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Systems and Free Body Diagrams

Readings: Chapter 1 [in text] Freebody.pdf [on website]

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Learning objectives By the end of this lecture, you should be able to:   Describe what a system is   Describe the difference between internal and

external forces   Identify and give examples of forces that cause

movement and forces that do not cause movement

  Draw a free body diagram:  identify and draw the system  identify and draw the appropriate forces

  Identify errors in free body diagrams 2

Systems

A system is a body or group of bodies or objects whose motion is to be examined.

3 ground reaction force

body weight + reaction for muscle force

(minus weight of foot)

muscle force

force of gravity (mg)

muscle force

Systems

Once you have correctly identified the system you can analyze what forces can cause the system to move.

Although muscular force is often required to move, it is not the actual force that propelled this goalkeeper into the air.

What force did?

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Internal and external forces

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System: The group of objects you are examining e.g. windsurfer, board, and sail

Internal forces: Forces acting within the system e.g. muscle forces, force of hands on boom, and…

External forces: Forces acting between the system and its environment e.g. force of water on board, air resistance, and…

joint forces

gravity

Internal and external forces

System: goalie

Internal forces:

External forces:

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muscle forces joint forces

ground reaction force force of ball on hand gravity air resistance

Reaction forces

Although internally initiated movements of the segments are required to move the body (due to force of muscular contraction), to move a system an external force must be present.

i.e. you have to push or pull against something to make your centre of gravity, or the CG of an object, move.

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Reaction forces

the diver pushes against the board

these movements are initiated by internal forces (muscle forces)

the board pushes against the diver with an equal and opposite force

this is what moves the diver

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Cause & Effect in Moving a System The cause of movement of a body or segment is an external force such as:

  muscle force (torque) (if the system is a body segment – if the system is the whole human – muscle forces are internal)

  gravitational force

  ground reaction force (you have to push first, though)

  another external force (e.g. an opponent in a wrestling match)

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Free Body Diagrams Free body diagrams (FBDs) are a crucial skill in the analysis of human movement

Read the pdf file on the 201 website: “Freebody.pdf”

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Free Body Diagrams

1.  Define the system

2.  Draw the outline of the system do not draw stick figures do not include any internal features

3.  Draw and label vector arrows for all external forces acting on the system, with correct:

•  point of application •  magnitude •  direction

don’t forget gravity, friction, and air resistance (if applicable) 11

Free Body Diagrams Example: Draw all forces acting on the runner.

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What would the diagram look like if you drew a free body diagram of the foot and shank?

It is best not to draw stick figures for general FBDs 13

Fg is ground reaction force. What does it include?

I would disagree with this diagram – why?

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Fj is joint reaction force. What does it include?

Free Body Diagrams

Example: Draw a free body diagram of the entire athlete.

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force of barbell (weight of barbell)

ground reaction force

force of gravity on body (mg)

If the athletes was NOT moving up or down, ideally the “up” vector would roughly represent the same magnitude as the two “down” vectors

Why?

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Free Body Diagrams Example: Draw a free body diagram of the head/trunk (not including arms).

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Note: This author draws stick figures. I do not like that method.

Free Body Diagrams Example: Draw a free body diagram of the subject’s hand-forearm segment.

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This text, unfortunately, has a whole series of “free body diagrams” that are incorrect.

Let us see if you can identify the problem.

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...more problems with FBDs in the text Fig. 10-20: A free body diagram of a runner's wrist weight system.

Problems?

  Friction is drawn in the wrong direction

  Do not draw the ground if it is not part of the system

  Wording of the caption: this is still a FBD of the runner and weight as the system!

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...more problems with FBDs in the text Fig. 10-21: A free body diagram of the forearm during a biceps curl.

Problems?

• Joint reaction force is drawn in the wrong direction • Do not draw objects that are not part of the system • Vectors are nowhere near reasonable representation of true forces

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...more problems with FBDs in the text Fig. 10-22: A free body diagram of a box on a table. The box is in equilibrium because there are no horizontal forces and the sum of the vertical forces is zero.

Problem?

  Do not draw the ground if it is not part of the system

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...more problems with FBDs in the text Fig. 10-25: A free body diagram of a sagittal view static lifting model showing the linear forces at the joints and segments.

Problems?

  Do not draw internal forces   the R vectors are actually NET

joint reaction forces, not true forces

  They have not drawn ground reaction force

(Adapted from Chaffin, D.B., Andersson, G.B.J. [1991]. Occupational Biomechanics [2nd ed.]. New York: Wiley).

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...more problems with FBDs in the text Fig.10-26: Free body diagram of the foot segment during the swing phase of a walking stride showing linear forces and accelerations.

Problems?

  Do not draw accelerations on a FBD

  Rx and Ry are actually NET forces, not true forces

  Close the system, do not leave lines open

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Main problem with FBDs in the text

The authors are not very stringent in their use of the term “free body diagram.”

They are using the term to describe a diagram on which you draw some information about the forces acting on the system.

I will be more precise in what I expect you to draw:   Only the system, nothing else   No internal forces   Reasonable representation of force vectors

where possible 29