TEKS Lesson 6.8E: Machines

Transcription

1 6.8E Investigate how inclined planes and pulleys can be used to change the amount of force to move an object. : Machines What is a machine? A machine is a device that allows you to do work in a way that is easier. You may think of machines as complex gadgets with motors, but a machine can be quite simple. For example, a shovel makes the work of moving soil easier, so a shovel is a machine. You use the same amount of work to move a pile of soil whether you use your hands or a shovel. The shovel, or any other machine, changes the way in which work is done. A machine makes work easier by changing at least one of three factors. A machine may: change the amount of force you exert, change the distance over which you exert a force, or change the direction in which you exert a force. 1. Identify What is the advantage of using a machine to do work? Input and Output Forces When you do work, you exert a force over some distance. Although a machine changes how you do the work, you still exert a force. For example, you exert a force on the shovel when you use it to lift soil. This force is called the input force. The input force moves the machine a certain distance, called the input distance. The machine, in turn, does work by exerting a force on an object. This is the output force. The output force is exerted over a distance called the output distance. Input and Output Work The input force multiplied by the input distance represents an amount of work called the input work. The output force times the output distance is the output work. When you use a machine, the amount of output work can never be greater than the amount of input work. 1

2 Changing Force For some machines, the output force is greater than work: Work = Force Distance If the amount of work does not change, a decrease in force requires an increase in distance. So, for a machine to do work with less input force, you must apply that input force over a greater distance. Suppose you want to lift a heavy box onto a stage. Lifting the box requires a lot of force because you are doing work on the box by moving it against the force of gravity. Instead of lifting the box, you could push it up a ramp. The length of the ramp is greater than the height of the stage, so you exert your input force over a greater distance. However, when you use the ramp, the work is easier because you can exert a smaller input force and receive a greater output force. 2. Explain How can a machine exert a force that is greater than the force you put into it? 2

3 Changing Distance For some machines, the output force is less than the input force. How does this type of machine help you do work? are holding the chopsticks a short distance. As a result, the other ends of the chopsticks move a greater distance, allowing you to pick up food. The food end moves a greater distance, but the force is less. Another example is when you ride a bicycle. In high gear, you apply a force to the pedals over a short distance. The bicycle, meanwhile, travels a much longer distance. Changing Direction Some machines don't change force or distance. Instead, they change the direction of the force. For example, a steel cable runs over a small wheel at the top of the weight machine in the figure below. You can raise the weights by pulling down on the cable. The force is the same but this cable system is a machine that makes your job easier by changing the direction in which you exert your force. 3

4 How does an inclined plane change force? As you read above, a ramp makes it much easier to lift something, such as pushing a grocery cart over a curb. A ramp is an example of a simple machine called an inclined plane. An inclined plane is a flat, sloped surface. An inclined plane allows you to exert your input force over a longer distance, as shown below, so the input force needed is less than the output force. The input force that you use on an inclined plane is the force with which you push or pull an object. The output force is the force that you would need to lift the object without the inclined plane. This force is equal to the weight of the object. 3. Apply Give an example of an inclined plane. Mechanical Advantage The mechanical advantage of a machine is the ratio by which it reduces the force needed to do work. You can determine the mechanical advantage of an inclined plane by dividing the length of the incline by its height. Mechanical advantage = Length of incline Height of incline For example, if you are loading a truck that is 1 meter high using a ramp that is 3 meters long, the mechanical advantage of the ramp is 3 meters 1 meter, or 3. The inclined plane increases the input force to three times as much. If the height of the incline does not change, increasing the length of the incline will increase the mechanical advantage. The longer the incline, the less input force you need to push or pull an object. 4

5 How does a pulley change force? When you raise a flag on a flagpole or when you open and close window blinds, you are using a pulley. A pulley is a simple machine made of a grooved wheel with a rope or cable wrapped around it, as shown below. You use a pulley by pulling on one end of the rope. The force of your pull is the input force. At the other end of the rope, the output force pulls up on the object you want to move. To move a heavy object over a distance, a pulley can make work easier in two ways: The pulley can change the direction of your input force. The pulley can decrease the amount of input force needed to lift the object. Types of Pulleys A fixed pulley has a single wheel that does not move. It changes the direction of the force but not the amount of force. For example, you pull down on the flagpole rope, and the flag moves up. Moveable pulleys and systems of pulleys, such as the block and tackle shown in the figure below, decrease the amount of force needed to lift an object. As the number of sections of rope that support the object increases, the input force decreases. The mechanical advantage of a pulley is equal to the number of sections of rope that support the object. As the force decreases, the distance increases. So, if the mechanical advantage is 2, you pull 2 meters of rope to lift the object 1 meter. 5

6 Lesson Check 1. Define What is a machine? 2. Identify Which of the following factors is not changed by a machine? A force B work C distance D direction 3. Identify A gardener uses a shovel, which is a simple machine, to dig a hole in the soil for planting a tree. Identify which part of the shovel represents the: a. input force b. output force c. input distance d. output distance 4. Calculate What is the mechanical advantage of a 2.0 m inclined plane that rises to a height of 0.5 m? 6

7 _ 5. Infer If the mechanical advantage of a machine is less than 1, what can you infer about the machine? A The input force is less than the output force. B The input distance is less than the output distance. C The input work is less than the output work. D The input distance is equal to the output distance. 6. Compare How is the effect on direction of force of a single fixed pulley like a single moveable pulley? 7. Analyze A ramp is used to lift a 250-kg piano onto a stage 1 m high. How does the ramp affect the: a. force that the piano mover must apply? b. distance over which the piano has to travel? _ 7