Work and Simple Machines

1. Work and Simple Machines

2. History of Work Before engines and motors were invented, people had to do things like lifting or pushing heavy loads by hand. Using an animal could help, but what they really needed were some clever ways to either make work easier or faster.

3. What is work? • In science, the word workhas a different meaning than you may be familiar with. • The scientific definition of work is: using a force to move an object a distance (when both the force and the motion of the object are in the same direction.)

4. Work or Not? • According to the scientific definition, what is work and what is not? • a teacher lecturing to her class • a mouse pushing a piece of cheese with its nose across the floor

6. More Notes • In order for you to do work on an object, the object must move some distance as a result of the force. • In order to do work on an object, the force you exert must be in the same direction as the object’s motion.

7. Formula for Work Work = Force x Distance • The amount of work done on an object can be determined by force times distance • The unit of work is newton-meters or a joule • One newton-meter is equal to one joule • One joule (J) is the amount of work you do when you exert a force of 1 Newton to move an object a distance of one meter • The unit of work is a joule

8. What’s work? • A scientist delivers a speech to an audience of his peers. • A body builder lifts 350 pounds above his head. • A mother carries her baby from room to room. • A father pushes a baby in a carriage. • A woman carries a 20 kg grocery bag to her car.

9. What’s work? • A scientist delivers a speech to an audience of his peers. No • A body builder lifts 350 pounds above his head. Yes • A mother carries her baby from room to room. No • A father pushes a baby in a carriage.Yes • A woman carries a 20 kg grocery bag to her car? No

10. Example Problem Work = Force x Distance Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done?

11. Example Problem Work = Force x Distance Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done? 200 joules (W = 20N x 10m)

12. Check for Understanding 1.Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 kg barbell over her head (approximately .60 m) 10 times in one minute; Ben lifts the 50 kg barbell the same distance over his head 10 times in 10 seconds. Which student does the most work? Explain your answers.

13. Ben and Bonnie do the same amount of work; they apply the same force to lift the same barbell the same distance above their heads. Yet, Ben is the most powerful since he does the same work in less time.

14. Section 2Mechanical Advantage

15. What is a Machine? • A Machine is a device with which you can do work in a way that is easier or more effective. • A machine makes work easier by changing the amount of force you exert, the distance over which you exert your force, or the direction in which you exert your force.

16. Mechanical Advantage • It is useful to think about a machine in terms of the input force (the force you apply) and the output force (force which is applied to the task). • Input force is also called the effort force. • Output force is sometimes called the resistance force because the machine must overcome some resistance. • When a machine takes a small input force and increases the magnitude of the output force, a mechanical advantage has been produced.

17. Mechanical Advantage • A machine’s mechanical advantage is the number of times a force is exerted on a machine and is multiplied by the machine. • Finding the ratio of output force to input force gives you the mechanical advantage. output force input force • If a machine increases an input force of 10 kg to an output force of 100 kg, the machine has a mechanical advantage (MA) of 10. • In machines that increase distance instead of force, the MA is the ratio of the output distance and input distance. Mechanical Advantage=

18. Calculating Efficiency • To calculate the efficiency of a machine, divide the output work by the input work and multiply the result by 100 percent. Output work Input work Efficiency = X 100 %

19. Actual and Ideal Mechanical Advantage • The mechanical advantage that a machine provides in a real situation is called the actual mechanical advantage. (AMA) • The mechanical advantage of a machine without friction is called the ideal mechanical advantage(IMA) of the machine. • The more efficient the machine is, the closer the actual mechanical advantage is to the ideal mechanical advantage.

20. Section 3 Simple Machines

21. Simple Machines Ancient people invented simple machines that would help them overcome resistance forces and allow them to do the desired work against those forces.

22. Simple Machines • The six simple machines are: • Lever • Wheel and Axle • Pulley • Inclined Plane • Wedge • Screw

23. Simple Machines • A machine is a device that helps make work easier to perform by accomplishing one or more of the following functions: • Multiplying the force • Changing the direction of a force • Multiplying the distance of a force • No machine can increase both the magnitude and the distance of a force at the same time.

24. Inclined Plane • The inclined plane makes it easier to move a weight from a lower to a higher elevation. • A ramp is an example of an inclined plane. • An inclined plane is an even sloping surface. • An inclined plane allows you to exert your input force over a longer distance. The input force needed to move an object will be less than the output force

25. Inclined Plane • You can determine the mechanical advantage of an inclined plane by dividing the length of the incline by its height. • While the inclined plane produces a mechanical advantage, it does so by increasing the distance through which the force must move. Length of incline Height of incline Ideal Mechanical Advantage =

26. ExampleAlthough it takes less force for car A to get to the top of the ramp, all the cars do the same amount of work. A B C Even though the work stays the same you reduce the amount of force by making the ramp more or less steep

27. Efficiency of an Inclined Plane • Even though an inclined plane has no moving parts, work is lost due to friction just as in any machine. • For example, if you pull a crate up an incline, friction acts between the bottom of the crate and the surface of the incline. Friction can be reduced by putting the crate on a dolly with wheels and then rolling it up the incline.

28. Wedge • The wedge is a modification of the inclined plane. • Wedges are used as either separating or holding devices.

29. Wedges • A wedge is a device that is thick at one end and tapers to a thin edge at the other end. • A wedge can be thought of as an inclined plane or two inclined planes stuck together that can move. • As in the case of the inclined plane, the longer and the thinner a wedge is, the less input force that is required to do the same work.

30. Examples of Wedges • The head of an axe-------------------- • A zipper---------------------- • Your teeth----------------------------

31. Screw • A screw can be thought of as an inclined plane wrapped around a cylinder. • This spiral inclined plane forms the threads of the screw. • While this may be somewhat difficult to visualize, it may help to think of the threads of the screw as a type of circular ramp (or inclined plane).

32. Screw-Inclined Plane Activity • Find a partner, Use a ruler to put a diagonal line from one corner of your paper to the other. Use a dark colored pencil or a maker to do this. • Cut on that line. You should be left with two equal triangles or inclined planes. • Tape the widest end of the paper to your pencil. • Twist your paper around your pencil and it will begin to look like a screw. Can you now see how an incline plane and a screw are similar?

33. Screws • If the screw threads are closer together, you need to turn the screw more times in order to screw into something. This means you are applying a force over a greater distance. • When you use a screwdriver to twist a screw into a piece of wood, you exert an input force on the screw. As the threads of the screw turn, they exert an output force on the wood. • As with all machines the increased distance results in an increased output force. • The closer together the threads are, the greater the mechanical advantage is.

34. MA of a screw can be calculated by dividing the number of turns per inch.

35. Examples of Screws • Bolts-------------------------------------- • Jars------------------ • Faucets-------------------------------

36. Wheel and Axle • A wheel and axle is a simple machine made of two circular or cylindrical objects that are fastened together and that rotate about a common axis. • The larger object is called the wheel and the smaller object is called the axle.

37. Wheel and Axle • When either the wheel or axle turns, the other part also turns. One full revolution of either part causes one full revolution of the other part.

38. Advantage of the Wheel and Axle • You apply an input force to turn the wheel, which is larger than the axle. As a result the axle rotates and exerts an output force to turn something. ( like a screwdriver and a screw) • The wheel and axle multiplies your force, but you must exert your force over a longer distance–in this case a circular distance.

39. Ideal Mechanical Advantage • You can calculate the ideal mechanical advantage of a wheel and axle using the radius of the wheel and the radius of the axle. Ideal Mechanical Advantage = Radius of wheel Radius of axle

40. Examples of Wheel and Axles • Doorknob---------------------------- • Screwdriver------------ • Steering wheel------------------------

41. The Lever • A lever is a rigid bar that is free to rotate or pivot, about a fixed point. • The fixed point that a lever pivots around is called the fulcrum. • The bar may be either straight or curved. • In use, a lever has both an effort (or applied) force and a load (resistant force).

42. How Levers Help! • Levers help in two ways. • First, it increases the effect of your input force. • Second, the lever changes the direction of your input force. (You push down or up on one side and the lever pushes down or up on the other side.)

43. The 3 Classes of Levers • There are three different types of levers, classified according to the location of the fulcrum relative to the input and output forces.

44. Types of Levers

45. First Class Lever • In a first-class lever the fulcrum is located at some point between the effort and resistance forces. • Common examples of first-class levers includecrowbars, scissors, pliers, tin snips and seesaws. • A first-class lever always changes the direction of force (I.e. a downward effort force on the lever results in an upward movement of the resistance force).

46. Fulcrum is between EF (effort) and RF (load)Effort moves farther than Resistance.Multiplies EF and changes its direction

47. Second Class Lever • With a second-class lever, the load is located between the fulcrum and the effort force. • Common examples of second-class levers includenut crackers, wheel barrows, doors, and bottle openers. • A second-class lever does not change the direction of force. When the fulcrum is located closer to the load than to the effort force, an increase in force (mechanical advantage) results.

48. RF (load) is between fulcrum and EF Effort moves farther than Resistance.Multiplies EF, but does not change its direction

49. Third Class Lever • With a third-class lever, the effort force is applied between the fulcrum and the resistance force. • Examples of third-class levers includetweezers, hammers, and shovels. • A third-class lever does not change the direction of force; third-class levers always produce a gain in speed and distance and a corresponding decrease in force.

50. EF is between fulcrum and RF (load) Does not multiply force Resistance moves farther than Effort.Multiplies the distance the effort force travels