The forearm as a third-class lever
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- The forearm as a third-class lever
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The forearm as a third-class lever
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[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
A lever is a simple machine that uses an applied force to rotate a rigid rod about a fixed pivot point / fulcrum with the purpose of lifting a weighted load. There are three major classes of levers, each defined by the location of the applied force/ effort in relation to the load / resistance and the fulcrum. A first-class lever has its fulcrum placed between the applied force and the load. An example of a first class-lever is a seesaw. In a second- class lever, the load is positioned between the fulcrum and the applied force. An example of a second-class lever is a wheelbarrow. Lastly, a third-class lever system has its applied force situated between the load and fulcrum. An illustration of the various classes of levers are shown in figure 1.
Figure 1: Examples of First, Second, and Third-class Levers
Most of the musculoskeletal levers of the human body are classified as third-class levers. For instance, the forearm, acts as a third-class lever. The biceps muscle, which originates from the scapula, inserts at the proximal part of the elbow. As the biceps contracts, it rotates the forearm about the elbow joint, bringing it closer to the body. This motion is called elbow flexion. Figure 2 shows a simplified lever representation of an arm holding a weighted load, W, in its hand. Other than the weighted load, the forces acting on this system are Fm , the force generated by the biceps muscle during contraction, and Fr , the reactive force at the fulcrum. These forces are oriented at a certain angle with respect to the horizontal axis as shown in figure 2.
Figure 2: Simplified Lever Representation of the Human Arm in Equilibrium With a Weighted Load
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[question] => In figure 2, if the weighted load (W) is equal to 1 kg and if the biceps muscle attaches 4 cm from the elbow joint at an angle (ϴ) 30 degrees from the horizontal axis, then what is the value of Fm?
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[description] => Reason for the Correct Answer:
Using the elbow joint as the fulcrum, derive the torque equation that describes the system depicted in figure 2.
Net torque = (Fm x 4cm x sin 30) – (10N x 40cm) =0
Solve for Fm: ( Fm x 4cm x sin 30) = (10N x 40cm) ; ( Fm x 2cm) = 400N * cm; Fm = 200 N
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[each_answer] => A. 100 N
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[each_answer] => B. 200 N
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[each_answer] => C. 300 N
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[each_answer] => D. 400 N
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[question] => Most musculoskeletal levers are classified as third-class levers; however, which of the following would NOT be considered an example of a third-class lever?
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[answer] => 1
[description] => Reason for the Correct Answer:
In a third-class lever system, the applied force is situated between the load and fulcrum.
During a push up, the foot serves as the fulcrum and the hands/ arms provides the applied force. The weight of the body/ load is situated between the applied force and fulcrum.
The body acts as second-class lever during a push up motion.
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[each_answer] => A. The body during a push up
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[each_answer] => B. The lower leg during knee flexion
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[each_answer] => C. The entire leg during hip extension
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[each_answer] => D. The arm during shoulder abduction
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[quiz_unique_key] => 83407773
[question] => Levers are simple machines that make it “easier” to do work by either decreasing the amount of force required to lift a load or by increasing the angular displacement and/or the velocity of a load. A third-class lever makes it “easier” to do work by doing which of the following?
I. Decreasing the amount of force required to lift a weighted load
II. Increasing the angular displacement of a load
III. Increasing the velocity of a load
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[answer] => 1
[description] => Reason for the Correct Answer:
Mechanical advantage is defined as the ratio of the output force produced by a machine to the applied force. If this ratio is greater than one, then there is a mechanical advantage in force. If this ratio is less than one, then there is a mechanical advantage in angular displacement and speed.
For a lever, this ratio is also equal to ratio of the lever arm of the applied force to the ratio of lever arm of the weighted load. Lever arm of applied / lever arm of applied force
In a third-class lever, the applied force is closer to the fulcrum than the weighted load.Therefore the lever arm of the applied force is shorter than the lever arm of the weighted load.
The mechanical advantage for third-class lever is always less than one.
The mechanical advantage of a third- class lever is in angular displacement and range of motion, thus II and III.
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[each_answer] => A. II and III
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[each_answer] => B. I, II and III
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[each_answer] => C. I and II
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[each_answer] => D. I only
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[question] => Assuming the lever system depicted in figure 2 is frictionless, which of the following statement is true concerning the amount of work and force that is required to lift the weighted load, W?
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[description] => Reason for the Correct Answer:
An ideal simple machine, a machine that does not increase internal energy through friction, does NOT change amount of work needed to move a load.
The applied force in a third-class lever has a shorter lever arm than the lever arm of the load/weight.
The applied force needs to be greater than the weight of the load in order for the system to be in equilibrium.
In an ideal third-class lever system, the amount of work will remain constant, but the force will increase.
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[each_answer] => A. Work remains constant, but force is increased
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[each_answer] => B. Work remains constant, but force is decreased
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[each_answer] => C. Work is decreased, but force remains constant
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[each_answer] => D. Work is increased, but force remains constant
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[question] => According to figure 2, which of the following graphs accurately represent how the lever arm changes with respect to the biceps muscle’s angle of application (ϴ)?
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[answer] => 1
[description] => Reason for the Correct Answer:
With a fatiguing restoring force, the amplitude would increase in successive cycles as the ankles increasingly fail to counteract gravity
The potential energy is greatest when the angular displacement is zero, towards which the signal decays.
The torque increases with the tilt angle, and the angle of the torque does not contribute to the amplitude of the signal in successive cycles.
The decay in amplitude of oscillations is most likely the result of frictional or feedback damping dissipating the energy stored in the oscillations.
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Most of the musculoskeletal levers of the human body are classified as third-class levers. For instance, the forearm, acts as a third-class lever. The biceps muscle, which originates from the scapula, inserts at the proximal part of the elbow. As the biceps contracts, it rotates the forearm about the elbow joint, bringing it closer to the body. This motion is called elbow flexion. Figure 2 shows a simplified lever representation of an arm holding a weighted load, W, in its hand. Other than the weighted load, the forces acting on this system are Fm , the force generated by the biceps muscle during contraction, and Fr , the reactive force at the fulcrum. These forces are oriented at a certain angle with respect to the horizontal axis as shown in figure 2.
Figure 2: Simplified Lever Representation of the Human Arm in Equilibrium With a Weighted Load
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