As shown in Figure 10.13 on page 306, toppling chimneys often break apart in midfall because the mortar between the bricks cannot withstand much shear stress. As the chimney begins to fall, shear forces must act on the topmost sections to accelerate them tangentially so that they can keep up with the rotation of the lower part of the stack. For simplicity, let us model the chimney as a uniform rod of length , pivoted at the lower end. The rod starts at rest in a vertical position (with the frictionless pivot at the bottom) and falls over under the influence of gravity. What fraction of the length of the rod has a tangential acceleration greater than g sin u, where u is the angle the chimney makes with the vertical axis?
Lecture objectives 16 1. Describe and identify the components of the motor unit A single motor neuron + fibers it controls = a motor unit 2. Describe the components of a muscle twitch Muscle twitch - a motor unit's response to a single action potential from the motor neuron Latent Period - first few milliseconds, excitation - contraction coupling starts, cross bridges formed but no measurable shortening Contraction Period - 10-100 ms, cross bridges active, reach peak of muscle tension, muscles may shorten Relaxation Period - 10-100 ms, initiated by Ca2+ re entry into the SR, decreased number of active cross bridges, muscle tension decreased to zero, muscle returns to initial length 3. What is muscle tension How is the difference from muscle tone Muscle tens