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Questions and Answers
What is kinetics primarily concerned with?
What is kinetics primarily concerned with?
Which of the following correctly distinguishes between mass and weight?
Which of the following correctly distinguishes between mass and weight?
Which of the following forces can be classified as an internal force?
Which of the following forces can be classified as an internal force?
How do magnitudes and directions relate in the context of kinetics?
How do magnitudes and directions relate in the context of kinetics?
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What distinguishes kinetics from kinematics?
What distinguishes kinetics from kinematics?
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What does the x-axis in human motion represent?
What does the x-axis in human motion represent?
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What type of motion is allowed by the anterior-posterior axis?
What type of motion is allowed by the anterior-posterior axis?
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Which force is commonly referred to as the pull of gravity on an object?
Which force is commonly referred to as the pull of gravity on an object?
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What happens to an object's mass when it is taken from Earth to the moon?
What happens to an object's mass when it is taken from Earth to the moon?
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What does a moment represent in physical terms?
What does a moment represent in physical terms?
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Which of the following accurately describes the term 'moment arm'?
Which of the following accurately describes the term 'moment arm'?
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Which type of force do muscles provide during contraction?
Which type of force do muscles provide during contraction?
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How does the weight of an object change when comparing its weight at sea level to the top of a mountain?
How does the weight of an object change when comparing its weight at sea level to the top of a mountain?
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What is the role of friction in biomechanics?
What is the role of friction in biomechanics?
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Which axis of rotation allows for the rotation of joints?
Which axis of rotation allows for the rotation of joints?
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Study Notes
Kinetics: Definition and Scope
- Kinetics studies forces acting on material bodies, including the human body, body segments (e.g., finger, ankle, joint), or an entire body.
- Forces produce, arrest, or stop motion in the whole body or individual segments.
Internal and External Forces
- Internal forces originate within the body; muscles are a primary example.
- External forces act on the body from outside sources. Gravity is the most common; others include pushes, pulls, and shoves.
Kinetics vs. Kinematics
- The lecture notes mention a prior kinematics lecture; the difference between kinetics and kinematics should be known from previous learning.
Force: Magnitude and Direction
- Forces have magnitude (quantity of motion, measured in units like feet, pounds, inches, etc.) and direction (positive and negative components).
- Human motion is three-dimensional (x, y, and z axes), with x representing left/right, y representing up/down (superior/inferior), and z representing anterior/posterior.
Osteokinematic Motion and Axes of Rotation
- Osteokinematic motion, movement produced by forces, is demonstrated using axes:
- Medial-lateral (ML) axis (x-axis): allows for flexion and extension.
- Anterior-posterior (AP) axis (z-axis): allows for abduction and adduction.
- Vertical axis: allows for rotation.
- Joints have varying degrees of freedom and orientations of axes of rotation determining the allowed motions.
Example: Tug-of-War
- Tug-of-war illustrates internal and external forces, their magnitudes, and directions.
- The outcome depends on whether forces are equal or unequal.
Types of Forces
- Gravity: the weight of an object, the most prevalent force.
- Muscle force: internal force; active (contraction) and passive (stretching) components due to viscoelastic properties.
- Externally applied resistance: pulleys, dumbbells, manual resistance (e.g., manual muscle testing).
- Friction: force between contacting objects in motion.
Mass vs. Weight
- Weight: force of gravity on an object; varies with gravitational pull (e.g., less on the moon or at higher altitudes).
- Mass: amount of matter in an object; remains constant regardless of location.
Units of Measurement
- Mass: typically measured in kilograms (SI unit); slugs are also used in British and US systems, but the lecture does not typically use slugs.
- Weight: usually measured in pounds or Newtons.
Moment, Moment Arm, and Torque
- Moment: product of force and linear distance; quantifies force size.
- Moment arm: perpendicular distance between the axis of rotation and the line of force.
- Torque: product of force and moment arm in rotary motion; similar to a moment but specifically for rotational movement.
Reducing Joint Stress: Example with Ankle Weight
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Reducing the moment arm by flexing the knee (shortening the distance between the weight and the axis of rotation) decreases torque on the hip joint.
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This is useful in rehabilitation to reduce stress after surgery or injury.
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Internal moment arm: perpendicular distance between muscle and axis of rotation.
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External moment arm: perpendicular distance between an external force and the axis of rotation.
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Reducing the moment arm (from 4.5 feet to 4 feet) reduced torque from 450 foot-pounds to 400 foot-pounds while keeping the force constant (10 pounds).
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Description
This quiz explores the essential concepts of kinetics, focusing on the forces acting upon material bodies, including the human body. It distinguishes between internal and external forces and compares kinetics with kinematics. Additionally, it addresses the significance of force magnitude and direction in human motion.
