Did you ever look closely at the moving parts of a bicycle? You can see some of them in the opening image. Do you see any simple machines, such as pulleys, wheels and axles, levers, or screws? A bicycle consists of many simple machines. It’s an example of a compound machine.
Q: What simple machines can you identify in the bicycle parts pictured above?
A: Find out if you are right at the following URL. Click on the parts of the bicycle in the animation to find the simple machines.
What Is a Compound Machine?
A compound machine is a machine that consists of more than one simple machine. Some compound machines consist of just two simple machines. You can read below about two examples—the wheelbarrow and corkscrew. Other compound machines, such as bicycles, consist of many simple machines. Big compound machines such as cars may consist of hundreds or even thousands of simple machines. You can see a student’s compound machine invention that includes several simple machines at this URL:
Look at the wheelbarrow in the Figure below. It is used to carry heavy objects. It consists of two simple machines: a lever and a wheel and axle. Effort is applied to the lever by picking up the handles of the wheelbarrow. The lever, in turn, applies upward force to the load. The force is increased by the lever, making the load easier to lift. Effort is applied to the wheel of the wheelbarrow by pushing it over the ground. The rolling wheel turns the axle and increases the force, making it easier to push the load.
The corkscrew in the Figure below is also a compound machine. It is used to pierce a cork and pull it out of the neck of a bottle. It consists of a screw and two levers. Turning the handle on top twists the screw down into the center of the cork. Then, pushing down on the two levers causes the screw to pull upward, bringing the cork with it. The levers increase the force and change its direction.
Efficiency of Compound Machines
Friction is a force that opposes motion between any surfaces that are touching. All machines have moving parts and friction, so they have to use some of the work that is applied to them to overcome friction. This makes all machines less than 100 percent efficient. Because compound machines have more moving parts than simple machines, they generally have more friction to overcome. As a result, compound machines tend to have lower efficiency than simple machines. When a compound machine consists of many simple machines, friction can become a serious problem, and it may produce a lot of heat. Lubricants such as oil or grease may be used to coat the moving parts of a machine so they slide over each other more easily. This is how friction is reduced in a car engine.
The mechanical advantage of a machine is the factor by which it changes the force applied to the machine. Many machines increase the force applied to them, and this is how they make work easier. Compound machines tend to have a greater mechanical advantage than simple machines. That’s because the mechanical advantage of a compound machine equals the product of the mechanical advantages of all its component simple machines. The greater the number of simple machines it contains, the greater its mechanical advantage tends to be.
Q: Assume that the lever and the wheel and axle of a wheelbarrow each have a mechanical advantage of 2. What is the mechanical advantage of the wheelbarrow?
A: The mechanical advantage of the wheelbarrow is the product of the mechanical advantage of the lever (2) and the mechanical advantage of the wheel and axle (2). Therefore, the wheelbarrow has a mechanical advantage of 4.
- A compound machine is a machine that consists of more than one simple machine.
- Compound machines such as a wheelbarrow or corkscrew consist of just two simple machines. Big compound machines such as cars consist of hundreds or thousands of simple machines.
- Compared with simple machines, compound machines generally have lower efficiency but greater mechanical advantage.
Do the simple and compound machines quiz at the following URL. Be sure to check your answers.
- What is a compound machine?
- What simple machines are found in a wheelbarrow? How do they make it easier to carry heavy objects?
- A pair of scissors is an example of a compound machine. Label the simple machines in the pair of scissors in the Figure below.
- Why do compound machines tend to have lower efficiency but greater mechanical advantage than simple machines?