14.1 Work and Power

2. 14.1 Work and Power • Work- Work is the product of force and distance. • Work Requires Motion • If there is no movement, no work is done. • Force must act on an object in the same direction the object moves.

3. What is Work? • W = F x d • Work is the product of force and distance • If the force does not act in the direction of motion, it does NO work. • The unit for force is newtons. The unit for distance is meters. • The SI unit for work is the newton-meter, also known as a joule (J) (A force of 1 N moving 1m)

4. What is power? • Power- the rate of doing work. • Doing work at a faster rate requires more power. Power is the rate of doing work. Because the snowblower can remove more snow in less time, it requires more power than hand shoveling does.

5. To increase power: • 1) increase the amount of work done in a given time • 2) do the given amount of work in less time • Power = work = joules Time second • The SI unit of power is the watt. (W)

6. Math Practice • 1. Your family is moving to a new apartment. While lifting a box 1.5 m straight up to put it on a truck, you exert an upward force of 200 N for 1.0 s. How much power is required to do this? • 2.You lift a book from the floor to a bookshelf 1.0 m above the ground. How much power is used if the upward force is 15.0 N and you do the work in 2.0s? • 3.You apply a horizontal force of 10.0 N to pull a wheeled suitcase at a constant speed of 0.5 m/s across flat ground. How much power is used? (Hint: The suitcase moves 0.5 m/s. Consider how much work the force does each second and how work is related to power.)

7. Another common unit of power is horsepower. • Horsepower- equals 746 watts. (First defined by James Watt

8. 14.2 Reading Strategy Copy the table, then as you take notes/ read, write a definition for each vocabulary term in your own words.

9. 14.2 Work and Machines Machines do Work • Machine- a device that changes force • When using the jack, you apply a force to the jack handle. The jack changes this force and applies a much stronger force to lift the car. Because the jack increases the force you exert, it is a machine. • Machines make work easier to do. They change the size of a force needed, the direction of a force, or the distance over which a force acts.

10. Increasing Force • Small force over a large distance equals a large force over a small distance. Turning the jack handle allows the man to raise the car. The distance moved by the handle is much greater than the distance the car is raised.

11. Increasing Distance • A machine that decreases the distance through which you exert a force increases the amount of force required. • Each oar operates as a machine that pushes the boat through the water. The arrows show how pulling the end of each oar through a small distance moves the end of the oar in the water through a large distance. Changing Direction • Some machines change the direction of the applied force. • Pulling back on oars causes the other end to move in the opposite direction.

13. Work Input and Work Output • Because of friction, the work done by a machine is always less than the work done on the machine. Work Input • Input force- the force you exert on a machine. • Input distance- the distance the input force acts through. • Work input- work done by the input force acting through the input distance.

14. Work input = input force x input distance

15. Work Output of a Machine • Output force- force exerted by a machine. • Output distance- the distance the output force is exerted. • Work output- output force multiplied by the output distance. • Can you increase the work output of the oar by positioning it differently? Unless the new position decreases friction, the answer is no. • The only way to increase the work output is to increase the amount of work you put into the machine

16. 14.3 Mechanical Advantage and Efficiency • Mechanical advantage - the number of times that the machine increases an input force. • Actual mechanical advantage • The actual mechanical advantage (AMA) equals the ratio of the output force to the input force. • AMA = Output Force Input Force • Ideal mechanical advantage • The ideal mechanical advantage (IMA) of a machine is the mechanical advantage in the absence of friction. • Because friction is always present, the actual mechanical advantage of a machine is always less than the ideal mechanical advantage • IMA = Input Distance Output Distance

17. Math Practice • 1. A student working in a grocery store after school pushes several grocery carts together along a ramp. The ramp is 3 meters long and rises 0.5 meter. What is the ideal mechanical advantage of the ramp? • 2.A construction worker moves a crowbar through a distance of 0.50 m to lift a load 0.05 m off of the ground. What is the IMA of the crowbar? • 3.The IMA of a simple machine is 2.5. If the output distance of the machine is 1.0 m, what is the input distance?

18. Efficiency • Some work input to a machine is always used to overcome friction so work output of a machine is always less than work input. • The percentage of the work input that becomes work output is the efficiency of a machine. • Because there is always some friction, the efficiency of any machine is always less than 100 percent. • Efficiency = Work output x 100% Work Input • if the efficiency of a machine is 75 percent, then you know that 75 percent of the work input becomes work output.

19. Reducing friction increases the efficiency of a machine. The flow pattern of a smoke trail is analyzed by computers to determine the fluid friction forces (air resistance) acting on the vehicle. Engineers use these test data to optimize a vehicle's shape for maximum fuel efficiency.

20. Math Practice • 8. You have just designed a machine that uses 1000 J of work from a motor for every 800 J of useful work the machine supplies. What is the efficiency of your machine? • 9.If a machine has an efficiency of 40%, and you do 1000 J of work on the machine, what will be the work output of the machine?

21. The idea for this labor-saving auto jack comes from Rube Goldberg (1883–1970), a sculptor, author, and Pulitzer Prize-winning cartoonist.

23. 14.4 Simple MachinesThere are six types of simple machines. • 1. Levers- a rigid bar that is free to move around a fixed point. • Fulcrum- the fixed point the bar rotates around. • Levers are classified into 3 categories based on the locations of the input force, the output force, and the fulcrum. • Input arm- the distance between the input force and the fulcrum. • Output arm- the distance between the output force and the fulcrum. • IMA= input arm/output arm

24. First Class Levers • The fulcrum of a first-class lever is always located between the input force and the output force. M.A. can be greater than, equal to, or less than 1. • Examples: seesaw, scissors, and tongs.

25. Second Class Levers • The output force is always located between the input force and the fulcrum. • Example: wheelbarrow. • The input distance you move your hand on a wheelbarrow is larger than the output distance the wheelbarrow moves to lift its load. M.A. is always greater than 1.

26. Third Class Levers • The input force is located between the fulcrum and the output force. The output distance is always larger than the input distance. M.A. is always less than 1. • Examples: baseball bats, hockey sticks, golf clubs.

27. Simple Machine #2 • Wheel and Axle – a simple machine that consists of two disks or cylinders, each one with a different radius. • The outer disk is the wheel and the inner cylinder is the axle. • They rotate together as a unit

28. M.A. of a Wheel and Axle • To calculate the ideal mechanical advantage of the wheel and axle, divide the radius (or diameter) where the input force is exerted by the radius (or diameter) where the output force is exerted • IMA = radius wheel radius axle • Mechanical advantage can be greater than or less than 1. • Cars have an MA greater than 1.

29. Simple Machine #3 • Inclined Plane – a slanted surface along which a force moves an object to a different elevation. • Ex: wheelchair ramps, ramps in general, switchback roads This long and winding road acts like a type of simple machine known as an inclined plane.

30. M.A. Inclined Planes • The distance along the ramp is the input distance • The change in height of the ramp is the output distance • IMA = dist. along plane change in height • Ex: 6-meter-long ramp that gains 1 meter of height has an ideal mechanical advantage of 6.

31. Simple Machine #4 • Wedge – a V-shaped object whose sides are two inclined planes sloped toward each other. • Ex: Sledgehammer, knife, zipper • MA is always greater than one. • The thinner the wedge, the greater the mechanical advantage The wedge being used to split the log consists of two inclined planes that slope toward each other. The inclined planes force the wood fibers apart as the wedge is driven into the log

32. Simple Machine #5 • Screw – an inclined plane wrapped around a cylinder. • Ex: Screws, nuts, bolts • The closer the threads, the greater the ideal mechanical advantage.

33. Simple Machine #6 • Pulley- a simple machine that consists of a rope that fits into a groove in a wheel. • Output force is different in size, direction, or both, from that of the input force. • The ideal mechanical advantage of a pulley or pulley system is equal to the number of rope sections supporting the load being lifted.

34. Types of Pulleys • Fixed Pulley • Wheel attached in a fixed location. • The direction of the exerted force is changed, but the size of the force is not. • IMA is always 1 • Input and output force are the same (assuming friction is small) • Ex: flagpole, blinds

35. Types of Pulleys • Moveable Pulley • Pulley is attached to the object being moved rather than a fixed location. • Since there are two rope sections supporting the load, MA is 2. • Used to reduce the input force needed to lift a heavy object. • Ex: sails, window washer platforms.

36. Types of Pulleys • Pulley System (a.k.a. Block and Tackle) • Combines both fixed and moveable pulleys into one system. • Large M.A. can be achieved • Ex: Large cranes

37. Data Analysis A facility engineer at a shipyard collected the data shown in the graph. The data give the measured output forces for a range of given input forces. • Using Graphs What system requires the smallest input force to lift a 2500-N load? • Calculating Determine the actual mechanical advantage for each of the systems for a 2000-N input force. • Applying Concepts Which of the three systems shown in the graph consists of a single fixed pulley? Explain how you know. • Inferring Describe what happens to system B's output force as the input force increases above 4000 N. How does this affect the mechanical advantage of the system at higher loads? Offer a possible cause for the performance shown in the graph. • Applying Concepts Using the mechanical advantage value from Question 2, determine the output force of system A for an input force of 8000 N.

38. Compound Machines • compound machine - is a combination of two or more simple machines that operate together. • Ex: Car, Washing Machine, Clock