CHAPTER 12:THE CONDITIONS OF

LINEAR MOTION

CHAPTER 12:THE CONDITIONS OF

LINEAR MOTION

KINESIOLOGYScientific Basis of Human Motion, 12th edition

Hamilton, Weimar & LuttgensPresentation Created by

TK Koesterer, Ph.D., ATCHumboldt State University

Revised by Hamilton & Weimar

Copyright © 2012 by The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill/Irwin

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The Nature of ForceThe Nature of Force

Force is that which pushes or pulls through direct mechanical contact or through the force of gravity to alter the motion of an object. Internal forces are muscle forces that act on

various structures of the body. External forces are those outside the body: Weight, gravity, air or water resistance, friction, or

forces of other objects acting on the body.

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Internal Force:

Muscular Force

Internal Force:

Muscular Force

In direct proportion to the number & size of fibers contracting in a muscle.

Muscles normally act in groups whose force or strength is measured collectively.

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Point of ApplicationPoint of Application

Point at which force is applied to an object.

Where gravity is concerned this point is always through the center of gravity.

For muscular force, this point is assumed to be the muscle’s attachment to a bony lever. The point of intersection of the line of

force and the mechanical axis of the bone.

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Mechanical AxisMechanical Axis

Fig. 12.3

• The mechanical axis of a bone is a straight line that connects the midpoint of the joints at either end of the bone.

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DirectionDirection

Direction of a force is along its action line. Gravity is a downward-directed vector

through the center of gravity of the object.

Direction of a muscular force vector is the direction of line of pull of the muscle.

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External ForcesExternal Forces

Accomplished in the same manner as muscular forces applied at an oblique angle.

Only horizontal force will move the table.

Vertical force serves to increase friction.

Fig 12.7

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Composite Effects of Two or More ForcesComposite Effects of Two or More Forces

Two or more forces can be applied to objects.

A punted ball’s path is the result of force of the kick, force of gravity, and force of wind.

Muscles work in groups, e.g. the 3 hamstrings.

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For forces applied in the same direction, the resultant is the sum of the forces:a + b = c

For forces applied in the opposite directions, the resultant is the sum of the forces:

a + (-b) = c

Linear ForcesLinear Forces

=+a b c

=+a b c

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Parallel ForcesParallel Forces

Forces not in the same action line, but parallel to each other.

Three parallel forces: two upward one downward

Fig 12.9

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Newtons’ Laws of MotionNewtons’ Laws of Motion

A body continues in its state of rest or of uniform motion unless an unbalanced force acts on it. An object at rest remains at rest. An object in motion remains in same motion, unless

acted upon by an outside force.

Friction & air resistance effect objects in motion.

1. Law of Inertia1. Law of Inertia

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2. Law of Acceleration2. Law of AccelerationThe acceleration of an object is directly

proportional to the force causing it and inversely proportional to the mass of the object.

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Impulse Impulse The product of force and the time it is applied.

Fig 12.12

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Momentum Momentum

The product of mass and velocity

Any change in momentum is equal to the impulse that produces it.

Force applied in direction of motion will increase momentum.

Force applied opposite to direction of motion will decrease momentum.

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Summation of ForcesSummation of ForcesForce generated by muscle may be summated from

one segment to another.

Typical throwing pattern:

Force from legs is transferred to the trunk.

Further muscular force increases momentum and is transferred to upper arm. Mainly as an increase velocity because mass is smaller.

Sequential transfer of momentum continues with mass decreasing and velocity increasing.

Finally, momentum is transferred to thrown ball.

Forces That Modify Motion

Weight

Forces That Modify Motion

Weight The force of gravity is

measured as the weight of the body applied through the center of gravity of the body and directed toward the earth’s axis.

W = mgFig 12.16

Weight

Contact Forces:FrictionContact Forces:Friction

Friction is the force that opposes efforts to slide or roll one body over another. In some cases we try to increase friction for a more

effective performance (Running Shoes). In other cases we try to decrease friction for a more

effective performance (Bowling Shoes)

The amount of friction depends on the nature of the surfaces and the forces pressing them together.

Elasticity and ReboundElasticity and Rebound

Objects rebound in a predictable manner.

The nature of rebound is governed by elasticity, mass, and velocity of rebounding surface, friction between surfaces, and angle of contact.

Elasticity is the ability to resist distorting influences and to return to the original size and shape.

Angle of ReboundAngle of Rebound

For a perfectly elastic object, the angle of incidence (striking) is equal to the angle of reflection (rebound).

Fig 12.22

Fluid ForcesFluid Forces

Water and air are both fluids and as such are subject to many of the same laws and principles.

The fluid forces of buoyancy, drag, and liftapply in both mediums and have considerable effect on the movements of the human body.

BuoyancyBuoyancy

Archimedes’ Principle: a body immersed in a liquid is buoyed up by a force equal to the weight of the liquid displaced.

Eureka, Eureka!

Drag and LiftDrag and Lift

Drag is the resistance to forward motion through a fluid (air or water).

Result of :

fluid pressure on the leading edge of the object.

amount of backward pull produced by turbulence on the trailing edge. Fig 12.24 b

Drag and LiftDrag and LiftLift is the result of changes in fluid pressure as

the result of difference in air flow velocities.

Fig 12.24 c

V P

V P Lift

Drag

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Direction & Point of Application of External Forces Direction & Point of Application of External Forces Force Direction of Force Point of Application

Weight (W) Downward Center of Gravity

Normal (R) Perpendicular Point of contact

Friction (F) Along surface Point of Contact

Buoyancy (B) Upward Center of buoyancy

Drag (D) Opposite flow Center of Gravity

Lift (L) Perpendicular to drag Center of Gravity

Work, Power, and EnergyWorkWork, Power, and EnergyWorkWork is the product of force expended and

the distance over which force is applied.

W = Fs Work (W), Force (F), Distance (s)

Units are any combination of force & distance: foot/pounds, joule = 107 x 1 gram / 1 centimeter

Positive & Negative Work Positive & Negative Work

Positive work – force acts in the same direction as that of the objects motion.

Negative work – force acts in the direction opposite to that of the objects motion.

PowerPower

The rate at which work is done.

EnergyEnergy

The capacity to do work.

Law of Conservation of Energy:

The total amount of energy possessed by a body or an isolated system remains constant.

2 Types of Energy: Potential Energy Kinetic Energy

Potential EnergyPotential Energy

Potential energy: energy based on position.

Potential energy is the product of the weight of an object and the distance over which it can act:

Kinetic EnergyKinetic Energy

Energy based on motion:

Work done is equal to the kinetic energy acquired

Analysis of Linear MotionAnalysis of Linear MotionFirst identify the nature of the

forces involved in the motion of interest: Weight Propulsive forces Ground Reaction Force Friction Buoyancy, Drag, & Lift

Analysis of Linear MotionAnalysis of Linear Motion The principles that govern the mechanical

aspects of a movement can be summarized by examining some of the basic concepts involved in the kinetics of linear motion: Inertia Impulse Work & Power Potential & Kinetic Energy