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TRANSCRIPT
Chapter Introduction
Lesson 1 Work and Power
Lesson 2 Using Machines
Lesson 3 Simple Machines
Chapter Wrap-Up The McGraw-Hill Companies
What do you think?
Before you begin, decide if you agree or disagree with each of these statements. As you view this presentation, see if you change your mind about any of the statements.
1. Work is done when you push a book across a table.
2. Doing work faster requires more power.
3. Machines always decrease the force needed to do a job.
Do you agree or disagree?
4. A well-oiled, low-friction machine can be 100 percent efficient.
5. A doorknob is a simple machine.
6. A loading ramp makes it easier to lift a load.
Do you agree or disagree?
• What must happen for work to be done?
• How does doing work on an object change its energy?
• How are work and power related?
Work and Power
In science, work is the *.
What is work?
workfrom Old English weorc, means “activity”
• Work is important in science because it is related to energy.
• A force that does not make an object move does no work.
What is work? (cont.)
• To calculate work, multiply the force applied to the object by the distance the object is moved by that force.
• The product of force and distance has the unit newton·meter. The newton·meter is also known as the joule (J).
Calculating Work
The work done on an object depends on the direction of the force applied and the direction of the motion.
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When the force and the motion are in the same direction, calculate work by multiplying the force and the distance.
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When the applied force and the motion of the object are NOT in the same direction, the applied force can be thought of as being two forces acting on the object at the same time.
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• When the applied force and the motion of the object are NOT in the same direction, only the horizontal part of the applied force is used in the work equation.
• The vertical part of the applied force does no work on the suitcase. Hutchings Photography/Digital Light Source
The work done to lift an object equals the weight of the object multiplied by the distance it is lifted.
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• Doing work on an object transfers energy to the object.
• This helps scientists predict how an object will act when forces are applied to it.
• Work done when you lift an object also increases the object’s energy.
Work and Energy
Doing work on a tray transfers energy to the tray. The added energy can be either kinetic energy or potential energy.
Work and Energy (cont.)
How does doing work on an object change its energy?
• Power is *.
• You can also think of power as how fast energy is transferred to an object.
What is power?
What is power? (cont.)
power
Science Use the rate at which work is done
Common Use the ability to accomplish something or to command or control other people
You can calculate power by dividing the work done by the time needed to do the work.
What is power? (cont.)
• Work is done on an object when the object moves in the direction of the applied force.
• When work is done on an object, energy is transferred to the object.
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• To increase power, work must be done in less time.
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1. Work is done when you push a book across a table.
2. Doing work faster requires more power.
Do you agree or disagree?
• What are three ways a machine can make doing work easier?
• What is mechanical advantage?
• Why can’t the work done by a machine be greater than the work done on the machine?
Using Machines
• A machine is any device that makes doing something easier.
• Some machines are simple and other machines are more complex.
What is a machine?
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• Machines make tasks easier, but do not decrease the amount of work required.
• A machine changes the way in which the work is done.
What is a machine? (cont.)
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• The force you apply to a machine is the input force.
• The machine changes the input force to an output force.
What is a machine? (cont.)
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The amount of input force multiplied by the distance over which the input force is applied is the input work.
What is a machine? (cont.)
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Machines convert input work to output work by applying an output force on something and making it move.
What is a machine? (cont.)
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A machine makes work easier by changing the size of the force, the distance the force acts, or the direction of a force.
How do machines make work easier to do?
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When the output force is greater than the input force, the output force acts over a shorter distance.
How do machines make work easier to do? (cont.)
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When the output force acts over a longer distance than the input force, the output force is less than the input force.
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Lesson 2-2
How do machines make work easier to do? (cont.)
In what three ways do machines make doing work easier?
A machine’s mechanical advantage is *.
What is mechanical advantage?
The mechanical advantage tells you how many times larger or smaller the output force is than the input force.
What is mechanical advantage? (cont.)
mechanicalfrom Greek mechanikos, means “machine”
• Mechanical advantage can be less than 1, equal to 1, or greater than 1.
• A mechanical advantage greater than 1 means the output force is greater than the input force.
• The ideal mechanical advantage is the mechanical advantage if no friction existed.
What is mechanical advantage? (cont.)
What is mechanical advantage? (cont.)
What is mechanical advantage?
• The output work done by a machine never exceeds the input work of the machine.
• Friction converts some of the input work to thermal energy and this converted energy cannot be used to do work.
What is efficiency?
• The efficiency of a machine is *.
• Because output work is always less than input work, a machine’s efficiency is always less than 100 percent.
What is efficiency? (cont.)
What is efficiency? (cont.)
Why can’t the work done by a machine be greater than the work done on the machine?
• A machine makes a task easier and it can be simple or complex.
• The mechanical advantage of a machine indicates how it changes an input force.
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• The efficiency of a machine is increased when a lubricant coats moving parts.
3. Machines always decrease the force needed to do a job.
4. A well-oiled, low-friction machine can be 100 percent efficient.
Do you agree or disagree?
• What is a simple machine?
• How is the ideal mechanical advantage of simple machines calculated?
• How are simple machines and compound machines different?
Simple Machines
• simple machine
• lever
• fulcrum
• wheel and axle
Simple Machines
• inclined plane
• wedge
• screw
• pulley
• Six types of simple machines do work using only one movement.
• lever
• wheel and axle
• inclined plane
• wedge
• screw
• pulley
What is a simple machine?
• A lever is a *.
• * called a fulcrum.
Levers
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• In a first-class lever, the fulcrum is between the input force and the output force.
• The direction of the input force is opposite the direction of the output force.
Levers (cont.)
A finger tab on a beverage can is a first-class lever.
Levers (cont.)
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• A second-class lever has the output force between the input force and the fulcrum.
• The output force and the input force act in the same direction.
• A second-class lever makes the output force greater than the input force.
Levers (cont.)
Levers (cont.)
A wheel barrow is an example of a second-class lever.
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• A third-class lever has the input between the output force and the fulcrum.
• The output force is less than the input force. Both the input force and the output force act in the same direction.
Levers (cont.)
Levers (cont.)
A rake is an example of a third-class lever.
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The ideal mechanical advantage of a lever equals the length of the input arm divided by the length of the output arm.
Levers (cont.)
Levers (cont.)
How is the ideal mechanical advantage of a lever calculated?
Levers (cont.)
The neck, foot, and arm are examples of first-, second-, and third-class levers in the human body.
• A wheel and axle is *
• For a wheel and axle, the length of the input arm is the radius of the wheel and the length of the output arm is the radius of the axle.
Wheel and Axle
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A screwdriver is a wheel and axle. The handle is the wheel and the shaft is the axle.
Wheel and Axle (cont.)
Even though no machine is 100% efficient, you can calculate the ideal mechanical advantage of a wheel and axle.
Wheel and Axle (cont.)
• A ramp, or inclined plane, is *.
• It takes less force to move an object upward along an inclined plane than it does to lift the object straight up.
Inclined Planes
Moving a sofa is easier using a ramp. Using a ramp only requires a 100-N force to move the 500-N sofa. Because of friction, no ramp operates at its ideal mechanical advantage.
The ideal mechanical advantage of an inclined plane equals its length divided by its height.
Inclined Planes (cont.)
• The longer or less-sloped an inclined plane is, the less force is needed to move an object along its surface.
• * is called a wedge.
• A wedge is really a type of inclined plane with one or two sloping sides.
Inclined Planes (cont.)
• A screw is *.
• When you turn a screw, the screw threads change the input force to an output force and the output force pulls the screw into the material.
Inclined Planes (cont.)
A fixed pulley only changes the direction of the force.
Pulleys (cont.)
Movable pulleys are attached to the object being lifted and decrease the force needed to lift the object.
Pulleys (cont.)
A pulley system is a combination of fixed and movable pulleys that work together.
Pulleys (cont.)
The ideal mechanical advantage of a pulley or a pulley system is equal to the number of sections of rope supporting the object.
Pulleys (cont.)
Two or more simple machines that operate together form a compound machine.
What is a compound machine?
How are simple machines and compound machines different?
• A gear is a wheel and axle with teeth around the wheel.
• Two or more gears working together form a compound machine.
What is a compound machine? (cont.)
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• When the teeth of two gears interlock, turning one gear causes the other to turn.
• Gears of different sizes turn at different speeds.
What is a compound machine? (cont.)
CORBIS
• The efficiency of a compound machine is calculated by multiplying the efficiencies of each simple machine together.
• Each simple machine decreases the overall efficiency of the compound machine.
What is a compound machine? (cont.)
• Six simple machines are the lever, wheel and axle, inclined plane, wedge, screw, and pulley.
• All levers rotate, or pivot, about the fulcrum.
• The kind of wedge used to split logs is a simple machine.
5. A doorknob is a simple machine.
6. A loading ramp makes it easier to lift a load.
Do you agree or disagree?
Key Concept Summary
Interactive Concept Map
Chapter Review
Standardized Test Practice
A machine makes work easier by changing the size of the applied force, changing the distance over which the applied force acts, or changing the direction of the applied force.
• For work to be done on an object, an applied force must move the object in the direction of the force.
• When work is done on an object, the energy of the object increases.
• Power is the rate at which work is done.
Lesson 1: Work and Power
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Lesson 2: Using Machines
• A machine can make work easier in three ways: changing the size of a force, changing the distance the force acts, or changing the direction of a force.
• The mechanical advantage of a machine is the ratio of the output force to the input force.
• Because of friction, the output work done by a machine is always less than the input work to the machine.
• Friction between moving parts converts some of the input work into thermal energy and decreases the efficiency of the machine.
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• A simple machine does work using only one movement.
• The ideal mechanical advantage of simple machines is calculated using simple formulas.
• A compound machine is made up of two or more simple machines that operate together.
Lesson 3: Simple Machines