Devil physics The baddest class on campus Pre-DP Physics

Giancoli Lesson 9-4 to 9-5 Application to muscles and joints Stability and balance

Objectives Apply the concepts learned in static equilibrium to problems involving the muscles and joints of the human body Calculate the force required by muscles to perform different functions Understand the physics behind why humans are susceptible to lower back pain Know the meaning of stable equilibrium, unstable equilibrium and neutral equilibrium

Introductory Video: How the Body Works - Muscles

Muscles and Joints Muscles attached to two different bones Attachment points called insertions Bones attached at joints Muscles can only contract and relax Muscles are normally paired to extend (extensor muscles) and to contract (flexor muscles)

Example Problem What force must be applied: (a) to hold a 15-kg dumbbell at your side, (b) to hold it at a 45° angle to your body, and (c) at a 90° angle to your body? Neglect the mass of the arm. The person is 1.8m tall.

Example Problem What force must be applied: (a) to hold a 15-kg dumbbell at your side, (b) to hold it at a 45° angle to your body, and (c) at a 90° angle to your body? Neglect the mass of the arm. The person is 1.8m tall. (a) With the dumbbell at your side, tension in your arm and shoulder support the weight of the dumbbell. There is no torque generated.

Example Problem (b) With the dumbbell at a 45° angle, there is a moment arm for both the insertion and the arm, but less than that for an arm at 90°.

Example Problem (c) With the dumbbell at a 90° angle, the moment arm for both the insertion and the arm is at a maximum. Will the force at 90° be greater than, less than, or the same as the force at 45°?

Example Problem (c) With the dumbbell at a 90° angle, the moment arm for both the insertion and the arm is at a maximum. The force is the same as at 45°.

Lower Back Pain Using the diagrams below, calculate the magnitude and direction of the force on the fifth lumbar vertebra.

Lower Back Pain Using the diagram on the right, calculate the magnitude and direction of the force on the fifth lumbar vertebra. First we need to find the force of the muscles using Στ. 18° 60° FM 12° 30°

Lower Back Pain Using the diagram on the right, calculate the magnitude and direction of the force on the fifth lumbar vertebra. First we need to find the force of the muscles using Στ. Find the perpendicular components of the weights wx 60°

Lower Back Pain Using the diagram on the right, calculate the magnitude and direction of the force on the fifth lumbar vertebra. First we need to find the force of the muscles using Στ. Find the perpendicular components of the weights Now find the perpendicular component of FM 18° 60° FM 12° 30°

Lower Back Pain Using the diagram on the right, calculate the magnitude and direction of the force on the fifth lumbar vertebra. First we need to find the force of the muscles using Στ.

Lower Back Pain Using the diagram on the right, calculate the magnitude and direction of the force on the fifth lumbar vertebra. Now we need to find Fv by finding its components. First find vertical and horizontal components of FM FM-x FM-y 18° 60° 12° FM 30°

Lower Back Pain Using the diagram on the right, calculate the magnitude and direction of the force on the fifth lumbar vertebra. Now we need to find Fv by finding its components. First find vertical and horizontal components of FM FM-x FM-y 18° 60° 12° FM 30°

Lower Back Pain Using the diagram on the right, calculate the magnitude and direction of the force on the fifth lumbar vertebra. Now we need to find Fv by finding its components. Now find vertical and horizontal components of FV through ΣFx and ΣFy FM-x FM-y 18° 60° 12° FM FV-x FV-y FY 30°

Lower Back Pain Using the diagram on the right, calculate the magnitude and direction of the force on the fifth lumbar vertebra. Now we need to find Fv by finding its components. Now find vertical and horizontal components of FV through ΣFx and ΣFy FV-x FV-y FY

Lower Back Pain Using the diagram on the right, calculate the magnitude and direction of the force on the fifth lumbar vertebra. Now we need to find Fv by finding its components. Pythagorize and Tangentiate FV-x FV-y FY

Lower Back Pain This is why you should lift with your legs and your back straight. While the vertebra and discs still have to support the weight, it drastically reduces required back muscle force. wx wx 60° FM-x FM-y 18° 60° 12° FM FV-x FV-y FY 30° 60°

Personal Testimony

Stability and Balance A body in static equilibrium will not move if left undisturbed What happens if it is disturbed? Depends on Balance and Stability Stable Equilibrium – body returns to its original position Unstable Equilibrium – body continues to move in the direction of displacement and may accelerate Neutral Equilibrium – body stays in its displaced position

Stability and Balance Stable equilibrium

Stability and Balance Unstable equilibrium

Stability and Balance Neutral equilibrium

Stability and Balance Three Cases

Stability and Balance Instability occurs when the center of gravity is no longer above its base of support Potential for instability increases as the distance between CG and base increases Potential for instability increases as the size of the base decreases

Stability and Balance General Rule: Stability can be increased if you can lower the center of gravity and/or increase the size of the base of support

Stability and Balance Reading Activity Question: Tilt the boxes and truck backward until the center of mass is over the axle of the hand truck. The hand truck is then supporting all of the weight of the boxes and itself.

Stability and Balance Which is more stable, a human or a dog?

Stability and Balance Which is more stable, a human or a dog? Does this imply that most humans are unbalanced? You be the judge.

Σary Review Can you apply the concepts learned in static equilibrium to problems involving the muscles and joints of the human body? Can you calculate the force required by muscles to perform different functions? Do you understand the physics behind why humans are susceptible to lower back pain? Do you know the meaning of stable equilibrium, unstable equilibrium and neutral equilibrium?

Questions

Homework #34-42 (skip #41)

Muscles of the Body