Chapter 2 Review of Forces and Moments - Brown University

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The value of μ0 is exactly 4π 10 −7 × H/m Electromagnetic forces between more generally shaped current carrying wires and magnets are governed by a complex set of equations. A full discussion of these physical laws is beyond the scope of this course, and will be covered in EN51. Applications of electromagnetic forces Electromagnetic forces are widely exploited in the design of electric motors; force actuators; solenoids; and electromagnets. All these applications are based upon the principle that a current-carrying wire in a magnetic field is subject to a force. The magnetic field can either be induced by a permanent magnet (as in a DC motor); or can be induced by passing a current through a second wire (used in some DC motors, and all AC motors). The general trends of forces in electric motors follow Ampere’s law: the force exerted by the motor increases linearly with electric current in the armature; increases roughly in proportion to the length of wire used to wind the armature, and depends on the geometry of the motor. Two examples of DC motors – the picture on the right is cut open to show the windings. You can find more information on motors at http://my.execpc.com/~rhoadley/magmotor.htm Hydrostatic and buoyancy forces When an object is immersed in a stationary fluid, its surface is subjected to a pressure. The pressure is actually induced in the fluid by gravity: the pressure at any depth is HMS Bounty p effectively supporting the weight of fluid above that depth. a d A pressure is a distributed force. If a pressure p acts on a surface, a small piece of the surface with area dA is subjected to a force dF=−p dA n p = p a + ρgd where n is a unit vector perpendicular to the surface. The total force on a surface must be calculated by integration. We will show how this is done shortly. The pressure in a stationary fluid varies linearly with depth below the fluid surface p = pa + ρ gd where p a is atmospheric pressure (often neglected as it’s generally small compared with the second term); ρ is the fluid density; g is the acceleration due to gravity; and d is depth below the fluid surface.
Archimedes’ principle gives a simple way to calculate the resultant force exerted by fluid pressure on an immersed object. The magnitude of the resultant force is equal to the weight of water displaced by the object. The direction is perpendicular to the fluid surface. Thus, if the fluid has mass density ρ , and a volume V I of the object lies below the surface of the fluid, the resultant force due to fluid pressure is F Center of mass of submerged portion V I = ρgV I j F Volume V I lies below fluid surface The force acts at the center of buoyancy of the immersed object. The center of buoyancy can be calculated by finding the center of mass of the displaced fluid (i.e. the center of mass of the portion of the immersed object that lies below the fluid surface). The buoyancy force acts in addition to gravity loading. If the object floats, the gravitational force is equal and opposite to the buoyancy force. The force of gravity acts (as usual) at the center of mass of the entire object. Aerodynamic lift and drag forces Engineers who design large bridges, buildings, or fast-moving terrestrial vehicles, spend much time and effort in managing aeroor hydro-dynamic forces. Hydrodynamic forces are also of great interest to engineers who design bearings and car tires, since hydrodynamic forces can cause one surface to float above another, so reducing friction to very low levels. V F L (Lift acts perpendicular to flow) F D Flow is asymmetric near airfoil (Drag acts parallel to flow) In general, when air or fluid flow past an object (or equivalently, if the object moves through stationary fluid or gas), the object is subjected to two forces: 1. A Drag force, which acts parallel to the direction of air or fluid flow 2. A Lift force, which acts perpendicular to the direction of air or fluid flow. The forces act at a point known as the center of lift of the object – but there’s no simple way to predict where this point is. The lift force is present only if airflow past the object is unsymmetrical (i.e. faster above or below the object). This asymmetry can result from the shape of the object itself (this effect is exploited in the
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Chapter 2 Review of Forces and Moments - Brown University

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