Force and force system By Dr M Abbas

Force and force system By: Dr. M. Abbas Jamil

Force: q Force is that which alters the state of rest of a body or its uniform motion in a straight line. It is simply a push (compression) or pull(tension).

Sources of Force: Four primary sources of force are: • • Application of these primary forces lead to: • Joint compression • Joint distraction • Pressure (force per unit area) on body tissues Gravity or weight of body parts Muscles Externally applied resistances Friction

Description: q Forces are vector quantities. A vector quantity describes both magnitude and direction. A person pulling a heavy load with a rope would be an example of a vector. The tension in the rope represents the magnitude of the vector, and the direction of the rope represents the direction of the vector.

Composition of Forces: q NET FORCES: When two or more forces are combined • Forces in the same direction: add the two forces together. + • = Forces in different directions: subtract the two and figure out which direction was the stronger of the two. - =

q Unbalanced: When the net force on an object is not zero. These produce a change in motion. q Balanced: When the net force on an object equals zero. These do not produce change in motion.

q A linear force results when two or more forces are acting along the same line.

q Parallel forces occur in the same plane and in the same or opposite direction.

q To produce concurrent forces, two or more forces must act from a common point but pull in different (divergent) directions, such as the two people pulling on the boat. q The net effect of these two divergent forces is called the resultant force. Because forces are vectors, they can be shown graphically using what is called the parallelogram method. First draw in vectors for the two forces (solid lines). Secondly, complete the parallelogram using dotted lines. Lastly, draw in the diagonal of the parallelogram (middle line and arrow). This diagonal line represents the resultant force.

§ An example of resultant force in the body is the anterior and posterior parts of the deltoid muscle. Both parts have a common attachment (the insertion) but they pull in different directions. When both parallel forces are equal, the resultant force causes the shoulder to abduct. If the pull of the two forces were not equal, that is, if the pull of the anterior deltoid were stronger than that of the posterior, the resultant force would show that the motion would be more in the direction of the anterior deltoid. The shoulder would flex and abduct in a forward, diagonal direction.

Units of Force: q Metric system (system international -SI) • Newton (N) • q the amount of force necessary to accelerate a mass of 1 kg at 1 m/s 2 English system • pound (lb) • the amount of force necessary to accelerate a mass of 1 slug at 1 ft/s 2 • equal to 4. 45 N

Newton’s First Law (law of inertia) q An object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an unbalanced force.

Newton’s Second and Third Law q Force equals mass times acceleration § Force = Mass x Acceleration q Newton’s Third Law § For every action there is an equal and opposite reaction.

Reaction: road pushes on tire Action: tire pushes on road

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