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Work, Power, and Gears

Work, Power, and Gears. Basic Physics Terms. In this lesson, we will review some basic physics terms and demonstrate them using Lego gears. Force and Torque Mechanical Advantage Newton’s Laws Work and Power Friction. Basic Terms - Force. What is a “force?”.

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Work, Power, and Gears

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  1. Work, Power, and Gears

  2. Basic Physics Terms • In this lesson, we will review some basic physics terms and • demonstrate them using Lego gears. • Force and Torque • Mechanical Advantage • Newton’s Laws • Work and Power • Friction

  3. Basic Terms - Force What is a “force?” A force causes something with mass to move (accelerate). This can be summed up with Newton’s 2nd Law. F = m x a (In fact, units of force are called “Newtons.”) Answer: The force of gravity. Question: What force do we experience every single day?

  4. Basic Terms - Torque What is “torque?” Torque causes something with mass to rotate. This motor produces a torque. We are interested in “torque” because we deal with rotational motors and axles.

  5. Force and Torque How are force and torque related? moment arm. A force can create a torque by acting through a moment arm. …produces a torque here. A force here...

  6. Basic Terms - Work Work (in physics) is defined as a force acting over a distance. W = F x d

  7. Mechanical Advantage Understanding the 2 components of Work is the key to understanding mechanical advantage. Question: Where would you hold the wrench for it to be most effective?

  8. Mechanical Advantage …but you don’t move very far. …but your hand moves a long way. If you hold the wrench here, you need a lot of force... If you hold the wrench here, you don’t need as much force...

  9. Mechanical Advantage It takes the same amount of work to turn the bolt. You can opt for a lot of force and little distance. W = F x d Or you can choose a little force but a lot of distance. W = f x D In many of our machines, we want to increase our force, so we don’t mind going the extra distance.

  10. Mechanical Advantage Question: Which ramp would you prefer to use to move a heavy weight to the top of the box? Answer: This ramp requires less force, but you have to move the weight a longer distance.

  11. Power Power is the rate at which work is done. It can be thought of as work per second. Power = Work / sec. Like work, power has 2 components, force and speed. P = F x v (v stands for velocity). Question: Can you name 2 units for power? Answer: Horsepower and Watts.

  12. Power Power has the same trade-offs as work. A motor produces the same amount of power. So, you can make a car that’s fast, but weak. Or you can make a car that’s slow, but strong. The total power in must equal the total power out (with an exception)...

  13. What is a gear? • A gear is a wheel with teeth that mesh together with other gears. • Gears change the • speed • torque (rot. force) • direction • of rotating axles.

  14. Different types of gears Spur gears Idler gears Worm gears Bevel gears Belts & Pulleys

  15. Spur Gears Most “common” type of gear, a wheel with teeth. Spur gears do three things. 1. Change rotational speed 2. Change torque 3. Change direction #6 axle 1x16 beam 8 tooth gear 40 tooth gear

  16. Gear Ratios The gear ratio is the ratio of the number of teeth on one gear to the number of teeth on the other gear. 40 teeth 8 teeth Gear ratio = 40 to 8 or, simplifying, 5 to 1. That means it takes 5 revolutions of the smaller gear to get 1 revolution of the larger gear.

  17. Gear Ratios The gear ratio tells you the change in speed and torque of the rotating axles. If it takes 5 turns of the 8 tooth gear for every 1 turn of the 40 tooth gear, that means the 40 tooth gear will rotate 5 times slower than the 8 tooth gear. BUT, it also means the 40 tooth gear’s axle has 5 times the torque (rotational force) as the 8 tooth gear’s axle.

  18. Bevel Gears Bevel gears are spur gears that mesh at a 90 degree angle. The gear ratio rules remainthe same, but the axles are perpendicular to one another. These 12 tooth bevel gears can only mesh with them-selves.

  19. Idler Gears An idler gear is a gear that is inserted between 2 other gears. idler gear How many turns of the 8 tooth gear on the left does it take to make 1 turn of the new 8 tooth gear on the right?

  20. Idler Gears Answer: 1! It’s as if the 8 tooth gears are meshed together. • Idler gears DO NOT change the gear ratio. • Idler gears DO… • make both 8 tooth gears rotate in the same direction, • add spacing between gears.

  21. Worm Gears Worm gears have some special properties. 1: The axles are perpen-dicular, like bevel gears. 2: How many rotations of the worm gear does it take for 1 rotation of the spur gear? ANSWER: 24! The worm gear acts like a gear with 1 tooth! This gives very large gear ratios.

  22. Belts & Pulleys Belts & pulleys are related to gears. They change speed and torque, but with a few differences... Pulleys transfer their force by the friction of the belts, rather than direct contact with the teeth of gears. This can cause the belts to slip. Unlike gears, the pulleys rotate in the same direction.

  23. Belts & Pulleys Belts can transfer force across long distances. Like gears, however, belts and pulleys do have a “gear ratio.” It is the ratio of the diameters of the pulleys.

  24. Simple Gear Ratio You know how to compute the gear ratio of a single pair of gears. 40 teeth 8 teeth Gear ratio = 40 to 8 or, simplifying, 5 to 1.

  25. Compound Gear Ratio A gear box with more than one pair of gears is called a compound gear box. A compound gear ratio is the overall gear ratio of the compound gear box.

  26. Compound Gear Ratio To compute a compound gear ratio, we must first find the gear ratios of each individual pair of gears. What is the gear ratio of this pair of gears? Answer: 3 to 1

  27. Compound Gear Ratio What is the gear ratio of this pair of gears? Answer: 3 to 1

  28. Compound Gear Ratio If this axle were to make 90 rotations, how many rotations would this axle make? Answer: 30

  29. Compound Gear Ratio And if this axle were to make 30 rotations, how many rotations would this axle make? Answer: 10

  30. Compound Gear Ratio 90 30 10 What does that imply about what the overall gear ratio is?

  31. Compound Gear Ratio Answer: 9 to 1 To compute the compound gear ratio, we multiply the gear ratios of each pair of gears in the gear box. (3 to 1) x (3 to 1) = 9 to 1

  32. Compound Gear Ratio What is the compound gear ratio of this gear box going from bottom to top? Gear ratio 3: 3 to 1 Gear ratio 2: 3 to 1 Gear ratio 1: 5 to 1 Answer: 45 to 1

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