Lecture 2: Statics Within the Body - Physics Lecture Notes PDF

# Lecture 2: Statics Within the Body ## Statics within the Body The image shows different classes of levers present in the human body. - The **first class lever** is shown on the left, where the fulcrum is located between the force and the resistance. - The **second class lever** is shown in the...

# Lecture 2: Statics Within the Body ## Statics within the Body The image shows different classes of levers present in the human body. - The **first class lever** is shown on the left, where the fulcrum is located between the force and the resistance. - The **second class lever** is shown in the middle, where the resistance is located between the fulcrum and the force. - The **third class lever** is shown on the right, where the force is located between the fulcrum and the resistance. ## Forces on the Body The forces on the body can be divided into 3 categories: - **Static Force:** Forces on the body that are at rest. - **Dynamic Force:** Forces on the body that are in motion. - **Frictional Force:** Forces on the body that oppose motion. ## Statics: Motion in One Plane and Levers ### Statics - Statics, in physics, is the subdivision of mechanics that is concerned with the forces that act on bodies at rest under equilibrium conditions. - Statics is the study of bodies and structures that are in equilibrium. For a body to be in equilibrium, there must be no net force acting on it. - Many problems involve motion in one plane, say the xy plane, for which z is a constant. For example, the motion of knees and elbows are in one plane. These types of problems can be classified as one of the three types of levers. ### Definition of Levers: A rigid bar that rotates around a fixed point or a fulcrum. - **Load:** The weight or resistance that is being moved. - **Effort:** The force that is being applied to the lever. - **Effort Arm:** The distance between the fulcrum and the point where the effort is applied. - **Resistance Arm:** The distance between the fulcrum and the point where the load is applied. ### Parts of Levers: - **Fulcrum:** Axis of rotation of the lever. - **Force (Effort) Arm:** The distance between the force and the axis. - **Resistance (Load) Arm:** The distance between the resistance and the axis. **Formula:** $Mdm = Wdw$ *Where*: - M = force - dM = force arm - W = Resistance - dW = Resistance arm. **Equation:** Force X Force Arm = Resistance X resistance Arm ## Classification of Levers There are 3 types of levers: ### 1. First Class Levers * The fulcrum is located between the weight (resistance) and muscle (force). * Both force and resistance are in the same direction. * Examples: - Seesaw - Scissor ## Examples of 1st Class Lever in Human Body - **The Head:** The head atop the spinal cord, where the weight of the head is balanced by the downward effective force of the muscles. - **Strength Training:** Seated dumbbell triceps extension. The triceps pull on the ulna about the elbow pivot balanced by the forces on the forearm. ## Levers in Human Body - The weight provides a torque of Wdw, while the muscle provides a torque of -MdM. So the total torque $T = Wdw - MdM = 0$ **Diagrams:** - **First Class Lever:** The fulcrum is located in the center. The load and effort are both on the same side of the fulcrum but are opposite each other. - **Second Class Lever:** The fulcrum is at the edge of the lever. The resistance is located in the center. The effort is applied on the other side of the fulcrum, but the effort is located closer to the fulcrum than the load. - **Third Class Lever:** The fulcrum is at the edge of the lever. The effort is applied closer to the fulcrum than the load and the resistance is located on the other side. ### 2. Second Class Levers * The force (muscle) and resistance (weight) act on the same side of the fulcrum. * The weight is nearer to the fulcrum. * M < W * Axis at one end, resistance in middle, force at other end. ## Examples of 2nd Class Lever in Human Body - **Standing on Tiptoes**: The rotation of the foot about the toes (the fulcrum), which would be caused by the weight of the foot, is balanced by the muscle force transmitted by the Achilles tendon. ## 3. Third Class Levers * The muscle and weight are on the same side of the fulcrum, but the muscle is nearer to the fulcrum than the weight. (Most common example in the body). * $dy < dw$, we see that M = (dw/dM)WW. ## Examples of 3rd Class Lever in Human Body - **The Elbow Joint:** When lifting a book, the elbow joint is the fulcrum across which the biceps muscle performs the work. ## Different Levers in Human Body The image shows all three types of levers in the human body. >*Note*: the image provides a visual example of lever classes in the human body, as well as some examples in the body, but it does not provide scientific evidence for the statements. **Additional Notes:** - Third class levers are the most common levers in the body because they allow for a greater range of motion and speed, but require more force. - The muscles in the body are often positioned to act as a third class lever, so that they can generate more speed and range of motion than if they acted as a first class lever.