Compound Machines
Look closely at a pair of scissors. You'll notice they consist of a couple of simple machines. The edges are sharpened like wedges. The blades and the handles together function as levers. In fact, most of the machines you use every day are actually a combination of simple machines working together. A compound machine is a combination of two or more simple machines that operate together. Many familiar compound machines, such as a car, a washing machine, or a clock, are combinations of hundreds or thousands of simple machines.
Look at the complex mechanisms of the watch shown in Figure 20. How do these simple machines interact to accomplish their task of keeping time? In a compound machine, the output force of one simple machine becomes the input force for another machine. Inside the watch, a complex series of gears is designed so that one gear drives the next. Each gear acts as a continuous lever. These gears help keep accurate track of days, hours, minutes, and seconds.
Figure 20 The complex workings of this watch consist of a series of machines. The output of one machine acts as the driving input for the next machine in the series.
Section 14.4 Assessment
Reviewing Concepts
Name six kinds of simple machines. Give an example of each.- Describe how to determine the ideal mechanical advantage of each type of simple machine.
How are the lever and the wheel and axle related to each other?
What is the ideal mechanical advantage of a ramp if its length is 4.0 m and its higher end is 0.5 m above its lower end?
Tightening a screw with a larger spacing between its threads requires fewer turns of a screwdriver than tightening a screw with smaller thread spacing. What is a disadvantage of the screw with larger thread spacing?
What class or classes of lever always have a mechanical advantage greater than 1?
Critical Thinking
Making Generalizations If you want to pry the lid off a paint can, will it require less force to use a long screwdriver or a short screwdriver? Explain.
Calculating When the pedals of a bicycle move through a distance of 0.25 m, the rear wheel of the bicycle moves 1.0 m. What is the ideal mechanical advantage of the bicycle?
Applying Concepts Explain why it could be useful for the mechanical advantage of a bicycle to be less than 1.
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