Work Work occurs when a force causes an object to move in the direction of the force.

Transcription

1 Work and Machines

2 Work Work occurs when a force causes an object to move in the direction of the force. You just finished reading and summarizing an entire chapter in your science textbook. In the scientific sense, you did NO work at all! Your dad just came home from a night out bowling with his friends. He had lots of fun and did lots of work. Applying a force to the bowling ball to make it move through a distance is WORK.

3 Working Hard or Hardly Working? Applying a force doesn t always result in work being done. Work is done on an object if two things occur: 1. The object moves as a force is applied. 2. The direction of the objects motion is the same as the direction of the force applied.

4 Calculating Work Work is a force applied through a distance. The greater the distance through which you exert a given force, the more work you do. The greater force you exert through a given distance, the more work you do. The amount of work (W) done in moving an object can be calculated by multiplying the force (F) applied to the object by the distance (d) through which the force is applied. W = F d Remember, force is expressed in newtons, and the meter is the basic SI unit for length or distance. Therefore, the unit used to express work is the newton-meter, which is more simply called the joule (J).

5 Power How Fast Work Is Done Power is the rate at which work is done. To calculate power (P), divide the amount of work done (W) by the time (t) it takes to do the work. P = The unit used to express power is joules per second (J/s), or the watt (W). W t

6 Questions 1. Your neighbor asks you to help push his stalled car. You push and push, but the car doesn t move. You are exhausted and sweaty. Have you done any work? 2. What is work? 3. You lift a chair that weighs 50 N to a height of 0.5m and carry it 10m across the room. How much work did you do on the chair? 4. Explain the work/power difference between driving nails with a hammer and using a nail gun. 5. How would it feel if you were power and your enemy was work?

7 Machines Making Work Easier A machine is a device that helps make work easier by changing the size or direction of a force. Examples: Using a jack and tire iron to change a flat tire Using a screwdriver to pry the lid off a paint can Using a wheelbarrow to haul a load of rocks Give 3 other examples of machines that make work easier.

8 Work Work input is the work you do on a machine. You apply a force, called the input force, to the machine to move it through a distance. Work output is the work done by the machine. The machine applies a force, called the output force, through a distance. Work output can never be greater than work input.

9 Machines do NOT increase the amount of work done. Climbing a Hill: If you wanted to get to point C on the illustration below you have two options: start at A and go straight up, or start at B and walk up the slope. Which is easier? Well, actually, they both take the same amount of work. The only thing you have control over is if you put forth a lot of effort over a short distance (situation A-C) or a little effort for a long distance (situation B-C). Lets exam what work really is (at least in scientific terms).

10 Machines DO NOT save work How is it that the path from A-C takes the same amount of work as path B-C? In A-C we have to exert a lot of force to get up the vertical side, but we don't have to go very far. Let's suppose that the force required is 100 Newtons and the distance is 4 meters, the amount of work done is 400 Joules: W=F d W=100 N 4 m W= 400 J Getting from B-C does not require near as much force as A-C, but we do have to move a considerably longer distance. In fact, the actual force and distance are 25 Newtons and 16 meters. Calculating the work gives: W= 25 N 16 m W= 400 J

11 The Force-Distance Trade-off When a machine changes the size of the force, the distance through which the force is exerted must also change. Force or distance can increase, but not together. When one increases, the other must decrease. The work output is never greater than the work input.

12 Mechanical Advantage Some machines can increase force more than others. A machine s mechanical advantage tells you how many times the machine multiplies force. Use the following equation to find mechanical advantage: MA = output force input force

13 Mechanical Efficiency The less work a machine has to do to overcome friction, the more efficient it is. Mechanical efficiency is a comparison of a machine s work output with the work input. Use the following equation: Mechanical Efficiency = output force input force 100 The 100 in the equation means that mechanical efficiency is expressed as a percentage. If a machine could be made that had 100 percent mechanical efficiency, it would be called an ideal machine.

14 Do the Math 1. You apply 200 N to a machine, and the machine applies 2,000 N to an object. What is the formula needed? What is the mechanical advantage? 2. You apply 10 N to a machine, and the machine applies 10 N to another object. What is the mechanical advantage? Can such a machine be useful? 3. Which of the following makes work easier to do? a. A machine with a mechanical advantage of 15 b. A machine to which you apply 15 N and that exerts 255 N

15 Questions 1. How does getting regular oil changes improve the mechanical efficiency of a car s engine? 2. Complete this sentence: Work is being done when 3. Why is the work output for a machine always less than the work input? 4. You and a friend together apply a force of 1,000 N to a 3,000 N automobile to make it roll 10m in 1 minute and 40 seconds. How much work did you and your friend do together? What was your combined power? 5. Mechanical Advantage is to cents as mechanical efficiency is to.

16 1. Lever Types of Machines a. First Class Lever b. Second Class Lever c. Third Class Lever 2. Inclined Plane 3. Wedge 4. Screw 5. Wheel and Axle 6. Pulley

17 Levers A lever is a simple machine consisting of a bar that pivots at a fixed point, called a fulcrum. Levers are used to apply force to a load. The three classes of levers are based on the locations of the fulcrum, the load, and the input force.

18 With a first class lever, the fulcrum is between the input force and the load. 1 st class levers change the direction of the input force. Depending on the location of the fulcrum, 1 st class levers can be used to increase force or increase distance. When the fulcrum is closer to the load than to the input force, the mechanical advantage is greater than 1. When the fulcrum is exactly in the middle, the MA is exactly 1. When the fulcrum is closer to the input force than to the load, the MA is less than 1.

19 With a second class lever, the load is between the fulcrum and the input force. 2 nd class levers allow you to apply less force than the force exerted by the load. Because the output force is greater than the input force, you must exert the input force over a greater distance. A 2 nd class lever has a mechanical advantage of greater than 1.

20 With a third class lever, the input force is between the fulcrum and the load. 3 rd class levers do not change the direction of the input force. 3 rd class levers do not increase the input force. The output force is always less than the input force. They increase the distance through which the output force is exerted. A 3 rd class lever has a mechanical advantage of less than 1.

21 Questions 1. A third class lever has a mechanical advantage of less than 1. Explain why it is useful for some tasks. 2. If you could choose to be a 2 nd class lever, what would you be? What would your job be? How does that feel? 3. How is a hammer both a first class lever and a third class lever? 4. True or False? A large stick used to move a heavy rock is a simple machine. 5. Classify each of the following as 1 st, 2 nd, or 3 rd class levers: garden shovel, wheelbarrow, seesaw, broom, baseball bat.

22 Inclined Planes An inclined plane is a simple machine that is a straight slanted surface a ramp. The mechanical advantage of an inclined plane can be calculated by dividing the length of the inclined plane by the height to which the load is lifted. An inclined plane allows you to apply a smaller force over a greater distance.

23 Wedges A wedge is a double-inclined plane that moves. A wedge applies an output force that is greater than your input force. The greater the distance you move the wedge, the greater the force it applies on the object. The mechanical advantage of a wedge is determined by dividing the length of the wedge by its greatest thickness.

24 Screws A screw is an inclined plane that is wrapped in a spiral. When rotated, a small force is applied over the long distance along the inclined plane of the screw. The screw applies a large force through the short distance it is pushed. Screws are most commonly used as fasteners. The longer the spiral on a screw is and the closer together the threads, the greater the screw s mechanical advantage. Archimedes Hydraulic Screw

25 Wheel and Axle The wheel and axle is a simple machine consisting of two circular objects of different sizes. The axle is the smaller of the two circular objects. As the wheel turns, so does the axle. Because the axle is smaller than the wheel, it rotates through a smaller distance, which makes the output force larger than the input force. The mechanical advantage of the wheel and axle can be determined by dividing the radius of the wheel by the radius of the axle.

26 Pulleys A pulley is a simple machine consisting of a grooved wheel that holds a rope or cable. A load is attached to one end of the rope, and an input force is applied to the other end. There are two kinds of pulleys: fixed and movable. Fixed Movable

27 Fixed Pulleys This is a fixed pulley. It doesn't move when the rope is pulled. It is fixed to the upper bar. You can pull down on the rope in order to lift the load up. A fixed pulley only spins. Therefore, there s a mechanical advantage of 1.

28 Movable Pulleys This is a movable pulley. As the rope is pulled up, it can also move up. The weight is attached to this moveable pulley. Each side of the rope is supporting the weight, so each side carries only half the weight. The force needed to hold up the pulley in this example is 1/2 the weight! The mechanical advantage of this system is 2. It is the weight (output force) divided by 1/2 the weight (input force).

29 More on Pulleys When a fixed pulley and a movable pulley are used together, the pulley system is called a block and tackle. You can see that the weight is now suspended by two pulleys rather than one. That means the weight is split equally between the two pulleys, so each one holds only half the weight, or 50 pounds (22.7 kilograms). That means that if you want to hold the weight suspended in the air, you only have to apply 50 pounds of force (the ceiling exerts the other 50 pounds of force on the other end of the rope).

30 Questions 1. Why is a set of stairs classified as an inclined plane? 2. Name and classify at least four simple machines found in a kitchen. 3. Pretend you are a simple machine reporter for the Machines Daily News. Write a short fictional account of your travels through at least two work related scenarios. 4. Assume there is a hit television show titled Machine Shop. What are the titles of five main characters that might star on the show? (For example, Handy McAnnic might be the leading character.)

31 Compound Machines Compound machines are made of two or more simple machines. Can you identify simple machines within a compound machine?

32 Answers Item Name Can opener Types of simple machines that make it up Inclined plane, wedge, wheel and axle Pencil sharpener Inclined plane, screw, wedge, wheel and axle Scissors Inclined plane, wedge, first class lever Stapler Inclined plane, wedge, second class lever Bicycle Wheel and axle, levers, pulleys, screws Block and Tackle 2 or more pulleys

33 Mechanical Efficiency of Compound Machines In general, the more moving parts a machine has, the lower its mechanical efficiency. Most compound machines have low mechanical efficiency. An automobile is an example of a compound machine that involves MANY simple machines. Too much friction could cause heating and damage to the simple machines involved. It s important that friction be reduced through use of lubrication.

34 Questions 1. You are one simple machine and your best friend is another simple machine. What are each of you and how do you work together? 2. The radius of the wheel of a wheel and axle is 4 times greater than the radius of the axle. What is the mechanical advantage of this machine? 3. A winding road is actually a series of inclined planes. Describe how a winding road makes it easier for vehicles to travel uphill. 4. What compound machine would you choose to be in order to assist hurricane victims? Explain. 5. Why do you think you would not want to reduce the friction involved in using a winding road?

35 Can you identify simple machines within a compound machine? 1. A zipper is made from. A. 2 inclined planes B. 3 wedges C. 3 levers D. 4 screws 2. Identify the simple machines that make up a pair of scissors. A. 2nd class levers and wedges B. Inclined planes and wedges C. 1st class levers and wedges D. A wheel and axle and a pulley 3. Identify the simple machines that make up a manual can opener. A. 3rd class lever, inclined plane, and wedge B. 2nd class lever, screw, and pulley C. Inclined plane, wheel and axle, and fixed pulley D. Inclined plane, wedge, wheel and axle

36 What is mechanical advantage? 1. Which knife gives you a better mechanical advantage? A. a short and fat knife B. a short and thin knife C. a long and fat knife D. a long and thin knife 2. Which of the following machines has the greatest mechanical advantage? A. a machine to which you apply a force of 50 N and the machine applies a force of 150 N B. a machine to which you apply a force of 60 N and the machine applies a force of 200 N C. a machine to which you apply a force of 25 N and the machine applies a force of 100 N D. a machine to which you apply a force of 40 N and the machine applies a force of 125 N 3. In 1994, a 3,000 kg pancake was cooked, and flipped, in Manchester, England. Suppose you had a giant spatula that you could use as a lever to flip this pancake. Where could you best position the fulcrum and the pancake in order to get the greatest mechanical advantage? A. place the fulcrum close to the pancake, as in a first-class lever B. place the pancake between the input force and the load, as in a second-class lever C. position the input force between the fulcrum and the pancake, as in a third-class lever D. all of these machines have equal mechanical advantage

37 How can you calculate the mechanical advantage of a machine? 1. You apply 200 N to a machine and the machine applies 2,000 N to an object. What is the mechanical advantage? A B. 10 C. 1,800 D. 400, What is the mechanical advantage of an inclined plane 3m long and 0.5m high? A. 0.5 B. 1.5 C. 3 D What is the mechanical advantage of a wheel and axle where the wheel s radius is 10 cm and the axle s radius is 2 cm? A. 2 B. 5 C. 10 D. 20

38 Who is Rube Goldberg? Reuben Lucius Goldberg Born in San Francisco Graduated with engineering degree from University of California Berkeley Worked as an engineer for City of San Francisco Water and Sewer Department Convinced his father he wanted to work as an artist Got a job as an office boy in sports department of a San Francisco newspaper Kept submitting cartoons until he was published Moved to New York to draw daily cartoons for Evening Mail Founding member of National Cartoonist Society Pulitzer Prize winner National figure, often-quoted television and radio personality 60-year career Now, time for a little comic relief

39 Invention for Opening the Garage Door Without Getting Out of the Car

40 How to Keep Shop Windows Clean Passing man (A) slips on banana peel (B) causing him to fall on rake (C). As handle of rake rises it throws horseshoe (D) onto rope (E) which sags, thereby tilting sprinkling can (F). Water (G) saturates mop (H). Pickle terrier (I) thinks it is raining, gets up to run into house and upsets sign (J) throwing it against nontipping cigar ash receiver (K) which causes it to swing back and forth and swish the mop against window pane, wiping it clean. If man breaks his neck by fall move away before cop arrives. Rube Goldberg - extremely complicated inventions used to perform an ordinary, simple tasks

41 More Rube Goldberg

42 Can you explain this one?

43 What s happening here?

44 So, what? Rube Goldberg machines are examples of complex machines. All complex machines are made up of combinations of simple machines. Rube Goldberg machines are usually a complicated combination of simple machines. By studying the components of Rube Goldberg machines, we learn more about simple machines. The cartoons symbolize man s capacity for exerting maximum effort to accomplish minimal results. The cartoons depict convoluted machines functioning in complex ways to perform simple tasks. Rube Goldberg has become synonymous with any complex system achieving a basic task. Next week you ll create your own Rube Goldberg machine!

45 Questions 1. How are machines an important part of our lives? 2. How might machines be important to animal survival? 3. How would your life change if inclined planes no longer existed? 4. Write a narrative poem about machines, work, power, and force.

46 Websites

47 Bibliography Silver, H. F., Hanson, J. R., Strong, R. W., Schwartz, P. B. Teaching Styles and Strategies: Interventions to Enrich Instructional Decision Making. Trenton: Thoughtful Education Press, Holt Science & Technology: Physical Science. Austin: Holt, Rinehart and Winston, 2001.