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Basic Physics Terms

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

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Basic Physics Terms

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

  2. 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?

  3. Work is only done by a force on an object if the force causes the object to move in the direction of the force. Objects that are at rest may have many forces acting on them, but no work is done if there is no movement.

  4. Basic Terms - Work Work (in physics) is defined as a force acting over a distance. W = F x d Work in terms of rotation is a torque acting over an angle. W = t x q

  5. Work • The scientific definition of work is … • The transfer of energy through motion (distance). • Work (like energy) is measured in Joules (J) • Formula: W = F * d

  6. 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?

  7. Two factors have to be considered when deciding if work is being done…. • Was a force applied? • Was there a change in distance?

  8. The SI unit of power is the Watt, named in honor of James Watt. One Watt, W, of power is the power achieved when 1.0 J of work is done or 1.0 J of energy is transferred in a time of 1.0 s.

  9. Work done against gravity mass (g) height object raised (m) W = mgh work (joules) gravity (m/sec2)

  10. a. lifting 1 book over your head a. holding 2 books at shoulder level a. holding a stack of books and walking them to the counter b. lifting 2 books over your head b. lifting 2 books over your head b. pushing a stack of books over to the counter In which case would you do more work?

  11. Work (W) Force (F) Distance (d) Joule (J) Newton (N) meter (M) When doing work problems, what units must be used for . . .

  12. The Basics of Machines

  13. Simple Machines “a device that is used to manipulate the amount and/or direction of force when work is done” A common misconception is that machines are used to do a task with less work than would be needed to do the task without the machine. They do not! In fact (mainly because of friction), you actually do more work with a machine than without it (for the same task). The major benefit of a machine is that the work can be done with less applied force, but at the expense of the distance through which the force must be applied.

  14. Basic Terms - Torque What is “torque?” Torque can be thought of as rotational force. 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.

  15. 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... The relationship is t = F x r. r is the length of the moment arm (in this case, the length of the wrench).

  16. Gears and Torque Up to now, we have been talking about how gears change speed. But they can also change torque. Question: What is the gear ratio of this gear box? Answer: 75 to 1 That means the last axle rotates 75 times slower than the first axle. It also means the last axle has 75 times the torque as the first axle.

  17. Gears and Torque Where does all this “torque” come from? Consider a pair of gears that are meshed together. F A torque on this axle... …produces a force at the tooth. t r The moment arm is the radius of the gear. Remember: t = F x r

  18. Gears and Torque The force from the small gear’s tooth pushes against the large gear’s tooth. This creates an equal (and opposite) force in the large gear. This is Newton’s 3rd Law. F …and produces a larger torque on this axle. r t The force acts through this larger moment arm...

  19. F1 = -F2 t1 r2 r1 t2 Gears and Torque t1 = F1 x r1 t2 = F2 x r2 Analyzing the forces... • F1 = t1 / r1 • F2 = t2 / r2 • F1 = - F2 • t1 / r1 = -t2 / r2 • -t2 / t1= r2 / r1 The ratio of torques is the ratio of the gear radii. This is the gear ratio!

  20. Gears and Torque Gears can increase the torque (and force) that they exert on something. This is known as mechanical advantage. torque increases BUT, it comes at a price. Do you know what it is?

  21. 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...

  22. 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.

  23. 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.

  24. Mechanical Advantage With our gear box, you were able to create a large torque here... …but you had to turn this handle many times. Remember: W = t x q

  25. Mechanical advantage (MA) is the ratio of output force to input force for a machine. • Foutput • MA = Finput

  26. PROBLEM • A force of 200 newtons is applied to a machine in order to lift a 1,000-newton load. What is the mechanical advantage of the machine? • MA = 1000 N / 200 N = 5 • note no label just five

  27. 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.

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

  29. Friction = Bad Friction is caused by two surfaces rubbing together. Friction in our gear box causes a loss in the input power. It is lost in the form of heat and sound energy.

  30. Friction = Good But friction is also what makes our robot move. The turning wheel produces a frictional force against the ground, which causes the robot to move.

  31. WHERE ARE WE? • A force causes something with mass to move. • Torque is rotational force. • Work has 2 components, force and distance. • Mechanical advantage lets you increase one component of work at the expense of the other. • Gears are one way to get mechanical advantage. They can increase the robot’s torque. • Power is the product of force and speed. • A robot can either be slow and strong, or fast and weak. • Friction causes both losses in power, but also makes robots move on the ground.

  32. Wheel and Axle Lever Simple Machines Pulley Screw Wedge Inclined Plane

  33. Example Have you ever tried to unscrew a nut, bolt, or screw from something with your bare hands and discovered that it was just too tight to loosen even if you had a good grip?

  34. You got the proper tool, such as a screw driver or wrench, and unscrewed it!

  35. The wrench and screw driver are examples of a wheel and axle, where the screw or bolt is the axle and the handle is the wheel. The tool makes the job easier by changing the amount of the force you exert. Wheel Axle

  36. All of the simple machines can be used for thousands of jobs from lifting a 500-pound weight to making a boat go. The reason why these machines are so special is because they make difficult tasks much easier.

  37. Let's Take a Look at Them

  38. Click Here to Learn More Levers Lever Load Fulcrum

  39. 3 classes of levers Determined by position of load, fulcrum, and effort

  40. 1st Class Lever The claw on a hammer is a lever. We call this kind of lever a first-class lever.

  41. 2nd Class Lever A bottle opener is a second-class lever, which means the fulcrum is at the end of the lever and the load is in the middle.

  42. 3rd Class Lever A third-class lever has its fulcrum at one end and the load at the other end, with the work you do in the middle. It's how a fishing pole works. You lift just a short distance at the handle, but the end of the pole pops up several feet

  43. Inclined Plane

  44. Screw A screw is an inclined plane wrapped around a post. Click on the light bulb to learn more.

  45. Wedge Two inclined planes make a wedge.

  46. Wheel and Axle Wheel Axle

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