What is a simple machine? Would you say this bicycle is a simple machine? It is certainly simpler than a car, but it does not fit the scientific definition of simple machine. A simple machine is a device with few or no moving parts. Simple machines make work easier by changing the amount or direction of a force. A force is a push or pull. Scientists measure force in units called newtons (N). We will look at two simple machines: inclined planes and pulleys. An inclined plane is a sloping surface like a ramp or wedge. A pulley usually called a pulley system consists of one or more wheels and a rope. Have you used either of these simple machines recently? The most obvious kind of inclined plane is a ramp. People use ramps to raise heavy or large objects that would be difficult to lift by hand. Have you ever seen workers use a ramp to load furniture into a moving van? They are using inclined planes to make their jobs easier. Inclined planes decrease the amount of force needed to move something. However, you have to apply the force over a greater distance. The longer the inclined plane is, the less force you use. All simple machines involve a trade-off between force and distance. Ramp B is steeper than Ramp A. Less force is needed to push boxes up Ramp A. However, you have to move the boxes over a greater distance. 1
Did you know knives and axes are also inclined planes? The cutting edges of these tools consist of two inclined planes that come together at an angle. The inclined planes push in opposite directions as they cut through an object. You could never pull a piece of firewood apart with your hands, but a downward stroke of an axe separates the wood easily. The longer the inclined planes are and the sharper the angle at which they come together, the less force you use to cut with them. An axe head is two inclined planes. Simple machines don t do your work for you. They don t even decrease the amount of work done. Work equals force times distance. If a simple machine decreases the force you apply to an object, the distance over which you apply the force must increase. In other words, the product of force times distance the work done does not change. To make a pulley system, a rope is looped around a wheel and an object (called a load) is attached to the rope. Pulling on the free end of the rope lifts the load. Adding wheels to a pulley system can make it easier to lift a load. Pulley systems can reduce the amount of force needed to lift a load. They can change the direction of force applied to a load. They can even do both at the same time. The rule of thumb is to count the number of ropes supporting the load. Divide the weight of the load by this number to get the applied force needed. In some pulley systems there can be as many as three ropes, though only two ropes may support the load. 2
Pulley systems make it easier to lift a load, because each part of the system supports part of the load s weight. Examine these three pulley systems. In which system do you use the least force to lift the load? In which system do you move the load over the greatest distance? Based on your answers, what can you conclude about pulleys? (The answer is at the end of the companion, following the What Do You Know? section.) Each pulley system is lifting a load equal to five newtons. Pulley Systems 1 and 3 change the direction of the force. Pulling down on the rope lifts up the load. This is helpful because it is easier to pull down than to lift up. In Systems 2 and 3, extra wheels and ropes support the loads. This reduces the force needed to lift each load. 3
This picture shows all the pulleys that raise and lower an elevator. Cables attached to the elevator pass over the yellow pulley wheels. One end of each cable attaches to the elevator. The other end of each cable is attached to a heavy weight that weighs about as much as the elevator. (Don t worry the cables are very strong!) The purpose of the weight is to balance the elevator. Adding a relatively small force to one side causes the weight to fall and the elevator to rise. Adding a relatively small force to the other side causes the elevator to fall and the weight to rise. Without the weight, the motor would need to exert much more force to move the elevator up and down. Earlier in this companion you analyzed three different pulley systems. Because System 3 contains the most pulleys, you use the least force to lift the load. However, you also lift the load over the greatest distance. Below you see three ramps. You want to lift an object from the lower level to the upper level. Circle the ramp that will require you to use the least force. Draw an X over the ramp that will require you to use the most force. 4
In this investigation you and your child will measure the force needed to lift a weight using several different pulley systems. This is what you will need: 1. About 3 meters of fairly heavy fishing line. (You need fishing line because in this investigation your pulley systems will not contain pulley wheels. Fishing line will not add much frictional force.) 2. A weight weighing several kilograms with a ring or handle on top. 3. A high point (e.g., a bar, ring, or knob) you can loop the fishing line over. (The knob or handle of an open door at the top of a cabinet will work. A ceiling light fixture will also work.) 4. A spring scale to measure force. If you don t have a spring scale, you can measure force indirectly using cups of pennies or other small masses. You will need to tie one end of the line to the weight and the other end to the cup; then, loop the cup over the high point and gradually add pennies to the cup until the weight is lifted off the ground. Record the number of pennies necessary to lift each weight a certain distance off the ground; this provides you with a reasonable estimation of comparable forces. With your child, follow this procedure: 1. Weigh the weight with the spring scale, and record the value. 2. Tie the line to the weight, loop the line over the high point, and tie the other end to the hook on the spring scale. Pull down and measure how much force it takes to lift the weight. Record the value. 3. Tie the line to the high point, loop the line through the ring on the weight, and tie the other end to the spring scale. Measure the force needed to lift the weight when you pull up on the spring scale. Record the value. 4. Compare the values you recorded in Steps 1 3. Which system required you to apply the least force to lift the weight? 5. Create other pulley systems by looping the line multiple times through the ring on the weight and over the high point. Measure the force needed to lift the weight in each case. Observe how the force varies with the number of lines supporting the weight. Discuss the results. Compare the force needed to lift the weight for the various pulley systems. Look for patterns related to the force needed to lift, number of lines supporting the weight, and distance over which you pulled the line to lift the weight. 5
Here are some questions to discuss with your child: 1. What are some examples of pulley systems or inclined planes you have seen in everyday life? 2. How does a pulley system help you lift something? How does an inclined plane help you lift something? 3. Does a pulley system or an inclined plane reduce the amount of work you have to do? Explain. 6