Work & Simple Machines Chapter 4

Section 1 Work & Power

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.

motion

Horizontal force

Vertical force

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.

Section 2 Using Machines

  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 x 100%

eff = 0.6 x 100%

eff = 60%

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

fulcrum

Rod or plank

  Levers can be divided into classes depending on the position of the fulcrum.

First class lever:

Input force

Output force

Ex. a seesaw or a pair of scissors

Second class lever

Input force Output 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.