Work and Machines

Work and Machines Chapter 5 Sections 1-3

C5- Work & Machines • Section 1- Work slides 3-13 • Section 2- Using Machines slides 14-30 • Section 3- Simple Machines slides 31-50

Section 1- Work • What You’ll Learn: • What work is • How work & energy are related • How to calculate work & power

Work • Work is done when a force causes an object to move in the same direction that the force is applied. • When a force makes an object move, energy is transferred from one object to another. <www.mcasco.com/p1wke.html>

How do you know if you are doing work? • Two things must happen: • A force must be applied to the object • The object must move in the same direction as the force

What does direction have to do with work? • When you pick up a stack of books, your arms apply a force upward. • You start walking with the books. • The direction of motion has changed; your arms are no longer working, instead your legs supply the forward direction <http://pixiestixkidspix.files.wordpress.com/>

Work & Energy • Energy is the ability to cause change or to do work. • When work is done energy is transferred from one object to another. <http://www.mnsu.edu/news/article>

Calculating Work • You multiply force times distance to calculate work. • Work (joules)= force (N) X distance (m) • W=Fd • Calculate the amount of work a painter does when he lifts a can of paint weighing 40 newtons 2 meters. • W=Fd=(40N) (2m)= 80 J

When is work done? • When a pitcher throws a ball to the catcher, he applies force to the ball only when it is in his hand. • The ball moves 10 m after it leaves his hand but work occurred only while it was in his hand for 1m. <http://www.statenews.com/media/>

Power • Power is the rate at which work is done. • Something has more power if it can do the same amount of work in less time. <http://www.nyu.edu/classes/>

How do you calculate power? • To calculate power, divide the amount of work done by the time it takes to do it. • Power (in watts)= work (in joules)/time(s) • P=W/t The SI unit for power is the watt. • Find the power of the machine that can do 5,000 joules of work in 20 seconds. • P=W/t=5000J/20 s=250 watts. The power of the machine is 250 watts.

How is power calculated when energy is transferred? • You can also calculate power by dividing the amount of energy transferred by the time it took to transfer it. • Power (in watts)=energy transferred (j)/ time (s) or P=E/t.

How is energy transferred when no work is done? • Suppose a light bulb changes electrical energy into light & heat at a rate of 100 j /2 s. How many watts of power will the light bulb have? • P=E/t =100j/2s= 50 watts

Section 2- Using Machines • What You’ll Learn: • How machines make work easier • How to calculate mechanical advantage • How to calculate the efficiency of a machine

What is a machine? • A machine is a device that makes work easier. • Knives, scissors, and doorknobs are simple machines. • Engines are more complex machines <http://www.mshp.dps.missouri.gov/MSHPWeb/PatrolDivisions>

Making Work Easier • There are 3 ways machines make work easier: • Increasing force on an object; • Increasing the distance over which a force is applied; or • Changing the direction of an applied force.

How can force be increased using a machine? • If Work= force X distance, then either force or distance increases. • If distance increases, then force decreases. • Machines such as a car jack, increase the distance a force is applied thus reducing the amount of force needed to do the same amount of work. <http://www.dynamicscience.com.au/tester/solutions/hydraulicus/gearrackandpinioncarjack1.gif>

How does increasing distance decrease force? • A rake makes the work easier by increasing the distance over which you apply force. • Your hands move a small distance at the top of the handle while the handle moves across a wider distance. <http://themongiatkellyclan.ca/wordpress/wp-content/uploads/2006/11/raking-leaves.jpg>

Why do you want a machine that will change direction? • Sometimes it’s easier to apply force in a different direction: • To raise a flag up a pole, it’s easier to pull down on a rope than to climb to the top. • When you use an axe to split wood, a downward force on the handle changes into a sideways force by the blade.

The Work Done by Machines • Pushing down on the handle raising the lid. • You are doing work on the crowbar that opposes the friction of the nails in the lid and crate. <http://images.jupiterimages.com/common/detail/82/80/23298082.jpg>

What are input forces & output forces? • A machine changes the way you do the work making it easier. • The force you apply to the machine is called the input force or Fin. • The force applied by the machine is called the output force or Fout. <http://img2.timeinc.net/toh/i/a/solutions/nail-pulling-01.jpg>

What are input work & output work? • When you use a machine there are 2 kinds of work: • Input work or Win , done by you • Output work or Wout, done by machine

How do machines use conservation of energy? • When you do work on a machine, you transfer your energy to the machine which then transfers energy to the object. • So, Wout is never greater than Win. • However, a machine does not transfer all its energy to the object. Due to friction, some of the energy changes to heat. • This means that Wout is always smaller than Win.

What is an ideal machine? • If you could build perfect machine without any friction, the input work would equal the output work. • Win=Wout or FinX din=FoutX dout • If the machine could increase the input force, then work output would be greater than the work input.

Mechanical Advantage • Some machines make work easier by making the output force greater than the input force. The number of times the applied force is increased by a machine is called the mechanical advantage (MA) of the machine. Mechanical advantage is the ratio of the output force to the input force. MA = Fout/Fin

Mechanical Advantage • Using a pulley system you only need 300 N to lift a piano that weighs 1500 N. What is the MA? • MA = Fout/Fin = 1500 N/300 N= 5 • Notice this ratio cancels out units of newtons.

What is ideal mechanical advantage? • The MA of a machine without friction is called the ideal MA or IMA. • You can calculate this by dividing the input distance by the output distance. <http://www.cpo.com/images/products/em-ropesandpulleygirl.jpg>

Efficiency • Some of the energy put into a real machine is changed into heat by friction. • So, the output work of a machine is always less than the work put into it. • Efficiency is the comparison of the amount of work put into a machine to the amount of work the machine puts out. • High-efficiency means less heat from friction!

How do you calculate efficiency? • Divide the output work by the input work to get a percentage. • Efficiency= Wout/in X 100% • To calculate the efficiency of a machine with Win of 50 joules & a Wout of 40 joules: 40/50=0.8, or 80%. • The efficiency of a real machine is always less than 100% due to friction.

How can machines be made more efficient? • Reducing friction by adding oil or grease to the surfaces that rub together fills the gaps between them so the surfaces slide across each other more easily.

Section 3- Simple Machines • What You’ll Learn: • Six types of simple machines • How simple machines make work easier • How to calculate the ideal mechanical advantage of simple machines

Types of Simple Machines • A simple machine is a machine that does work with only one movement of the machine. • There are six types: lever, pulley, wheel & axle, inclined plane, screw, and wedge. • The screw and wedge are different forms of the inclined plane.

Levers • A wheelbarrow, a rake & a baseball bat are all examples of levers. • A lever is a bar that pivots, or turns around, a fixed point called the fulcrum. • The input arm is the distance from the fulcrum to the point where the input force is applied; the output arm is the distance from the fulcrum to the point where the lever exerts the output force.

What are the 3 classes of levers? • The class of a lever is based on the location of the fulcrum, the input force and the output force. <http://www.daviddarling.info/images/levers.jpg>

First-Class Lever • The top figure shows a first-class lever with the fulcrum located between the input and output forces. • The first-class lever always changes the direction of the force. • Examples include a crowbar, scissors, and a seesaw. <http://www.daviddarling.info/images/levers.jpg>

Second-Class Lever • The middle figure represents a second-class lever with the output force between the input force and the fulcrum. • Both input and output forces move in the same direction. • The wheelbarrow is a classic example. <http://www.daviddarling.info/images/levers.jpg>

Third-Class Lever • The bottom figure, a third-class lever, shows the output force is farther away from the fulcrum than the input force. • The output force is always less than the input force in a third-class lever such as a baseball bat, but the advantage is that it increases the distance over which the output force is applied. <http://www.daviddarling.info/images/levers.jpg>

How is ideal mechanical advantage of a lever calculated? • To calculate IMA of any machine, divide the input distance by the output distance. • For a lever, the input distance is the length of the input arm & output distance is the length of the output arm. • IMA= Lin/Lout

Pulleys • To raise a sail upward, a sailor pulls down on a rope wrapped around a pulley. • A pulley is a grooved wheel with a rope, chain or cable wrapped around it. • Pulleys may be fixed or movable or in systems.

What is a fixed pulley? • This modified first-class lever changes the direction of the input force like on a sail or a flagpole. • An elevator also uses a fixed pulley with a cable. <http://www.legoeducation.com/sharedimages/content/Large/L_Pulley_dia1_fixed.gif>

What is a movable pulley? • A movable pulley has one end of the rope fixed & the wheel is free to move. • The movable pulley doesn’t change the direction of the force, but it does decrease the amount of input force needed to lift the object. MA=2 <http://goldridge08.com/pictures/simple/actpulmov.gif>

What is a block and tackle? • A block and tackle is a system of fixed & movable pulleys used together. • The more sections of the rope a system uses to pull up an object, the greater the output force is. IMA=# of sections <http://student.britannica.com/eb/art-4560>

Wheel and Axle • Simple machine with an axle attached to the center of a larger wheel and both turn together. • Doorknobs and ferris wheels are examples. <http://www.dkimages.com/discover/previews/741/131247.JPG>

What is the IMA of the wheel & axle? • A wheel and axle is a modified lever with the center of the axle as the fulcrum. • To calculate the IMA of a wheel and axle, use this equation: IMA= radius of wheel (m)/radius of axle (m) or IMA= rw/ra • To increase IMA, simply increase the radius of the wheel.

How do gears work? • A gear is a wheel and axle with teeth around the rim of the wheel. • One gear makes the other turn with the smaller gear turning more times than the larger one. Output force & direction can be changed with a gear. <http://www.coeshow.com/shop/images/uploads/CRF150-Gears.jpg>

Inclined Planes • An inclined plane is a sloping surface that reduces the amount of force it takes to do work. • Examples include ramps and stairways. <http://www.jaha.org/edu/inclined_plane/images/IP1_000.jpg>

How does an inclined plane make work easier? • You do the same work by lifting a box straight up or pushing it up a ramp. • As the inclined plane becomes longer, the force needed to move the object becomes less. • The input force is applied over a longer distance, so it takes less input force. IMA=length of slope(m)/ height of slope(m)

The Screw • A screw is an inclined plane wrapped in a spiral around a post. • The inclined plane forms the threads on the screw. • Apply force by turning the screw; friction holds it in place. • Examples: jar lid, corkscrew, drill bit, light bulb. <http://cnx.org/content/m13594/latest/screw.gif>

The Wedge • A wedge is an inclined plane with one or two sloping sides. • Like the screw, the inclined plane moves through the object. Knives are wedges. • The IMA increases as it gets longer & thinner. <http://www.canadianhomeworkshop.com/stuff/photos/oct03b.jpg>

Compound Machines • Some machines, like this can opener, are made of several simple machines. • Two or more working together are called a compound machine. • The handles are levers, a wedge pierces the can, a wheel & axle turns to open the can. <http://www.focuspg.com/itm_img/709-lg.jpg>

Work and Machines