Work and Simple Machines

1. Work and Simple Machines

2. What is work? • In science, the word work has 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.)

3. 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

5. 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?

6. 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

7. Formula for work Work = Force x Distance Work = (newton x meter) • One newton-meter is equal to one joule (1 J) • The unit of work is a joule (J)

8. W = F x d 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?

9. W= F x d 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? W= f x d W = 20N x 10m W= 200 Joules

10. Power Power is the rate at which work is done. • Power = Work(Joules) • Time (second) • The unit of power is the watt. • Named by James Watt. He was trying to • find a way to compare how much power his • steam engine produced, compared to a • known unit. At the time people used horses.

11. Check for Understanding 1.A man applies a force of 500 N to push a truck 100 m down a street. How much work does he do?

12. And the answer is….. Set up your formula: Work = force x distance = 500N x  100 m = 50,000 J

13. In which situation do you do more work? • You lift a 75 N bowling ball 2 m off the floor. • You lift two 50N bowling balls 1 m off the floor.

14. A. Work = force x distance = 75 N x 2m = 150 J B. Work = force x distance = 50N x 1m = 50 J per ball (= 100 J for both) So you do more work lifting the 75 N bowling ball 2m off the floor- A is correct!

15. Review for Power • You and a friend together apply a force of 1000N to a 3000N car to make it roll 10m in 1 minute 40 seconds. • 1.What was your work? • 2.What was your power?

16. Answer to Power question • W = f x d • = 1000N x 10m • = 10,000 J • P = work /t • = 10,000 J • 100 sec • = 100 watts

17. 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.

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

19. Simple Machines All machines come from six simple machines. They are: Lever Wheel and Axle Pulley Inclined Plane Wedge Screw

20. Simple Machines A machine is a device that helps make work easier to perform by accomplishing one or more of the following functions: • transferring a force from one place to another, • changing the direction of a force, • increasing the magnitude of a force, or • increasing the distance or speed of a force.

21. 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). When a machine takes a small input force and increases the magnitude of the output force, a mechanical advantage has been produced.

22. Mechanical Advantage MA = output force input force Ex.If you apply an input force of 5 pounds to wall with your hammer but the output force is 25 pounds onto the nail, the machine has a mechanical advantage of 25 or 5. 5

23. The Lever A lever is a rigid bar that rotates around a fixed point called the fulcrum. The bar may be either straight or curved. Levers are used to apply a force to a load.

24. The 3 Classes of Levers The class of a lever is determined by the location of the fulcrum, load and input force.

25. First Class Lever In a first-class lever the fulcrum is located between the input force and the load. • Examples of first-class levers include crowbars, scissors, pliers, and seesaws. • A first-class lever always changes the direction of force (ex. a downward input force on the lever results in an upward movement of the output force).

26. Fulcrum is between input and the load. Multiplies output force and changes its direction.

27. Second Class Lever With a second-class lever, the load is located between the fulcrum and the input force. Examples: nutcrackers, wheelbarrows, doors, and bottle openers.

28. Load is between fulcrum and input force. Multiplies input force, but does not change its direction

29. Third Class Lever With a third-class lever, the input force is applied between the fulcrum and the load. • Examples of third-class levers include tweezers, hammers, and shovels. • A third-class lever does not change the direction of force and it does not increase the input force.

30. Input force is between fulcrum and load. Does not multiply the input force Multiplies the distance through which the output force is exerted.

31. 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 objects. When either the wheel or axle turns, the other part also turns.

32. Pulley A pulley consists of a grooved wheel that holds a rope or cable. A load is attached to one end and the input force is applied to the other.. Two kinds: Fixed or movable. They can combine to make a block and tackle. Fixed- CHANGES direction of force, does NOT increase it. Movable- does NOT change direction of force but DOES increase force

33. Inclined Plane An inclined plane is an straight sloping surface. The inclined plane makes it easier to move a weight from a lower to higher elevation.

34. Wedge A wedge is a double inclined plane that moves.When you move a wedge through a distance you apply force on an object. Greater distance= greater force. Examples-doorstop, chisels, axe head, knife

35. Screw The screw is an inclined plane wrapped in a spiral. • 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).

36. Here is how…

37. Remember: MACHINES DO NOT “SAVE” WORK THEY ONLY MAKE IT A WHOLE LOT EASIER TO DO!!

38. Practice Questions 1. Explain who is doing more work and why: a bricklayer carrying bricks and placing them on the wall of a building being constructed, or a project supervisor observing and recording the progress of the workers from an observation booth. Work is defined as a force applied to an object, moving that object a distance in the direction of the applied force. The bricklayer is doing more work. 2. How much work is done in pushing an object 7.0 m across a floor with a force of 50 N and then pushing it back to its original position? How much power is used if this work is done in 20 sec? Work = 7 m X 50 N X 2 = 700 N-m or J; Power = 700 N-m/20 sec = 35 W 3. Using a single fixed pulley, how heavy a load could you lift?Since a fixed pulley has a mechanical advantage of one, it will only change the direction of the force applied to it. You would be able to lift a load equal to your own weight, minus the negative effects of friction.

39. Practice Questions 4. Give an example of a machine in which friction is both an advantage and a disadvantage. One answer might be the use of a car jack. Advantage of friction: It allows a car to be raised to a desired height without slipping. Disadvantage of friction: It reduces efficiency. 5. Why is it not possible to have a machine with 100% efficiency? Friction lowers the efficiency of a machine. Work output is always less than work input, so an actual machine cannot be 100% efficient. 6. What is effort force? What is work input? Explain the relationship between effort force, effort distance, and work input. The effort force is the force applied to a machine. Work input is the work done on a machine. The work input of a machine is equal to the effort force times the distance over which the effort force is exerted.