Work and machines
Physical Science - Middle School
Work and machines
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Study Guide Work and machines Physical Science - Middle School
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WORK AND MACHINES What Is Work? Work is the exertion of force through a distance. The formula for its calculation is: work =force x distance. For an example, if 5 newtons of force were applied for 5 meters, the amount of work done would be 25Nm. Simple Machines Simple machines are devices designed to make work easier for us. Normally, a machine takes the effort force put into it and multiplies it to produce a greater output force. A machine can also make work a lot easier by changing the direction of the input force or the distance through which the input force is applied. In all of these cases, the total work put out by the machine can never exceed the total amount of work put in. Lesson Checkpoint: Name two ways a machine can make work easier for us. Mechanical Advantage How much help the machine gives is measured by the mechanical advantage of the machine. As indicated before, a machine sometimes multiplies the amount force put into it, and the number of times it does this is the mechanical advantage. The formula for calculating mechanical advantage is: Mechanical advantage = output force/input force For example, if a machine takes the force coming in and multiplies it three times, the mechanical advantage would be 3. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
Ideally, the total amount of work done by the machine is supposed to equal the total amount of work put into it. However, because of friction, in many cases this is not true. Machines with greater efficiency do a better job overcoming the resisting force of friction. The formula for calculating the efficiency of a machine is: Efficiency=work output/work input x 100% Therefore, a machine having a 90% efficiency rating is using only 10% of its work output to overcome the force of friction. Types of Simple Machines There are several types of simple machines. The first is the inclined plane or ramp. By increasing the distance over which the input force is applied, it reduces the amount of input force needed. A wedge is a moving inclined plane and it helps in the same manner. A screw, which is an inclined plane wrapped around a cylinder, also works in the same way. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
A lever is a bar the pivots around a fixed point called a fulcrum. Since the position of the fulcrum can vary, there are three classes of levers. In a first class lever, like a crowbar, the fulcrum is between the resistance and the effort. In a second class lever, like a wheel barrel, the resistance is between the effort and fulcrum. A third class lever, like a broom or hockey stick, has the effort in between the fulcrum and the resistance. See the pictures below. Next, is the wheel and axle. This simple machine is made up of one larger cylinder, the wheel, attached to a longer but much smaller cylinder, the axle. An example of this is a screwdriver. This common machine reduces the amount of input force by adding to the distance the effort is applied through. The final simple machine is the pulley which consists of a cable placed around a grooved wheel. The different types are shown below. The mechanical advantage of this machine is simply the number of ropes or cables in the pulley system which support the weight being lifted. <use the diagrams on the top half of page 431 of Prentice Hall. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
Lesson Checkpoint: Describe three types of simple machines. Simple machines are normally thought of as tools used by us but they are also part of our bodies. Our arms and legs are types of levers where the kneecap and elbow serve as fulcrums and our teeth work as wedges. When simple machines are put together, this combination is now called a compound machine. One of the best examples of this would be a sewing machine. Lesson Checkpoint: Name two simple machines in your body. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
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