


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 M4



Section 1 Work and PowerChapter M4




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 M4




Machines: Making Work Easier

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

Chapter M4




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 M4




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 M4



Section 2 What Is a Machine?Chapter M4




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 forceinput force

Chapter M4




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 M4




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 M4




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 M4




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 M4




Levers, continued

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

Chapter M4




Levers, continued

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

Chapter M4




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 M4




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 M4




Pulleys, continued

Chapter M4




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 M4




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 M4




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 M4




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 M4




Compound 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 M4



Work and Machines

Concept Map

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

work input output forceworklever

distanceinput forcemechanical efficiencymechanical advantage

Chapter M4



Work and MachinesChapter M4



End of Chapter M4 Show

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