Work and Machines Chapter 5 Sections 1-3.

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Presentation transcript:

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>