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2.3 Act 3 Force Diagram’s and Friction

Directions: Read this excerpt then complete the questions and force diagrams.

Excerpt adapted from: Teach Engineering Discovering Friction; written by Pratt School of Engineering, Duke University.

What happens when one vigorously rubs their palms together, or rubs their palms against their thighs? One should notice that heat is generated. This is always true of friction: friction generates heat. The reason is that the force of friction reduces some of the kinetic energy of the moving object. Since energy cannot be lost only transferred in or out of a system, that kinetic energy is converted to heat energy.

We take advantage of the heat generated by friction every time we light a match. And although we keep our mammalian bodies warm by the metabolic, fuel-burning activity of our millions of cells, we feel particularly hot during strenuous activity. This is not only due to the fact that we are burning fuel faster when we exercise, but it is also partly due to the friction created by large blocks of muscles moving back and forth next to each other. When we run, for example, the muscles in the fronts of our thighs, known collectively as the quadriceps, or "quads", rub back and forth against those in the backs of our thighs, known as the "hamstrings". Heat is also generated by the movement of hundreds of thousands of muscle cells and their protein components as they slide past one another when we alternately contract and relax our muscles. These sources of friction build up heat and cause us to sweat and fan ourselves in an effort to cool off.

On the other hand, if we are too cold, we shiver. Shivering is a special type of involuntary, cyclical pattern of muscle contraction and relaxation. It is a physiological adaptation that causes us to burn fuel and produce heat whether we want to or not, but it also lets friction help us maintain our body temperature when our clothing and shelters are not sufficient.

For better or for worse, friction is an inescapable force we encounter every moment of our lives. We depend on friction in order to walk, we take advantage of friction in order to light a match, we try to reduce friction in our car engines and door hinges, and friction is generated as the muscle fibers of our hearts contract and relax with each heart beat. Usually physicists and engineers invest a lot of time and energy into trying to reduce or eliminate friction within the moving parts of machinery, but others sometimes look for ways to increase friction. Whether an engineer is trying to design a better set of automobile brakes or a more efficient wind turbine, a thorough understanding of friction is a vital prerequisite.

  1. What is one object you’d like designed to have more or less friction? Why?

2. Complete the chart.

#1. / A
A bowling ball is at rest. / B
A bowling ball is given a push and begins rolling across a floor / C
A bowling ball rolls across a very smooth tile floor through the “no-touch” zone. / D
A bowling ball rolls onto some grass and slows to a stop
Sketch of Situation:
List of Forces:
Force Diagram:
Balanced or Unbalanced
  1. Describe the motion when there are no unbalanced forces acting on an object. (For example, bowling ball in “no touch zone”)
  1. Describe the motion when there is some outside, unbalanced force on an object. (For example, bowling ball when you pushed it with mallet.)
  1. For each of the following draw a force diagram to show the forces acting on the object, explain how you know if the forces are balanced or unbalanced?

a)A baseball player begins sliding into 2nd base.

b)A bike hanging in a storage garage.

c)A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces.

©American Modeling Teachers Association 2015 1 MSS Gr 8 U 2.3 Act 3