Forces
• Coefficient of friction
  • static
  • dynamic