Workshop 3 Web Highlights
Scientists often use the concept of energy to think about how objects
and systems behave. They would say that the moving car possesses
kinetic energy and the stretched rubber band possesses potential
energy. Rather than talk about the forces involved, as we have done
in these workshops, we could describe the students' rubber-band
car experiments in terms of energy. The potential energy in the
stretched rubber band is changed into the kinetic energy of the
car's motion. Friction dissipates the car's energy and brings it
"Forces Affecting Motion"
As Sallie and Katy indicate, there are many forces acting on the
rubber-band-powered cars. Since the car has mass, its weight is
that force that acts downward toward the center of the Earth. The
floor is also pushing up with an equal and opposite or normal force.
We could even consider the weight of the atmosphere acting on the
car. Rather than clutter our diagrams with all the forces acting,
we chose to focus on the forces that act along the line of the car's
Leonardo da Vinci was the first to study friction systematically.
He developed some ideas about this force that scientists still use.
Da Vinci distinguished between sliding and rolling friction and
also studied the effects of lubrication. Regardless of the type,
friction always opposes motion. Friction is the resistance to the
sliding, rolling, or flowing motion of a substance due to its contact
with another substance. We sometimes see the term "static friction,"
which refers to the greater force required to start an object sliding
than that needed to keep it sliding along a surface. Rolling friction
is caused primarily by the small indentations formed as one surface
rolls over another. Actually friction is what makes an object roll
down on an incline. If friction were not acting, the object would
slide down the incline. Air resistance is really a type of fluid
friction, since both liquids and gases are considered fluids. For
more on friction, visit the following Web sites: