University of Virginia
Physics Department

## Compound Machines

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.10

Objectives

Students will

• understand the relationship between the mechanical advantage of a compound machine and that of its components.
• construct a compound machine that will do work with the greatest mechanical advantage.
• recognize the applications of compound machines in daily activities.

Motivation for Learning

Discrepant Event-Name the Simple Machines

Materials
• Scissors
• Knife
• Axe-if possible
• Typewriter-if possible
• Piano keyboard-if possible
• Bicycle-if possible

Procedure

1. Examine the scissors with the class. Ask them to identify the simple machines that make up the scissors (a lever and a wedge). Discuss the mechanical advantage that each of these provides.
2. Examine the knife. Repeat the identification (a lever and a wedge).
3. Repeat this with other compound machines available. If you do not have access to the suggestions above, find others that are present in the household or classroom, such as a shovel, a pencil sharpener, pliers or nail clippers. The purpose of this activity is to demonstrate the necessity for machines, both simple and compound, in everyday life. See Background Information for identification of the simple machines in each of these items.

Background Information

A compound machine is a combination of two or more simple machines. There are six types of simple machines: a lever, a pulley, an inclined plane, a wheel and axle, a wedge and a screw. One example of a compound machine is a bicycle. The wheels and pedals each form a separate wheel and axle system. The brakes and brake handles are levers, and the seat adjustment is a screw. Others include tools such as an axe or a shovel, which are made of a wedge and a lever; a typewriter or piano keyboard, which utilize a system of two or more levers; a pencil sharpener, which is composed of a wedge and a wheel and axle; and pliers or nail clippers, which are both made of multiple levers.

The mechanical advantage (MA) of a given machine is the magnitude by which it reduces the required force. For example, a mechanical advantage of two would cause the required force to be half that without the machine. The total mechanical advantage of a bicycle is calculated as the product of the individual mechanical advantages of each of its simple machines that cause motion.

This activity involves a comparison of two simple machines, an inclined plane and a pulley, and a compound machine. The mechanical advantage of an inclined plane is equal to its length divided by its height. The mechanical advantage of a pulley is equal to the number of ropes that lift an object. In this experiment, the mechanical advantage of the pulley is two, because there are two pulleys involved (and therefore two sections of rope that lift the object). As above, the mechanical advantage of the plane-pulley system should be equal to the product of these individual mechanical advantages:

MAsystem = MApulley * MAplane

= 2 * length/height

This is the theoretical value for the system. The actual advantage can be determined by the following equation:

Weight of object (N) / Force required by machine (N)

The students will be asked to compare the theoretical and actual values for the mechanical advantage of their compound machine. A possible explanation for a discrepancy between the two could be friction, which lowers the efficiency of each individual machine slightly.

### Student Activity

To print out the Student Copy only, click here.
At least two students are necessary per group for this activity.

Materials

• Inclined plane, 50-60 cm long, at least 20° incline angle
• Spring scale (10 N max)
• Small car with wheels, 400-800 g (Suggestion: Wheeled Cart from Frey Scientific Catalog 1-888-222-1332, item number SO4480)
• 2 pulleys
• String, 70 cm long with loops on both ends
• Metric ruler or meter stick

Procedure

1. Have one student hold the spring scale from the top, and another hang the cart from the bottom. Record the force reading from the spring scale (approx. 5 N) on the data chart.
2. Using the ruler, measure the length and height of the inclined plane. Record this.
3. Lay the cart on the bottom of the inclined plane, with the spring scale attached to the top. Pull the cart up the plane at a constant speed while another student reads the force from the spring scale. Record this on a data chart.
4. Construct a two pulley system. Hook one end of the string to the bottom of a pulley. Loop this string through the second pulley, back around the first pulley, and hook it to the end of the spring scale. Hang the cart from the second pulley (see figures below).
5. While one student holds the hook at the top of the first pulley, another should lift the cart by pulling the spring scale and read the force. Record this on the data chart.
6. While maintaining the two pulley system, set the cart at the bottom of the inclined plane. One student should hold the top of the first pulley near the top of the inclined plane. Another student should pull the cart using the spring scale and read the force. Record this on a data chart (See figures below).

 Step 1 Step 3 Step 4 Step 6

Data Sheet

To print out the Data Sheet only, click here.

 Weight of cart (N) Height of inclined plane (m) Length of inclined plane (m) Force on inclined plane (N) Force on pulley (N) Force on pulley-plane system (N)
1. What is the theoretical mechanical advantage of the pulley? This is calculated by the number of ropes used to lift the cart.

2. What is the theoretical mechanical advantage of the plane? Divide the length of the plane by the height of the plane.

3. What is the theoretical mechanical advantage of the compound machine (the plane-pulley system)? This is the product of the individual mechanical advantages.

4. What force was required to lift the cart originally (in step one)?

5. What is the experimental mechanical advantage of the plane? Divide the weight of the cart by the force on inclined plane. Compare this to your theoretical advantage calculated above.

6. What is the experimental mechanical advantage of the pulley? Divide the weight of the cart by the force on the pulley. Compare this number to your theoretical advantage calculated above.

7. What is the experimental mechanical advantage of the compound machine? Divide the weight of the cart by the force on the plane-pulley system. Compare this number to your theoretical advantage calculated above.

Extensions

Repeat activity but use a steeper angle and/or mass attached to the cart.

Students with Special Needs

Some students may have difficuilty manipulating the objects necessary for the assembly of the apparatus. This activity can be done with partners or in small groups.

Click here for further information on laboratories with students with special needs.

Assessment

Data sheet to be completed during the laboratory.