12.3 - Principles 4 & 5 - Linear Motion

Principles Related to Linear Motion

  • Two biomechanical principles are related to linear (or translational) motion:

    • Principle 4 “The Impulse-Momentum Relationship”

    • Principle 5 “The Direction in Which Movement Usually Occurs”

Principle 4

  • THE IMPULSE-MOMENTUM RELATIONSHIP

    • “The greater the applied impulse, the greater the increase in velocity.”

Interpreting Principle 4

  • When an object such as a cricket ball, field hockey ball, or tennis ball is in motion, it is said to have momentum.

    • The momentum of the ball or any other object in motion is equal to its mass multiplied by its velocity.

      • To get a ball moving, a cricket, field hockey, or tennis player will use a striking implement to apply a pushing force to the ball over a period of time.

      • The greater the pushing force, and the greater the amount of time over which it is applied to the ball, the greater the impulse.

    • This is a restatement of biomechanical principle 4.

Imparting High Velocity to a Cricket Ball

Example of Principle 4 in Action

  • Elite athletes and their coaches often rely on biomechanical principle 4 to improve their techniques and performance.

    • For example, today’s high jumpers commonly use a technique called the Fosbury Flop.

      • As jumpers near the bar, they arch their neck and back and push against the ground to create a powerful impulse force.

    • An equal and opposite ground reaction force is generated, which propels the high jumper into the air.

Example of Principle 4 in Action

  • The “jump serve” in volleyball provides another good example of biomechanical principle 4.

    • Players begin well back behind the service line, lob the ball forward, and run and jump into the air in order to “spike” the ball to the opposing team.

      • The forward running motion of the server’s body transfers momentum to the ball, making it move through the air at a high velocity.

        • This increase in velocity, combined with a high flight path, makes it difficult for the ball to be returned by the opposing team.

Applying Impulse in a Bobsled Race

Principle 5

  • THE DIRECTION OF APPLICATION OF THE APPLIED FORCE

    • “Movement usually occurs in the direction opposite that of the applied force.”

Interpreting Principle 5

  • The fifth biomechanical principle is closely related to Newton’s third law of motion, which states that for every action there is an equal and opposite reaction.

    • People at work and at play rely on this principle constantly.

      • For example, when a person sitting in an armchair stands up, the individual will place his or her hands on the armrests and push down.

        • A reaction force that is equal in magnitude but opposite in direction will be generated by the chair arms.

Principle 5 and Aquatic Events

  • Biomechanical principle 5 is evident in many aquatic events.

    • When completing a length of a pool, for example, free-style swimmers turn and push against the wall of the pool with their legs.

      • The swimmers’ bodies are propelled forward—in the direction opposite that of the applied force.

Example of Principle 5 in Action

Example of Principle 5 in Action

  • Biomechanical principle 5 can be seen in action in many team sports.

    • In making a cut, for example, an ultimate player or a soccer player will push his or her foot against the ground to make a change in direction away from an opponent.

    • Similarly, an ice hockey player will push off using the edge of the skate blade to make the same type of movement to either avoid a hit or make one.