Chapter 7
Published on Mar 18, 2016
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PRESENTATION OUTLINE
LEVERS
Mechanical work(W) occurs when a force acts on an object to move it in a direction parallel to the force vector, W=Fd.
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Power(P) is the rate of doing work. It is proportional to the amount of work done and inversely proportional to the amount of work done and inversely proportional to the time needed to do it. P=W/∆t
Photo by Ole Houen
The basic mechanical parts that compose more complex machines are called simple machines.
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According to the first law of thermodynamics, you cannot get more work out of a machine than you put it in.
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A machine's efficiency is a measure of how well it converts input energy into usable energy. efficiency=W(out)/W(in) x 100%
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A system moved by a machine is called the load or resistance. The direct force needed to just begin moving the resistance with negligible acceleration is the resistance force(Fr). This also sometimes considered to be the maximum force the resistance can exert on the simple machine. The force exerted by the user of a simple machine or by the source of energy for the machine is the effort force (Fe).
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A machine's mechanical advantage (MA) is a measure of its effectiveness in reducing the effort required to do work. A machine's ideal MA (IMA) can be calculated in a number of ways. One method compares the distances over which the effort is exerted to the distance the resistance moves. IMA=De/Dr
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Because no machine is 100%efficient, a simple machine's actual mechanical advantage (AMA) is lower than its ideal mechanical advantage. The AMA is calculated by comparing the resistance force to the actual effort force exerted to move the resistance. AMA=Fr/Fe.
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A simple machine compensates for a smaller effort force by exerting that force over a longer distance. This is because work and energy put into a machine must at least equal the work out. This is called the distance principle.
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A lever is a rigid bar resting on a pivot point called fulcrum.
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Systems that can rotate around a central point are affected by torques or moments. A torque or Moment(T) is the product of a force acting perpendicularly to the lever arm the distance it s applied from the point of rotation.
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A balanced lever that has no net torque acting on it experiences rotational equilibrium. This condition is described by the law of moments. F1 L1=F2 L2
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In a first-class lever, the fulcrum is located between the effort and the resistance. Its MA may be greater than, less than, or equal to 1.
Photo by Hubble Space Telescope / ESA
The fulcrum of a second-class lever is at the end of the lever arm, and the resistance is between the fulcrum and the effort. Its MA is always greater than 1.
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A third-class lever is arranged so that its effort is between the resistance and the fulcrum.Its MA is always less than 1.
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Wheels and axles, gears, and pulleys are related simple machines. Wheel and axle
systems overcome the motion limits of simple lever systems by being able to continuously rotate.
Photo by Pierre J.
A pulley is a wheel with a groove in its rim in which a rope, cable, or chain runs,A system of pulleys and cables is called a block and tackle or a pulley gang.
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An inclined plane is a two-dimensional tilted surface that allows a resistance load to be moved with less effort form a lower to a higher position than lifting it vertically between the two positions. The IMA of an inclined plane compares the length of its surface (L) to the vertical distance between the lower and upper ends of the surface.(IMA=L/H).
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A wedge is a three- dimensional object similar in appearance to an inclined plane that exerts a force to spread material apart when it is driven into the material.
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A screw is a fastener that exerts a large force to hold objects together. It consists of a long, thin wedge wrapped around a cylinder to form a helix.
Photo by Nigel_Brown
