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Physics

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Physics

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    1. How would you lift a very heavy object if you had to do it alone? The answer is simple machines.How would you lift a very heavy object if you had to do it alone? The answer is simple machines.

    2. Balancing Act Using only a meter stick and a wooden block, balance two masses in a seesaw kind of structure. How did you get them to balance? Could you do it in one try? Compare your setup with other groups Use a wooden block as a pivot point. Teachers will either put the masses on each end and move the block, or they will place the block in the middle and move the masses along the stick.Use a wooden block as a pivot point. Teachers will either put the masses on each end and move the block, or they will place the block in the middle and move the masses along the stick.

    3. Lever Forces Materials Computer/calculator Force Probe 500g mass String Meter stick Wooden Block

    4. Lever Forces Measure the Weight of the 500g mass (in Newtons). Balance the middle of the meter stick on the wooden block. Place the 500g mass at the 90 cm line. Attach the string to the meter stick at the 10 cm line. Attach the string to the force meter and pull down on the sensor until the meter stick is balanced. Record the force needed to balance the meter stick. Repeat the above steps with the 500g mass at the 70 cm line and the 60 cm line.

    5. Lever Forces After recording your data in a table, perform the following calculations for the three trials: Divide the weight of the 500g mass by the force required to balance the meterstick. Divide the distance between the force meter and the wooden block by the distance between the 500g mass and the wooden block. How do these numbers compare? What do these numbers indicate about the lever system?

    6. Lever Lingo As a class, develop a working definition for a lever Lever: a bar that is free to pivot or turn about a fixed point Fixed point of a lever is called a fulcrum. Two important forces: Effort force is applied to the effort arm (by you) Resistance force is applied to the resistance arm So there is a trade off: a small force for a big distance, and a large force for a small distance.So there is a trade off: a small force for a big distance, and a large force for a small distance.

    7. Why use a Simple Machine? Simple Machines make work easier by giving the user a mechanical advantage. How do we calculate the mechanical advantage for a lever system? Ideal Mechanical Advantage (IMA) = Leffort / Lresistance Why do we stipulate that the MA is ideal? Because weve assumed that the machine puts out exactly as much work as we put in. This implies 100% efficiency This situation is never possiblewhy? Leffort is the distance between the effort force and the fulcrum Lresistance is the distance between the resistance force and the fulcrum 100% efficiency is never possible because of FRICTION.Leffort is the distance between the effort force and the fulcrum Lresistance is the distance between the resistance force and the fulcrum 100% efficiency is never possible because of FRICTION.

    8. Lever Example A worker uses an iron bar to raise a manhole cover that weighs 90 Newtons. The effort arm of the bar is 60 cm long and the resistance arm is 10 cm long. Draw a picture of this scenario Calculate the IMA of the lever system IMA = Le/Lr = 60 cm/ 10cm = 6 What force would the worker need to apply to lift the manhole? We need 90 N of force to lift the manhole cover, but we have a mechanical advantage of 6. Now we only need 15 N of force to lift the manhole.We need 90 N of force to lift the manhole cover, but we have a mechanical advantage of 6. Now we only need 15 N of force to lift the manhole.

    9. Three Lever Classes Class is based on the position of the effort force, resistance force, and fulcrum. First class lever Fulcrum located between effort force and resistance force Usually used to multiply a force Example: Seesaw This kind changes the direction of force.This kind changes the direction of force.

    10. Three Lever Classes Second class lever Resistance is located between the effort force and the fulcrum. Always multiplies a force Example: Wheelbarrow Always multiplies a force.Always multiplies a force.

    11. Three Lever Classes Third class lever Effort force located between the resistance and the fulcrum. Effort arm is always shorter than resistance arm MA is always less than one Example: Broom There is an increase distance moved and speed at the other end. Other examples are baseball bat or hockey stick.There is an increase distance moved and speed at the other end. Other examples are baseball bat or hockey stick.

    12. Natural Levers Identify an example of a 1st class lever in the human body Remember to relax the body and feel the muscle groups working to move the bonesRemember to relax the body and feel the muscle groups working to move the bones

    13. Natural Levers Identify an example of a 2nd class lever in the human body Remember to relax the body and feel the muscle groups working to move the bonesRemember to relax the body and feel the muscle groups working to move the bones

    14. Natural Levers Identify an example of a 3rd class lever in the human body Remember to relax the body and feel the muscle groups working to move the bonesRemember to relax the body and feel the muscle groups working to move the bones

    15. Lever Application While driving along a mountain road after a severe windstorm, you come upon a fallen tree blocking the road. The tree weighs 9000 Newtons (i.e. there is no chance that you could move it yourself. Nearby, you notice a 10 foot long thick branch that broke away from the main trunk and a large rock. Draw a realistic lever system that would allow you to move the tree. Is the lever 1st, 2nd, or 3rd class? What is the IMA of the lever? What force would you need to apply? First class If we have the effort arm at 9 ft. and the resistance arm at 1 ft., the IMA = 9 Force to apply = 1000 N First class If we have the effort arm at 9 ft. and the resistance arm at 1 ft., the IMA = 9 Force to apply = 1000 N

    16. Ancient Pulleys Instructor: Read this in your best theatrical voice ? What else did Archimedes study?Instructor: Read this in your best theatrical voice ? What else did Archimedes study?

    17. Archimedes Advantage Have two volunteers attempt to hold two metal rods apart while the instructor attempts to force them together with a pulley system. Discuss the Mechanical Advantage offered by pulleys Ask volunteers to keep the bars parallel and to hold the ends not the middleAsk volunteers to keep the bars parallel and to hold the ends not the middle

    18. Pulley Systems Materials Computer/calculator Force Sensor 500g mass Pulleys String Ring Stand

    19. Pulley Systems Measure the Weight of the 500g mass (in Newtons). Attach a single pulley to the ring stand and thread the string over the groove. Attach the force sensor to one end of the string and the 500g mass to the other. Raise the 500g mass 10 cm from the table top. Record the effort force. Attach the 500g mass to a single pulley. Attach one end of the string to the ring stand, thread it under and through the pulley groove and attach the other end to the force sensor. Raise the 500g mass 10 cm from the table top. Record the effort force.

    20. Pulley Systems Attach a double pulley to the ring stand and thread the string through it and a single pulley attached to the 500g mass as shown in figure ? Pull down on the force sensor to raise the 500g mass 10 cm from the table top and record the effort force. Identify the number of supporting ropes for each pulley system.

    21. Pulley Systems After recording the data for the three pulley systems in a table, perform the following calculation: Divide the weight of the 500g mass by the force required to raise it off the table top. Compare this value to the number of supporting ropes for each pulley system. What can you conclude about the Mechanical Advantage offered by a pulley system? Which value indicates IMA? Which value indicates AMA?

    22. Pulley Vocabulary Develop a working definition of a pulley Pulley: A grooved wheel with a rope chain or cable running along the groove. Different types of pulleys Fixed: modified first class lever, IMA = 1 Movable: attached to resistance, IMA = 2 Compound (block and tackle): system of pulleys IMA of pulleys: based on number of supporting cables

    23. Science in the Movies : Pulleys

    24. Pulley Engineering Design, Create, and Verify a pulley system with a mechanical advantage of: 4 5 6 Be sure to include drawings of your systems and how you verified the MA of the systems For block and tackle systems, what effect most greatly reduces the MA?

    25. Pulley Pictures Insert pictures of pulley systems with IMA of 2,3,4,5,6 from Pat See LP

    26. Pulley Puzzlers The girl in the illustration is trying to lift herself and the swing using the rope. What would you predict would happen if she pulled down on the rope? If she weighs 90 pounds and the swing weighs ten pounds, how much force does she need to apply? We should assume that it is an extensionless, massless rope, a massless pulley that does not hinder its rotation, and a rigid support. We should imagine a box around the girl and the swing with only the rope extending out from the box to isolate the forces acting on the objects inside the box from the external forces. The rope supports the box once, goes through the pulley and supports the box again. The tension of the rope going upwards is T on each side, so the total tension is 2T. The combined force from the tension in the rope must be greater than the combined weight of the girl and the chair for the chair to be lifted. For a 90 pound girl and 10 pound chair, she must use a force of 50 pounds of more to be able to lift herself off the ground.We should assume that it is an extensionless, massless rope, a massless pulley that does not hinder its rotation, and a rigid support. We should imagine a box around the girl and the swing with only the rope extending out from the box to isolate the forces acting on the objects inside the box from the external forces. The rope supports the box once, goes through the pulley and supports the box again. The tension of the rope going upwards is T on each side, so the total tension is 2T. The combined force from the tension in the rope must be greater than the combined weight of the girl and the chair for the chair to be lifted. For a 90 pound girl and 10 pound chair, she must use a force of 50 pounds of more to be able to lift herself off the ground.

    27. Pulley Puzzlers The man in the illustration below is trying to lift himself and the block on which he is standing off the ground. Is this possible? This movable pulley allows the man to pull upwards and the block on which he stands will rise with him on top. He must apply a force equal to his weight and the weight of the block because there is only 1 supporting rope outside the system. There is a mechanical advantage of 1. Yes, it can be done. This movable pulley allows the man to pull upwards and the block on which he stands will rise with him on top. He must apply a force equal to his weight and the weight of the block because there is only 1 supporting rope outside the system. There is a mechanical advantage of 1. Yes, it can be done.

    28. Pulley Puzzlers A long rope passes over a single pulley and has bananas on one end and a monkey on the other. If the monkey and the bananas have the same mass, can the monkey ever reach the bananas if he climbs the rope? Assume that the rope is massless, there is no friction, and the rope is limitless. The opposite external torques produced by the bananas and the monkey will cancel each other out. Any upward movement of the monkey will be equaled by an upward movement of the bananas. If the monkey started below the bananas, the distance between the two will always stay the same. Because we assume that the rope is limitless and the bananas will never wedge into the pulley, the monkey will never be able to reach the bananas. The opposite external torques produced by the bananas and the monkey will cancel each other out. Any upward movement of the monkey will be equaled by an upward movement of the bananas. If the monkey started below the bananas, the distance between the two will always stay the same. Because we assume that the rope is limitless and the bananas will never wedge into the pulley, the monkey will never be able to reach the bananas.

    29. Summary Simple Machine Lever 1st class, 2nd class, 3rd class Pulley Fixed, Movable, Block and Tackle Ideal Mechanical Advantage (IMA) Work in = Work out, 100% efficiency All real systems have efficiency < 100% How is some of the input energy lost? The energy lost is from friction and heat lost into the environmentThe energy lost is from friction and heat lost into the environment

    30. Reading Assignments NSES & BSL Find an example from each of your textual sources related to levers and pulleys. Note the page number(s) and provide an explanation of the relationship. Glencoe Integrated Physics and Chemistry Chapter 5, pp.132-146

    31. Simple Machines List 3 examples of levers and pulleys in or around your home and how they make work easier Glencoe Integrated Physics and Chemistry, Ch 5 assessment, pp154-155 Checking Concepts 1-10 Thinking Critically 11-13 Developing Skills 15

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