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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?

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 PowerA 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 PowerPractice 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 PowerSuppose 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 & CostExperiment 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

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.

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)

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

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

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.

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.

The rate at which work gets done is

- Very slow if I’m in charge.
- Force.
- Work.
- Power.

To calculate power, you divide work (or force x distance) by

- Work.
- Time.
- Force.
- Distance.

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)

2000 W = ___________ kW

- 2 kW
- 20 kW
- 200 kW
- 2 cans of A & W

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

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.

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

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

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

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.

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

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 WorkSuppose 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 WorkLearning 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.

Input and Output Forces

- Examine the input and output forces for a shovel.
- The input force is also called the applied force.

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.

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

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)

Which of the following is a simple machine?

- Diagram 1
- Diagram 2
- Diagram 3
- Diagram 4
- Diagram 5

The force you apply when you first use a machine is called the

- Output force.
- Input or Applied force.
- Inner force.
- Jedi Knight force.

Machines make work easier by

- Lowering the initial effort required to do the work.
- Lowering the applied force.
- Changing directions.
- All of the above.

Which of the following machines USUALLY causes a change in direction?

- pulleys
- Saying mean things to someone stronger than you
- ramps
- tweezers

Which of the following lowers the applied force?

- A bike in high gear compared to lower gears
- tweezers
- screwdriver
- A pulley

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!

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

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)

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

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.

- 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.

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.

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.

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 EfficiencyYou 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?

Plan and Solve

What quantity are you trying to calculate?

The efficiency of the lawn mower = __

What formula contains the given quantities and the unknown quantity?

Efficiency = Output work/Input work X 100%

Perform the calculation.

Efficiency = 200,000 J/250,000 J X 100%

Efficiency = 0.8 X 100% = 80%

The efficiency of the lawn mower is 80 percent.

- How Machines Do Work

Calculating EfficiencyYou 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?

Look Back and Check

Does your answer make sense?

An efficiency of 80 percent means that 80 out of every 100 J of work went into cutting the lawn. This answer makes sense because most of the input work is converted to output work.

- How Machines Do Work

Calculating EfficiencyReal vs. Ideal Machines

- Ideal machines would operate at 100% efficiency, while real machines operate at less than 100% efficiency due to friction.

Real Machine < 100% Efficiency

Ideal Machine = 100% Efficiency

The force the machine exerts on an object is called the ___________ force.

- Output
- Input
- Applied
- Same

The output force divided by the applied force is the

- Efficiency of the machine.
- Mechanical advantage of the machine.
- Ratio of good to bad parts of the machine.
- Only calculation that has to be greater than 1.

Which of the following will have a mechanical advantage = 1?

- Shovel
- Screwdriver
- A fixed pulley
- Broom or 3rd class lever

If the output force is greater than the input or applied force, then the M.A. is

- Less than 1 like a broom
- Greater than 1 like a screwdriver
- Equal to 1 like a fixed pulley
- None of the above are completely true.

The efficiency of a ramp is 76%. Why is it not 100% since input work is supposed to equal output work?

- The reaction force of the machine on the person causes this difference.
- It is 100%, the first statement is a lie!
- Friction causes the output work to be less than the input work.
- Gravity causes the output work to be less than the input work.

Contrast real and ideal machines.

- Real machines < 100% efficiency, while ideal machines = 100% efficiency.
- Real Machines = 100% efficiency, while ideal machines < 100& efficiency.
- Real Machines > 100% efficiency, while ideal machines = 100% efficiency.
- Real Machines keep it real, while the only ideal machine is my 8th grade science teacher.

Noggin Knockers/Hwk.- p. 113- 1c, p. 121: 1b, 1c, 2b, 2c, 3c 12 pts. total- 2 points each)

- 1-Rolling a bowling ball & kicking a football.
- 2- Screwdrivers lower the applied force (the amount of force or effort you exert).
- 3- M.A. = 1
- 4- M.A. = 80 N/40 N = 2
- 5- Real Machines have less than 100 % efficiency due to friction.
- 6- (b) 70 N (applied force/force you exert on the ax should be less than the output force/force the ax exerts on the piece of wood)

Pulley Demo (2 Pulley- Fixed and Movable/Block & Tackle)

- Output Force = the person’s ___________.
- Note that every time a machine lifts/moves an object, the object’s weight is the output force.
- Applied Force = Person’s ________ on the rope downward.
- Results: Output force is (greater than, less than, or equal to) the input force.
- Conclusion: So, the Mechanical Advantage of this pulley system and others with 2 or more pulleys is (greater than, less than, or equal to)1.

Learning Objectives

- Describe the 6 types of simple machines including the different pulley setups and different classes of levers.
- Describe the mechanical advantage (relative to 1) for each simple machine in terms of output vs. applied force.(See Mechanical Advantages of Machines in your lab notebook)

Pulley

- A pulley is a simple machine made of a grooved wheel with a rope or cable wrapped around it.

Wedge

- A wedge is a device that is thick at one end and tapers to a thin edge at the other end.

Wheel and Axle

- A wheel and axle is a simple machine made of two circular or cylindrical objects fastened together that rotate about a common axis.

Lever Experiment

- Goal- Draw and model the 3 classes of levers shown below & determine how they make work easier by comparing the input or applied force to the output force (weights = 2.8 N).
- Results- Record the applied force for the 1st class lever (left- fulcrum closer to load/output force), 2nd class lever (middle), and 3rd class lever (right). Calculate the M.A.
- Conclusion- State which levers lower the applied force and which levers make work easier by changing the direction of the force. Are there any levers that do both (lower the applied force and change the direction of the force)? If so, which one(s)?

Lever Experiment Extension (No lab write-up)

- Goal- Determine how lifting a bunch of books (with a heavy load weight) compares to using a 1st class lever to lift the books.
- Procedure
- Lift the books and remember how much force it felt like you were exerting.
- Then repeat using a 1st class lever.
- Results/Conclusions
- Did the lever make it easier to lift the books? If so, then how? Hint- Compare your applied force using the lever to the amount of force that it took to just lift the books (output force/weight).

Levers

- Levers are classified according to the location of the fulcrum relative to the input and output forces.

Identification of Real World Examples

- Identify the following examples of simple machines as 1 of the 6 previously discussed (be specific with any levers):
- Shoving a shovel straight into the ground
- Steering system of a bike or car
- Ramp or a screw
- Wheelbarrow
- Pliers
- A construction crane

Simple Machines in the Body

- Most of the machines in your body are levers that consist of bones and muscles.

More Simple Machines in the Body

- Teeth- Wedges
- Turn your forearm at the elbow- Wheel & Axle
- Muscle used to raise your eyes- Pulley

Compound Machines

- A compound machine is a machine that utilizes two or more simple machines.

If the input force for the lever below is 100 N, then the output force or load weight would have to be

- Less than 100 N.
- Greater than 100 N.
- Equal to 100 N.
- Equal to 0 N.

For a 1st class lever to lower the applied force, where must the fulcrum or pivot point be?

- Closer to the input force.
- Closer to the output force.
- Directly in the middle.
- At the other end.

If the weight of the load is 100 N, then which of the following is a possible value for the input or applied force?

- 50 N
- 100 N
- 150 N
- 200 N

Your body includes several simple machines such as

- Teeth acting as wedges.
- Eye raising via a pulley.
- Rotating your forearm is an example of a wheel and axle.
- Lifting an object up using your arm and bending your elbow is an example of a lever.
- All of the above are examples of simple machines in your body.

A machine composed of 2 or more simple machines is a

- Simpler machine.
- Complex machine.
- Compound machine.
- Machine that operates at 100% efficiency.

Which of the following is an example of a compound machine?

- Using a meter stick as a 1st class lever
- Fixed pulley
- Ramp
- Scissors

Work & Machines Practice Quiz Answers

- When a force is applied to an object and it moves in the same direction as the force.
- Friction
- Applied force is lower for machines with M.A.’s greater than 1.
- Greater than 5 N (because the applied is lower than the output force)
- M.A. = 1, then that machine ONLY changes the direction of the force.
- 1st class lever: lowers the applied force and changes the direction of the force. 2nd class lever: lowers the applied force.
- Applied force is less than 100 N (because the applied force is lower than the output force/load weight).
- A LOWER applied force is exerted over a GREATER distance (on the wheel) while a larger output force is over a shorter distance (on the axle).

Work & Machines Practice Quiz Answers

- Multiple pulleys result in a lower applied force (so it would be easier to lift an object with a heavier weight)
- (a) Ramp
- (b) Screw
- (c) Door stopper, knife, ax, teeth
- (d) Doorknob, steering wheel, rotating your forearm
- (e) Seesaw, pliers, scissors, lifting your head
- (f) Wheelbarrow, door, lifting your heel
- (g) Raising a flag on a flagpole, construction cranes, eye raising

Previewing Visuals

- Before you read, preview Figure 17. Then write two questions that you have about the diagram in a graphic organizer like the one below. As you read, answer your questions.

Three Classes of Levers

Q. What are the three classes of levers?

A. The three classes of levers are first-class levers, second-class levers, and third-class levers.

Q. How do the three classes of levers differ?

A. They differ in the position of the fulcrum, input force, and output force.

Graphic Organizer

Mechanical Advantage

Example

Simple Machine

Length of incline ÷ Height of incline

Ramp

Inclined plane

Ax

Wedge

Length of wedge ÷ Width of wedge

Length around threads ÷ Length of screw

Screw

Screw

Distance from fulcrum to input force ÷ Distance from fulcrum to output force

Seesaw

Lever

Radius of wheel ÷ Radius of axle

Screwdriver

Wheel and axle

Pulley

Flagpole

Number of sections of supporting rope

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