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# UNC-Chapel Hill PHYS 115 - Exam 1 Study Guide

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PHYS 115 1st EditionExam # 1 Study Guide Lectures: 1 - 5Lecture 1 - What are the formulas for work, kinetic energy, potential energy, and total energy?o W = Fdcos(theta)o K = ½ mv2o Ugrav = mgyo Uspring = ½ kx2o E = K + U- Describe forces in relation to the vector.o Opposite to vector = negativeo Perpendicular to vector = zeroLecture 2- Are gases fluids?o Yes.- What does incompressible mean?o Molecules can get as close to water as possible.- What is density? Does it change?o Ratio of mass to volume.o The density of a particular substance, ie. Water does not. - Describe pressure.o Force over areao Increases over a smaller area if the same force is applied.- What is the formula for change in pressure?o Density x gravity x depth- What direction does pressure come from?o All directionso Walls of the container, fluid below, above, and beside.- What is the formula for pressure at a given depth?o- How does pressure across a horizontal line compare at different points?o It is the same in one fluid.o It only increases with depth.- What is Pascal’s Principle?o Change the pressure at one point; all points experience a pressure change.- What is Gauge Pressure?o Is __ in excess of atmospheric pressureo 10 psi of gauge pressure is actually 25 psi in a basketball.Lecture 3- What is the Volume flow rate Q?o Aka “flux”o The rate at which a volume of fluid flows through a tubeo- What is the Equation of Continuity?o- What are the properties of an ideal fluid?o incompressible (i.e., density is constant)o flow is steady (i.e., flow is laminar not turbulent)o nonviscous (i.e., the fluid has no resistance to flow)- The aorta has a cross-sectional area of 3.0 cm2. Blood moves through the aorta at approximately 30 cm/s. A capillary has a cross-sectional area of 2.8 x 10-7 cm2. Blood moves through a capillary at approximately 0.05 cm/s. Compare the speeds.o The aorta’s cross-sectional area must be greater than the sum of all the cross-sectional areas of all the capillaries in the body.o The speed of the capillaries is slower than the aorta, so the cross sectional area must be greater to keep the two volume flow rates equivalent.o The speed of the capillaries is slower than the aorta, so the cross sectional area must be greater to keep the two volume flow rates equivalent.Lecture 4- Describe viscosity (η).o Resistance to flowo Increase in viscosity, slower the flowo Units of Pa*so Units of centipoise (10-3 Pa*s)o Requires a change in pressure to overcome drag- Does viscosity have a single speed?o No, the speed increases away from the wall.o Friction is created from the wall.- How do you measure speed in a viscous fluid?o Finding the average speed (Q)o L decreases the flow rate because a longer tube has more frctionLecture 5- What is a Reynolds number?o The ratio of inertial to drag forces- Viscosity increases, drag __.o Increases - Size of object increases, drag __.o Increases- Speed increases, drag __.o Increases- Drag force of a fluid of viscosity η on a sphere of diameter d moving at slow speed v:o- What is inertial force?o The force needed to accelerateo What you exerto The harder to change motion, the more inertia- Which is harder to change motion? Fast or slow?o Fast- Increase velocity, __ inertia.o Increase - Increase density, __ inertia.o Increase - Describe Reynolds number.oo Where n =hr- When Re is small, __ forces dominate.o Viscous- When RE is large, __ forces dominate.o

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