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14.1: Machines help people do work  Machine:  Any device that helps people do work.  Work is the use of force to move an object  Does not decrease.

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Presentation on theme: "14.1: Machines help people do work  Machine:  Any device that helps people do work.  Work is the use of force to move an object  Does not decrease."— Presentation transcript:

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2 14.1: Machines help people do work  Machine:  Any device that helps people do work.  Work is the use of force to move an object  Does not decrease the amount of work that is done  Only changes the way the work is done  Example:  Ramp  pulley

3 How do machines help?  Machines make the work easier by changing:  The size of the force needed to do the work  the distance over which the force is applied  The direction in which the force is exerted  Can be powered by different types of energy depending on the type of machine:  Electronic machines use electrical energy  Mechanical machines use mechanical energy  Mechanical energy is usually supplied by the person using the machine

4 Changing size and direction  Some machines help by changing the size of the force needed  If a machine allows you to exert less force, you must apply that force over a greater distance:  Total amount of work remains the same whether a machine is used or not  Work = force x distance  Because a machine does not decrease the amount of work to be done, less force must mean a greater distance

5  Some machines allow you to apply a greater input force over a shorter distance (rake)  You will move your hand a shorter distance to move the end of the rake a longer distance  Input force:  The force exerted on a machine  When using a rake, the input force is the force from the boy on the rake  Output force:  The force that the machine exerts on an object  This is the force that the rake exerts on the leaves

6  Machines can help you do work by changing the direction of a force:  Flagpole:  When raising the flag up the flagpole, you pull down on the rope to raise the flag up the pole  The rope system does not change the size of the force, only the direction.  The force pulling the flag up is equal to the force you apply in your downward pull  Shovel:  Once you have the shovel in the ground, you push the handle down to lift the dirt up.  A shovel also changes the size of the force you apply-you need less force to lift the dirt

7 Mechanical advantage of a machine  Mechanical advantage  MA  The number of times a machine multiplies the input force  Formulas to know:  MA = output force ÷ input force  F out = MA x F in  F in = F out ÷ MA

8  If your machine allows you to apply less force over a greater distance (doorknob) the output force is greater than the input force; MA is greater than 1  For machines that allow you to apply greater force over a shorter distance (rake) the output force is less than the input force; MA is less than 1  For machines that change only the direction of a force (rope system of a flagpole) the input and output forces are the same; MA is equal to 1

9  The output force of a machine is 600N and the input force is 200N. What is the MA of the machine?  A machine has an input force of 150N and a MA of 0.5. What is the output force?  The output force of a machine is 135N and the MA is 2.5. What is the input force?

10 Work transfers energy  Machines transfer energy to objects on which they do work.  If the machine lifts an object it gives off potential energy  The higher you lift an object, the more work you do and the more energy you give the object  A machine that causes an object to move gives the object kinetic energy

11 Output work is always less than input work  Efficiency:  The ratio of a machine’s output work to the input work  An ideal machine would be 100% efficient so all of the input work would be converted to output work (not possible due to friction)  Calculate efficiency:  Efficiency (%) = (output work ÷ input work) x 100

12  If someone does 500J of work on a pair of pliers and the pliers do 300J of work on a wire, what is the efficiency of the pliers? E = Output ÷ Input x 100 Output = Input =

13  The more efficient the machine, the less mechanical energy is lost  Some energy is lost to heat (friction)  The more moving parts the machine has, the more energy is lost to friction  Car engine:  Efficiency is only about 25% due to the heat generated  Typical electric motors are about 80% efficient  Increase efficiency:  Decrease friction  Decrease air resistance

14 14.2:Six simple machines  There are six machines on which all other mechanical machines are based:  Inclined plane  Lever  Wheel and axle  Pulley  Wedge  screw

15 Lever:  A solid bar that rotates, or turns, around a fixed point (fulcrum)  Bar can be straight or curved  Can multiply the input force  Can also change the direction of the input force  3 classes of levers all with different arrangements of the fulcrum, input force (effort), and output force (resistance)

16  First-class lever:  Fulcrum is located between the input force and the output force  Used to change the direction and size of the force  Second-class lever:  Output force is located between the input force and the fulcrum  Used when a greater output force is needed  Third-class lever:  Input force is between the output force and the fulcrum  Used to reduce the distance over which you apply the input force OR increase the speed of the end of the lever

17 1 st class lever system 2 nd class lever system 3 rd class lever system

18 Wheel and Axle  Made of a wheel attached to a shaft or axle  Act as a rotating collection of levers  Axle at the wheel’s center is like a fulcrum  Screwdrivers, steering wheels, doorknobs, electric fans

19 Pulley  A wheel with a grooved rim and a rope or cable that rides in the groove  As you pull the rope, the wheel moves  Fixed pulley:  Pulley that is attached to something that holds it steady  Makes work easier by changing the direction of the force  You must apply enough force to overcome the weight of the load and any friction  Distance you pull the rope is the same distance that the object is lifted

20  Movable pulley:  One end of the rope is fixed but the wheel can also move.  Load is attached to the wheel  Person pulling the rope provides the output force that lifts the load  Single movable pulley does not change the direction of the force-it multiplies the force  You would need only half the force which means you need twice the distance  Block and tackle system  Contains both fixed and movable pulleys  Used to haul and lift very heavy objects

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22 Inclined plane  A simple machine made of a sloped surface (ramps)  Makes work easier by supporting the weight of the object over the distance it travels  The less steep the incline, the less force you need  Which means the distance will increase

23 Wedge  Simple machine with one thick edge and one thin edge  Used to cut, split, or pierce objects; also to hold objects together  Can be as simple as a doorstop, a chisel, or an ice scraper, blade of an ax  Angle of cutting edge determines the input force needed (thick wedge with large angle needs more force to cut)  Thin edges provide a smaller surface area for the force to act upon

24 Screw  Inclined plane wrapped around a cylinder or cone to form a spiral  Used to raise and lower weights and to fasten objects  Distance between the threads of the screw determine the amount of force needed:  Threads close together = less force over greater distance

25 Calculating MA of specific machines  Inclined plane:  Ideal MA = length of incline ÷ height of incline  Wheel and Axle:  Ideal MA = Radius of input ÷ Radius of output  Lever:  Ideal MA = distance from input force to fulcrum distance from output force to fulcrum


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