chapter 21 preview section 1 work and powerwork and power section 2 what is a machine?what is a...

Chapter 21

Preview

Section 1 Work and Power

Section 2 What Is a Machine?

Section 3 Types of Machines

Work and Machines

Concept Mapping

Chapter 21 Section 1 Work and Power

Bellringer

First, in your science journal, define what specific kind of work is being done in each activity below. Then, select the activities that require the least amount of work.• carrying heavy books home• reading a 300-page novel • skiing for 1 hour • lifting a 45 kg mass• holding a steel beam in place for 3 hours• jacking up a car

Chapter 21

Objectives

• Determine when work is being done on an object.

• Calculate the amount of work done on an object.

• Explain the difference between work and power.


Chapter 21

What Is Work?

• Work is the transfer of energy to an object by using a force that causes the object to move in the direction of the force.

• Transfer of Energy One way you can tell that work is being done is that energy is transferred.


Chapter 21

What Is Work?, continued

• Difference Between Force and Work Applying a force doesn’t always result in work being done.

• Force and Motion in the Same Direction For work to be done on an object, the object must move in the same direction as the force.


Chapter 21

How Much Work?

• Same Work, Different Force Work depends on distance as well as force.


Chapter 21

How Much Work?, continued

• Calculating Work The amount of work (W) done in moving an object can be calculated by multiplying the force (F) applied to the object by the distance (d) through which the force is applied:

• The unit used to express work is the newton-meter (N m), which is more simply called the joule.

W F d


Chapter 21

Power: How Fast Work Is Done

• Calculating Power Power is the rate at which energy is transferred. To calculate power (P), you divide the amount of work done (W) by the time (t) it takes to do that work:

W ÷ t = P

• The unit used to express power is joules per second (J/s), also called the watt. One watt (W) is equal to 1 J/s.


Chapter 21

Power: How Fast Work Is Done, continued

• Increasing Power It may take you longer to sand a wooden shelf by hand than by using an electric sander, but the amount of energy needed is the same either way. Only the power output is lower when you sand the shelf by hand.


Chapter 21 Section 2 What Is a Machine?

Bellringer

Write a one-paragraph answer in your science journal to the following question:

Why do we use machines?

Chapter 21

Objectives

• Explain how a machine makes work easier.

• Describe and give examples of the force-distance trade-off that occurs when a machine is used.

• Calculate mechanical advantage.

• Explain why machines are not 100% efficient.


Chapter 21

Machines: Making Work Easier

• A machine is a device that makes work easier by changing the size or direction of a force.


Chapter 21

Machines: Making Work Easier, continued

• Work In, Work Out The work that you do on a machine is called work input. The work done by the machine on an object is called work output.

• How Machines Help Machines allow force to be applied over a greater distance, which means that less force will be needed for the same amount of work.


Chapter 21

Machines: Making Work Easier, continued

• Same Work, Different Force Machines make work easier by changing the size or direction of the input force.

• The Force-Distance Trade Off When a machine changes the size of the force, the distance through which the force is exerted must also change.


Chapter 21 Section 2 What Is a Machine?

Chapter 21

Mechanical Advantage

• What Is Mechanical Advantage? A machine’s mechanical advantage is the number of times the machine multiplies force.

• Calculating Mechanical Advantage You can find mechanical advantage by using the following equation:

( )mechanical advantage MA output force

input force


Chapter 21

Mechanical Efficiency

• The less work a machine has to do to overcome friction, the more efficient the machine is. Mechanical efficiency is a comparison of a machine’s work output with the work input.

• Calculating Efficiency A machine’s mechanical efficiency is calculated using the following equation:

mechanical efficiency work outputwork input

100


Chapter 21

Mechanical Efficiency, continued

• Perfect Efficiency? An ideal machine would be a machine that had 100% mechanical efficiency.

• Ideal machines are impossible to build, because every machine has moving parts. Moving parts always use some of the work input to overcome friction.


Chapter 21

Mechanical Efficiency

Click below to watch the Visual Concept.

Visual Concept


Chapter 21 Section 3 Types of Machines

Bellringer

What type of machine can be found on at least half the students in this room right now? What kinds of machines were common 50 years ago? 100 years ago? Are any of the same machines around today that were common in the 1800s? What has changed about those same machines today?

Record your thoughts in your science journal.

Chapter 21

Objectives

• Identify and give examples of the six types of simple machines.

• Analyze the mechanical advantage provided by each simple machine.

• Identify the simple machines that make up a compound machine.


Chapter 21

Levers

• A lever is a simple machine that has a bar that pivots at a fixed point, called a fulcrum.

• First-Class Levers With a first-class lever, the fulcrum is between the input force and the load.


Chapter 21

Levers, continued

• Second-Class Levers The load of a second-class lever is between the fulcrum and the input force.


Chapter 21

Levers, continued

• Third-Class Levers The input force in a third-class lever is between the fulcrum and the load.


Chapter 21

Pulleys

• A pulley is a simple machine that consists of a wheel over which a rope, chain, or wire passes.

• Fixed Pulleys A fixed pulley is attached to something that does not move.


Chapter 21

Pulleys, continued

• Movable Pulleys Unlike fixed pulleys, movable pulleys are attached to the object being moved.

• Blocks and Tackles When a fixed pulley and a movable pulley are used together, the pulley system is called a block and tackle.


Chapter 21

Pulleys, continued


Chapter 21

Wheel and Axle

• What Is a Wheel and Axle? A wheel and axle is a simple machine consisting of two circular objects of different sizes.


Chapter 21

Wheel and Axle, continued

• Mechanical Advantage of a Wheel and Axle The mechanical advantage of a wheel and axle can be found by dividing the radius (the distance from the center to the edge) of the wheel by the radius of the axle.


Chapter 21

Inclined Planes

• An inclined plane is a simple machine that is a straight, slanted surface.

• Mechanical Advantage of an Inclined Plane The mechanical advantage (MA) of an inclined plane can be calculated by dividing the length of the inclined plane by the height to which the load is lifted.


Chapter 21 Section 3 Types of Machines

Chapter 21

Inclined Planes, continued

• Wedges A wedge is a pair of inclined planes that move.

• Mechanical Advantage of Wedges can be found by dividing the length of the wedge by its greatest thickness.


Chapter 21

Inclined Planes, continued

• Screws A screw is an inclined plane that is wrapped in a spiral around a cylinder.

• Mechanical Advantage of Screws The longer the spiral on a screw is and the closer together the threads are, the greater the screw’s mechanical advantage is.


Chapter 21

Compound Machines of Machines

• What Are Compound Machines? Compound machines are machines that are made of two or more simple machines.

• Mechanical Efficiency of Compound Machines The mechanical efficiency of most compound machines is low, because compound machines have more moving parts than simple machines do. Thus, there is more friction to overcome.


Chapter 21

Compound Machine of Machine

Click below to watch the Visual Concept.

Visual Concept


Chapter 21 Work and Machines

Concept Mapping

Use the terms below to complete the concept map on the next slide.

work input distanceoutput force input forcework mechanical efficiencylever mechanical advantage

Chapter 21 Work and Machines

chapter 21 preview section 1 work and powerwork and power section 2 what is a machine?what is a...

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