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Table of Contents. What Is Work? How Machines Do Work Simple Machines. A tow truck exerts a force of 11,000 N to pull a car out of a ditch. It moves the car a distance of 5 m in 25 seconds. What is the power of the tow truck? What quantity are you trying to calculate?

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  1. Table of Contents • What Is Work? • How Machines Do Work • Simple Machines

  2. A tow truck exerts a force of 11,000 N to pull a car out of a ditch. It moves the car a distance of 5 m in 25 seconds. What is the power of the tow truck? What quantity are you trying to calculate? The Power (P) the tow truck uses to pull the car = __ What formula contains the given quantities and the unknown quantity? Power = Work/Time =(Force X Distance)/Time Perform the calculation. Power = (11,000 N X 5.0 m)/25 s Power = (55,000 N•m)/25 sor 55,000 J/25 s Power = 2,200 J/s = 2,200 W 1 Joule per second = 1 Watt 1000 Watts = 1 kilowatt or 1000 W = 1 kW - What Is Work? Calculating Power

  3. A tow truck exerts a force of 11,000 N to pull a car out of a ditch. It moves the car a distance of 5 m in 25 seconds. What is the power of the tow truck? Look Back and Check Does your answer make sense? The answer tells you that the tow truck used 2,200 W to pull the car. This value is about the same power that three horses would exert, so the answer is reasonable. - What Is Work? Calculating Power

  4. Practice Problem A motor exerts a force of 12,000 N to lift an elevator 8.0 m in 6.0 seconds. What is the power produced by the motor? (12,000 N x 8.0 m)/6.0 s = 16,000 W or 16 kW - What Is Work? Calculating Power

  5. Suppose you use your DVD player & TV with a combined power rating of 250 W for 40 hours over the course of a month. How many kilowatt-hours did you add to the electric bill? Remember to convert units. How much money did you add to the electric bill if the electric company charges 7 cents ($0.07) per kWh? 250 W = 0.250 kW Amount of kWh = 0.250 kW x 40 hours = 10 kWh Cost = 10 kWh x $0.07 per kWh = $0.70 or 70 cents - What Is Work? Calculating kilowatt-hours & Cost

  6. Experiment Problem from the Forces Test • Suzie and Markie are attempting to discover how to make moving large objects easier. They both believe that lighter objects are easier to move across a surface. They design an experiment to test out their prediction using a small wooden cart, a force sensor, and weights. • Hypothesis-Lighter objects are easier to move across a surface. • Ind. Variable- weight or mass • Dep. Variable- frictional force or force required to move the object or distance moved in a certain amount of time • Constants- same surface, same incline, same distance moved, same force sensor, same amount of pull/time for each measurement

  7. Experiment Problems • Determine the hypothesis, independent variable, dependent variable, and 2 or moreconstants for the experiment: • A student believes that bacteria grows quicker in warmer environments and slower in a cooler environment. This student is using petri dishes (little plastic dishes) and incubators of varying temperatures to cultivate the bacteria. • Hypothesis- Bacteria will grow quicker the warmer it gets (as temperature goes up). • Ind. Variable- Temperature • Dep. Variable- Amount of bacteria grown • Constants- Same size petri dishes, same amount of bacteria in each dish to start with, same amount of light, etc.

  8. Plant Experiment • Determine the independent variable(s), dependent variable, 2 or moreconstants, and the control group: • Flowers in a greenhouse are fertilized with a mixture of nitrogen (N), phosphorus (P), and potassium (K). A student has used different amounts of these parts of fertilizer to determine which component is most responsible for good growth. Examine the table. • Ind. Variables- Amount of different fertilizers (N, P, & K) • Dep. Variable- Amount of Plant growth • Constants- Amount of soil, amount of water added to the plant, amount of sunlight • Control Group-Plant C (b/c it doesn’t have any fertilizer, so the student is seeing how much the plant would grow normally- without fertilizer)

  9. Noggin Knockers

  10. Electricity Usage and Power • The amount of money you add to the electric bill can be determined by how long you use certain appliances and the power rating of those appliances. • Power is the rate at which the work gets done, so power is the amount of work done in a certain amount of time. • Power = Work/Time or • (Force x Distance)/Time • And Power = strength of the electric current x voltage • Power is measured in Watts (W) or kilowatts (kW). • Examples- Light Bulbs range from 40 W to 100 W. • 1 Watt = 1 (N x m)/s or 1 J/s • 1000 Watts = 1 kilowatt

  11. Electricity Usage and Power • Electric companies charge about 7 cents ($0.07) per kilowatt-hour (kWh). • So, if use an appliance with a 1000 W (or 1 kW) power rating for 100 hours over the course of a month, then you used 1 kW x 100 hours • = 100 kWh. • To determine the money added to the bill, multiply the kWh by the money per kWh… • 100 kWh x 0.07 dollars/kWh = $7.00

  12. Which of the following is NOT an example of doing work? • Pushing a cart around in the grocery store. • Lifting your books. • Holding a person straight above your head. • Pulling a person out of quicksand. • Me in 10 years.

  13. If a 100 N force to the right is used to move a couch 5 m to the right, then how much work was done? • 500 N x m or 500 Joules • 20 N x m or 20 Joules • 500 N • No work was done.

  14. The rate at which work gets done is • Very slow if I’m in charge. • Force. • Work. • Power.

  15. To calculate power, you divide work (or force x distance) by • Work. • Time. • Force. • Distance.

  16. Which of the following are units for power? • Newton x meters (N x m) • Newtons • Joules (J) or Joules x seconds (J x s) • Watts (W) or kilowatts (kW)

  17. 2000 W = ___________ kW • 2 kW • 20 kW • 200 kW • 2 cans of A & W

  18. How much power is required of you if you use 50 N to lift your books 1 m in 2 seconds? • 100 W • 50 W • 25 W • 0 W

  19. Your electric bill is determined by multiplying a cost of about 7 cents ($0.07) for every • Watt-seconds. • Kilowatt-hour. • Kilowatt-seconds. • Watt-minutes.

  20. Suppose you play Call of Duty: Modern Warfare 3 for 700 hours over the course of a month. The combined power rating of the TV and the X-Box is 500 Watts. What is the number of kWh for your gaming? Remember to convert units if needed. • 350,000 kWh • 1200 kWh • 350 kWh • 0.350 kWh

  21. So if you had to pay 7 cents ($0.07) per kWh and your gaming racked up 350 kWh, then how much money did you add to the electric bill due to your gaming addiction? • $24.50 • $2.45 • $2450 • $50.00

  22. Homework- p. 113: 1a, 1b, 1c, 2b, 2c, 3b, & 4 • 1a- Work is when you apply a force on an object and this causes the object to move a certain distance. • 1b- The object has to move in the same direction in which the force is applied. • 1c- Work is done for rolling a bowling ball and kicking a football. • 2b- Work = Force x Distance (in same direction as the force) • 2c-Same amount of work b/c 2 N x 3 m = 6 J and so does 3 N x 2 m • 3b- Power is Work divided by the time it takes to get the work done. • 4- P = (Force x Distance)/Time = (22 N x 3.0 m)/6.0 s = 11 Watts

  23. Noggin Knockers

  24. Learning Objectives • Identify when work is done on an object. • Force, Movement in the same direction as the force • Calculate the work done on an object. • Define and calculate power.

  25. - What Is Work? The Meaning of Work • Work is done on an object when the object moves in the same direction in which the force is exerted. Work= Force x distance

  26. A tow truck exerts a force of 11,000 N to pull a car out of a ditch. It moves the car a distance of 5 m. What is the work done by the tow truck? Work = Force x distance (in the direction of the force) Work = 11,000 N x 5.0 m = 55,000 N x m (Newton meters) 1 N x m = 1 Joule = 1 J So, Work of the tow truck = 55,000 Joules or 55,000 J - What Is Work? Calculating Work

  27. Suppose you get super strong exert a force of 500 N by moving a person 2 m out of the way of a moving truck. How much work did you do? Work = 500 N x 2 m = 1000 N x m (Newton-meters) So, Work = 1000 Joules or 1000 J - What Is Work? Calculating Work

  28. Learning Objectives • Explain how machines make work easier. • Lowering the applied force and/or Changing direction • Determine the mechanical advantage of a machine (relative to 1). • Calculate the efficiency of a machine.

  29. - How Machines Do Work Input and Output Forces • Examine the input and output forces for a shovel. • The input force is also called the applied force.

  30. Rise of the Machines Activity • In your lab notebook (this is not a FULL lab write-up): • Determine which of the following are machines: ramp, pliers, screwdriver, baseball, ruler, coat zipper, paper, tweezers, gear system of a bike. • For the ones that are machines, draw a diagram of the machine and draw the input (or applied) force and output force arrows.

  31. Diagrams- Ramp & Pliers

  32. Diagrams- Screwdriver & Coat Zipper

  33. Diagrams- Tweezers & Bike

  34. - How Machines Do Work What Is a Machine? • A machine makes work easier by LOWERING the amount of force you exert (by increasing thedistance over which you exert your force), or the direction in which you exert your force. • Examples: • Lowering the applied force- Turning the knob to turn the hose on • Changing Direction- Lifting weights using a pulley

  35. Rise of the Machines (Part 2) • Determine if the machine lowers the applied force OR changes direction: ramp, pliers, screwdriver, coat zipper, seesaw, & putting up a flag on a flag pole. Hint: If it’s difficult to use your hands for a task (making it so you need to use the machine for a task), then that machine probably lowers the applied force. • Ramp-lowers the applied force(output force is greater than the input force of pushing an object up a ramp) • Pliers-lowers the applied force(output force>input force) • Screwdriver-lowers the applied force(output force>input force) • Coat Zipper-lowers the applied force(input force is low compared to the output force pushing outward) • Seesaw & Flag pole-Changing directions (pull/push downward & the flag or other side of the seesaw goes up)

  36. Which of the following is a simple machine? • Diagram 1 • Diagram 2 • Diagram 3 • Diagram 4 • Diagram 5

  37. The force you apply when you first use a machine is called the • Output force. • Input or Applied force. • Inner force. • Jedi Knight force.

  38. Machines make work easier by • Lowering the initial effort required to do the work. • Lowering the applied force. • Changing directions. • All of the above.

  39. Which of the following machines USUALLY causes a change in direction? • pulleys • Saying mean things to someone stronger than you • ramps • tweezers

  40. Which of the following lowers the applied force? • A bike in high gear compared to lower gears • tweezers • screwdriver • A pulley

  41. Which of the following is true about why a steering wheel connected to an axle is used in vehicles? • More force on the steering wheel is needed over a shorter distance to make the vehicle turn. • Less force on the steering wheel is needed over a larger distance to make the vehicle turn. • More force on the steering wheel is needed over a larger distance to cause the vehicle to turn. • Less force on the steering wheel is needed over a shorter distance to cause the vehicle to turn. Arrows show distance traveled, not force!

  42. Suppose you are using a screwdriver, and the output force is 100 N. Which of the following is a possible applied force? Hint: Keep in mind how this machine makes work easier and double check to ensure your answer makes sense. • 200 N • 150 N • 40 N • 0 N

  43. Learning Objectives • Calculate the mechanical advantage of a machine. • Output Force/Input Force, Relative to 1 (Less than 1, Equal to 1, Greater than 1)

  44. - How Machines Do Work Input and Output Work • The amount of input work done by the gardener equals the amount of output work done by the shovel. • Mechanical Advantage of a machine = output force/applied force • M.A. = Fo/Fa

  45. Mechanical Advantages of Ramps • Goal: Determine the mechanical advantage for inclined planes (ramps) with varying steepness by using M.A. = Fo/Fa • Hypothesis: For the inclined planes, determine if you believe the mechanical advantage will be greater than 1, equal to 1, or less than 1. Explain why you predict this based upon how the machines work and the equation for M.A. • Background: • Output force = the ____________ of the cart = 2.5 N. • Procedure (Organize your results in a Table- on the next slide): • Determine the applied force (by pushing the go-car up the ramp with the force sensor) and output force for 3 different steepnesses of the ramp. • Calculate the mechanical advantage for the 3 ramp setups.

  46. Data Table & Conclusions • Conclusions (answer in complete sentences): • Which ramp had the greatest mechanical advantage? Explain why. • Did any setup have a mechanical advantage less than 1? Explain why or why not. Hint- Use the M.A. equation & the terms applied force & output force. • Based upon your data, determine which M.A. corresponds to the machine that lowers the applied force: 0.6, 2.0, & 1.0.

  47. Mechanical Advantage of a Fixed Pulley • After the Conclusions from the previous experiment (Mechanical Advantages of Ramps), record your data and conclusions for the M.A. of a fixed pulley. • Background: The output force (once again) = the _________ in N. • Setup: Tie a long piece of string to the force sensor hook. Make sure the weights are not hanging and the string is loose with some slack. Next, tie the untied end of the string to the rubber band around the weights. Determine the output force. Then untie the string and thread it through the pulley track. Tie it to the weights. • Results: • Measure the applied force by pulling the force sensor down (which should pull the weight up). • Calculate the mechanical advantage (Fo/Fa). • Conclusions: • Was the mechanical advantage close to 1? If so, then explain why in terms of the input force compared to the output force. • So if the M.A. = about 1, then the machine probably makes work easier by which of the following: lowering the applied force OR changing direction.

  48. Mechanical Advantages of Machines • Procedure (In groups) • Record the following in your lab notebook with the title above. Determine if the machine lowers the applied force OR changes the direction of the force; then determine which is greater- the output or the applied force; lastly, determine it’s M.A. relative to 1 (<, >, or =) for… • Inclined Plane (a ramp)- Refer to the ramp experiment. • A fixed pulley (like a flagpole)- Refer to the Fixed Pulley experiment. • A wedge (like a coat zipper or an ax) • Wheel and axle (like a screwdriver) • A screw (a winding inclined plane)- Refer to the ramp experiment.

  49. Graphic Organizer (Table) for Machines

  50. You do 250,000 J of work to cut a lawn with a hand mower. If the work done by the mower is 200,000 J, what is the efficiency of the lawn mower? What is the main force that will resist the motion of the parts of a machine and cause the efficiency to be less than 100%? FRICTION What information have you been given? Input Work (Winput) = 250,000 J Output Work (Woutput) = 200,000 J - How Machines Do Work Calculating Efficiency

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