Biomechanics Exam 3

Force

A mechanical disturbance or load that may deform a body or change its state of motion

Is force a vector or scalar quantity?

Vector

• Has: 

  • Magnitude  

  • Direction  

  • Point of application 

  • Line of action (line of application) 

Point of application

• Where to the tendon attaches to the bone for internal forces

• For external forces, point of application is where body is in contact with some external body (ex: ground reaction force)

Examples of contact forces

• Ground reaction forces 

• Joint reaction force 

• Friction 

• Fluid resistance 

• Muscle force 

Examples of non-contact forces

• Gravity 

• Centripetal 

Newton’s First Law

• Law of Inertia 

• A body will maintain a state of rest or constant velocity unless acted on by an external force that changes the state 

• In absence of net force, the state of motion of an object will not change 

Inertia depends on ______

mass of object

What is inertia

Inertia is the resistance of an object to changes in its state of motion

Newton’s Second Law

• Law of Acceleration 

  • Acceleration of an object is: 

    • Directly proportional to the net force 

    • Inversely proportional to the mass 

    • In the same direction as net force 

    • Defined by the equation: F=ma 

If net force is increased, what happens to acceleration?

It increases (F= ma)

Newton’s Third Law

• Law of reaction

• For every action, there is an equal and opposite reaction

Law of Gravitation

All bodies are attracted to one another with a force proportional to the product of their masses and inversely proportional to the distance between them 

Ex: weight (W = mg) 

Ground Reaction Force (GRF)

The equal and opposite reaction force the ground exerts on the body during stance  (stance phase of walking)

How do you measure GRF 

• Force plate used to measure GRF 

• GRF is assumed to act on the center of
  mass of the individual

If a person is standing still on a force plate, what would their GRF read?

It would be equal to their weight (need acceleration to change the GRF)

Center of mass (COM)

object's mass is equally distributed in all directions

When acceleration is negative = vertical GRF is….

less than a person’s bodyweight

When calculating GRF, is gravity negative or positive?

Positive

Describe vertical GRF in walking 

• Highest in stance phases 

• Highest in the pre-swing phase, specifically

When acceleration is positive = vertical GRF is….

greater than a person’s bodyweight

Speed impact on GRF

• Stance phase is shortened with greater speeds 

• Max GRF is higher in running than in walking

• Steeper slope with greater speed

• With running, one defined peak whereas walking has two peaks

Why is there only one peak in the vertical ground reaction force graph of running?

Heel does not hit ground

How does mass affect GRF?

• Larger mass means higher max values for VGRF 

• Shape of graph does not change with mass 

Joint Reaction Force 

• Equal and opposite force between two articulating bones (ie joint) caused by the weight and inertial force on the bones 

• From compressive force and shear force, we can calculate joint reaction force (resultant joint reaction force) 

Muscle Force

Resultant of all forces generated by muscle force