http://www.ck12.org Chapter 4. Motion and Forces
MEDIA
Click image to the left for use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/82421Wheelbarrow
Look at the wheelbarrow in theFigure4.93. 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.
FIGURE 4.93
Corkscrew
The corkscrew in theFigure4.94 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.
Mechanical Advantage of Compound Machines
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