Work and Machines

What Is Work?

How Machines Do Work

Simple Machines

Table of Contents

Work and Machines - 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 and Machines

Work and Power

The amount of work performed on an object can be determined by multiplying force times distance.

Work = Force x Distance

*The SI unit of work is the Joule (J = Newton x meter)

Power is the rate at which work gets completed or the amount of work done on an object in a given amount of time.

Power = work / time

Or

Power = Force x Distance / Time

*The SI unit for power is measured in Joules/second or Watts (W)

1 J/s = 1 W

A watt is a small unit of power so large amounts are measured in larger units called kilowatts (1,000 watts). 746 watts = 1 horsepower (non SI unit)

Work and Machines

So why is measuring power useful?

Work and Machines

Horsepower looks much different today!

Work and Machines

Calculating Power

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?

Read and Understand

What information have you been given?

Force of the tow truck (F) = 11,000 N

Distance (d) = 5.0 m

Time (t) = 25 s

- What Is Work?

Work and Machines

Calculating Power

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?

Plan and SolveWhat 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 = (Force X Distance)/Time

Perform the calculation.

Power = (11,000 N X 5.0 m)/25 s

Power = (55,000 N•m)/25 s or 55,000 J/25 s

Power = 2,200 J/s = 2,200 W

- What Is Work?

Work and Machines

Calculating Power

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?

Work and Machines

Calculating Power

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?

16,000 W or 16 kW

- What Is Work?

Work and Machines

Calculating Power

Practice Problem

A crane lifts an 8,000-N beam 75 m to the top of a building in 30 seconds. What is the crane’s power?

20,000 W or 20 kW

- What Is Work?

Work and Machines

Question Answer

Asking QuestionsBefore you read, preview the red headings. In a graphic organizer like the one below, ask a what or how question for each heading. As you read, write answers to your questions.

What is work? Work is done when an object moves in the same direction in which the force is exerted.

How can you calculate work?

Work = Force X Distance

What is power? Power is the rate at which work is done.

- What Is Work?

Work and Machines

End of Section:What Is Work?

Work and Machines - 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.

Work and Machines

Machines and WorkA machine is a device that allows you to do work in a way that is easier.

Machines can be simple or complex.

Input force is the force you exert on the machine.

Output force is the force the machine exerts on an object.

The input force x input distance = input work

The output force x output distance = output work

*The amount of output work can never be greater than the amount of input work when using a simple machine!

Work and Machines - How Machines Do Work

What Is a Machine?A machine makes work easier by changing at least one of three factors:

1. A machine may change the amount of force you exert.

2. A machine may change the distance over which you exert your force.

3. A machine may change the direction in which you exert your force.

*In each case, the amount of work stays the same!

Work and Machines

Mechanical Advantage

A machines mechanical advantage is the number of times a machine increases a force exerted on it.

Mechanical advantage = Output force / Input force

When output force is greater than input force, the mechanical advantage of a machine is greater than 1 and this increases the force.

If the mechanical advantage is less than 1, then distance over which the force is exerted will increase.

If the mechanical advantage is equal to 1, then direction has changed but output and input force will remain the same.

Work and Machines

Mechanical Advantage

The input force and output force for three different ramps are shown in the graph.

- How Machines Do Work

Work and Machines

Mechanical Advantage

Input force

Reading Graphs:

What variable is plotted on the horizontal axis?

- How Machines Do Work

Work and Machines

Mechanical Advantage

400 N

Interpreting Data:

If an 80-N input force is exerted on Ramp 2, what is the output force?

- How Machines Do Work

Work and Machines

Mechanical Advantage

Ramp 1: 10; Ramp 2: 5; Ramp 3: 2

Interpreting Data:

Find the slope of the line for each ramp.

- How Machines Do Work

Work and Machines

Mechanical Advantage

The slope of each ramp’s graph equals the change in output force divided by the change in input force. This is the formula for mechanical advantage. Ramp 1 has the greatest mechanical advantage.

Drawing Conclusions:

Why does the slope represent each ramp’s mechanical advantage? Which ramp has the greatest mechanical advantage?

- How Machines Do Work

Work and Machines

Efficiency of Machines• The work you put into a machine is equals the work done by the

machine (in an ideal situation).• However, output work is always less than the input work for any

machine in reality!• Efficiency is a comparison of output work to input work and is

expressed as a percentage (%).• The higher the percentage, the greater the efficiency and vice

versa.• To calculate the efficiency of a machine, divide the output work by

the input work and multiply the result by 100 percent

Efficiency = Output work / Input work x 100%

Ideally all machines strive towards 100% efficiency

*No machine is 100% efficient due to friction!

A machine’s measured mechanical advantage is called actual mechanical advantage

Work and Machines

Calculating Efficiency

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?

Read and Understand

What information have you been given?

Input Work (Winput) = 250,000 J

Output Work (Woutput) = 200,000 J

- How Machines Do Work

Work and Machines

Calculating Efficiency

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?Plan and SolveWhat 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

Work and Machines

Calculating Efficiency

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?

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

Work and Machines

Calculating Efficiency

Practice Problem

You do 20 J of work while using a hammer. The hammer does 18 J of work on a nail. What is the efficiency of the hammer?

90%

- How Machines Do Work

Work and Machines

Calculating Efficiency

Practice Problem

Suppose you left your lawn mower outdoors all winter. Now it’s rusty. Of your 250,000 J of work, only 100,000 J go to cutting the lawn. What is the efficiency of the lawn mower now?

40%

- How Machines Do Work

Work and Machines

Main Idea

Detail Detail Detail

Identifying Main IdeasAs you read the section “What Is a Machine?” write the main idea in a graphic organizer like the one below. Then write three supporting details that further explain the main idea.

The mechanical advantage of a machine helps by…

changing the amount of force you exert

changing the distance over which you exert your force

changing the direction of the force

- How Machines Do Work

Work and Machines

End of Section:How Machines

Do Work

Work and Machines - Simple Machines

Inclined Plane

An inclined plane is a flat, sloped surface.

Work and Machines - Simple Machines

Wedge

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

Work and Machines - Simple Machines

Screws

A screw can be thought of as an inclined plane wrapped around a cylinder.

Work and Machines - Simple Machines

Levers

A lever is a ridged bar that is free to pivot, or rotate, on a fixed point.

Work and Machines - Simple Machines

Levers

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

Work and Machines - Simple Machines

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.

Work and Machines - Simple Machines

Wheel and Axle

You can find the ideal mechanical advantage of a wheel and axle by dividing the radius of the wheel by the radius of the axle.

Work and Machines - Simple Machines

Pulley

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

Work and Machines - Simple Machines

Simple Machines in the Body

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

Work and Machines - Simple Machines

Compound Machines

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

Work and Machines

Previewing VisualsBefore 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.

- Simple Machines

Work and Machines

End of Section:Simple Machines

Work and Machines

Graphic Organizer

Wheel and axle

Simple Machine Mechanical Advantage Example

Inclined plane Length of incline ÷ Height of incline Ramp

Radius of wheel ÷ Radius of axle Screwdriver

Wedge Length of wedge ÷ Width of wedge Ax

ScrewLength around threads ÷ Length of screw Screw

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

Pulley Number of sections of supporting rope Flagpole

Work and Machines

End of Section:Graphic Organizer