medp1 biomechanics

8/11/2019 MedP1 Biomechanics

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Biomechanics IMedP1August 22nd, 2014

Physics for the Life SciencesMedical Physics Component

Kim Dirks

School of Population Health

Faculty of Medical and Health Sciences

Ext: 89755

[email protected]


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Lecture Objectives

To understand the concept of a lever arm and beable to calculate it for a range of simplescenarios involving forces and axes of rotation

To understand the conditions necessary toachieve equilibrium (both the net external forceand the net external torque on a system must bezero)

Be able to solve simple equilibrium problemsinvolving lever systems in the human body


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The Door


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The Concept of Torque

The turning effect of a force on the lever depends on:

1) The magnitude of the force

2) The position relative to the pivot (fulcrum) atwhich the force is being applied

3) The angle at which the force is being

applied

Positive torque Counter-clockwiseNegative torque Clockwise


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The Lever Arm

Fl=!

The lever arm is thedistance between theline of action(direction of theforce) and the axisof rotation (pivotpoint)


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The Lever Arm

Fl=!



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The Lever Arm

Fl=!



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Class Example: Different lever arms,different torques (p.241)

In the figure on the previous page, a force ofmagnitude 55 N is applied to the door. However,the lever arms are different in the three parts of

the drawing:

a) 0.80 mb) 0.60 m and

c) 0 m.

Find the torque in each case. Are the torquespositive or negative?


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F = 55 Nl = 0.80 m


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F = 55 Nl = 0 m


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F = 55 N

l = 0.80 m


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Class Example: The archilles tendon (p. 242)

The tendon exerts a force of magnitude 720 N.Determine the torque (magnitude and direction)of this force about the ankle joint.


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Rigid Objects in Equilibrium

! = 0"

and

Rotational equilibrium is achieved as long as:

0=

! xF

0=

! yF


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Class Example: A diving board (p. 243)

A woman whose weight is 530 Nis poised at the right end of adiving board with length 3.90 m.

The board has negligible weightand is supported by a fulcrum1.40 m away from the left end.

Find the forces that the boltand the fulcrum exert on theboard.


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Class Example: The physics of bodybuilding (p. 246)

The arm is horizontal and weighs 31.0 N.The deltoid muscle can supply 1840 N of force.

What is the weight of the heaviest dumbbell he can hold?


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Lecture Objectives

To understand the concept of a lever arm and beable to calculate it for a range of simplescenarios involving forces and axes of rotation

To understand the conditions necessary toachieve equilibrium (both the net external forceand the net external torque on a system must bezero)

Be able to solve simple equilibrium problemsinvolving lever systems in the human body


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Recommendation for Homework

Rework class examples

Try Example 4 (p. 244 tricky but solution provided)

Try End of Chapter Problems 9.3, 9.15, 9.22

medp1 biomechanics

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