Work & Simple Machines Chapter 4
Work & Power Section 1
Work Work - occurs when a force causes an object to move in the same direction that the force is applied. Work involves motion, not just effort. Work is done only when the force you exert on an object is in the same direction as the objectʼs motion. Lifting a clothes basket is work Carrying it while walking is not work
When a force is exerted at an angle, only the part of the force that is in the same direction as the motion does work. Horizontal force Vertical force motion
Calculating work (formula) Work = force x distance W = Fd The SI unit for work is the Joule (J) W = F = 40N d = 2m W = Fd W = (40N)(2m) W = 80 N. m W = 80 J
The distance in the work equation is the distance an object moves only while the force is being applied.
Power Power - how quickly work is done. power = Work or p = W time t The SI unit for power is the watt
p = W = 200 J t = 12 s p = W t p = 200 J 12 s p = 17 watts
Doing work on an object increases the objects kinetic energy The amount of work done is the amount of energy transferred and can be expressed in the power formula in place of work done: (power = energy transferred / time needed) Power is always the rate at which energy is transferred.
Using Machines Section 2
Machine - device that makes doing work easier. Machines change the way a person does work, not the amount of work that needs to be done. Input force - the effort, or work, force you exert on a machine Output force - the resistance force, or the work a machine does to move an object over some distance.
When using a machine, the output work can never be GREATER then the input work. Do not get output work confused with output force!
Mechanical Advantage - number to times the input force is multiplied by a machine. Mechanical Advantage = output force Input force Mechanical Advantage = 500N 50N Mechanical Advantage = 10 There are no units for mechanical advantage; they cancel out
Some machines make work easier by allowing you to exert a smaller force over a longer distance, resulting in a mechanical advantage of more than one. Other machines allow you to exert your force over a short distance resulting in a mechanical advantage of less than one. Still other machines allow you to change the direction of input force resulting in a mechanical advantage equal to one.
Efficiency - ability of a machine to convert input work to output work Efficiency = output work input work x 100% eff = Wout Win x 100% Wout = 4500 J Win = 7500 J eff =
Using a pulley system a crew does 7500 J of work to load a box that requires 4500 J of work. What is the efficiency of the pulley system? Wout = 4500 J Win = 7500 J eff = eff = Wout Win x 100% eff = 4500J 7500J eff = 0.6 x 100% eff = 60% x 100%
Friction - reduces efficiency by converting some work into heat. The efficiency of a real machine is always less than 100% because of friction. Oil, or another lubricant, can increase efficiency by reducing the number of contact points between surfaces.
Section 3 Simple machines
Simple machine - does work with only one movement Compound machine - a machine made of a combination of simple machines.
Inclined plane - a flat, sloped surface Less force is needed to move an object from one height to another using an inclined plane than is needed to lift the object As the inclined plane gets longer, the force needed to move the object gets smaller. The mechanical advantage of an inclined plane is the length of the inclined plane divided by its height.
Wedge - an incline plane that moves. It changes the direction of the applied force. Ex. your teeth, an axe.
Screw - inclined plane wrapped around a cylinder or post. The screw threads form the inclined plane on a screw mechanical advantage: of the screw is the length of the inclined plane wrapped around the screw divided by the length of the screw. *the more tightly the threads are together the easier it is to turn the screw.
Lever - any rigid rod or plank that pivots about a point. Fulcrum - the point about which the lever pivots Mechanical advantage: divide the distance from the fulcrum to the input force by the distance from the fulcrum to the output force Rod or plank fulcrum
Levers can be divided into classes depending on the position of the fulcrum. First class lever: Output force Input force Ex. a seesaw or a pair of scissors
Second class lever Output force Input force Ex. Wheelbarrow
Third class lever Input force Output force Ex. Baseball bat or rake
Wheel and axle - two circular objects of different sizes that rotate together. Mechanical advantage: divide the radius of the wheel by the radius of the axle.
In some cases, the input force turns the wheel, and the axle exerts the output force, resulting in a mechanical advantage greater than 1 Ex. Doorknob, a steering wheel, and a screwdriver In other cases, the input force turns the axle, and the wheel exerts the output force, resulting in a mechanical advantage of less than 1 Ex. Fan, Ferris wheel, Wheel
Pulley - grooved wheel with a rope or chain wrapped around it Fixed pulleys, such as on window blinds or flag poles, are attached to an overhead structure and change the direction of the force you exert; they have a mechanical advantage of 1. Moveable pulleys are attached to the object being lifted and allow you to exert a smaller force; they have a mechanical advantage of 2. Pulley system - combination of fixed and moveable pulleys.