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Concept Day

FRICTION-2-1

Note: The main focus of the Lab for this Investigation is to determine the effect of velocity on friction. However, as there are no additional theoretical or conceptual components required to understand the Lab and its results, we will therefore devote the major portion of this Concept Day presentation to examples of friction in the real world.

• Ask yourself: Why does friction cause heat?
• Remember that friction is a force that opposes motion. Motion is kinetic energy.
• Since friction acts against the motion, the kinetic energy that would have resulted in free or unimpeded motion is released as heat. This heat can be tremendous, depending on the amount of kinetic energy involved.

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FRICTION-2-2

• The first example of the tremendous amount of heat that can be produced by friction is shown in the reentry of the Space Shuttle into the Earth’s atmosphere.

Note: This slide was also used in the CELL Heat and Heat Transfer. In that instance, we focused on the insulating properties of the tiles that line the Shuttle’s bottom surface.

• Remembering that friction is caused by kinetic energy, consider the enormous amount of energy that the moving Space Shuttle brings to bear on the gas molecules in the atmosphere.
• The Space Shuttle has a mass of over 75,000 kilograms (165,000 pounds) empty. As it hits the Earth’s atmosphere at reentry, it is typically moving at about 27,000 km/hr (17,000 mph).
• Taken together, the shuttle has enough energy (about 3.23 x 1012joules) to heat an average house in a cold climate such as Colorado for some 41 years! Due to friction, this causes temperatures on the Shuttle’s reentry surface that reach 1,650oC.

Note: Ask yourself why excessive heat production is not a problem as the Shuttle moves in orbit at the same speed and with the same mass as at reentry into the Earth’s atmosphere.

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FRICTION-2-3

• The next example of the tremendous amount of heat that can be produced by friction under more everyday conditions is shown in the heat generated by a piston inside an internal combustion engine.
• In an automobile engine, the piston fits snuggly in the cylinder and moves up and down thousands of times per minute.

• The image on the left of the slide is a computer model showing how heat is actually distributed in a piston (the temperatures are given in oF).
• The photo on the lower right is a piston that is partially melted by the temperatures associated with engine friction.
• Much of the heat generated by internal combustion engines is due to the moving of metal surfaces against each other. In addition, of course, heat is generated as a result of the combustion reaction itself. It is also noteworthy that the function of motor oil is to reduce friction between moving metal parts in the engine.

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FRICTION-2-4

• This slide illustrates one of the earliest and most important uses of friction by humans, namely the ability to start a fire on demand.
• The photo on the right depicts a scene where friction is used to produce heat and start a fire. There is debate, however estimates of the earliest control of fire by man range from 200,000 years ago to over one million years ago.
• The image on the left is of a modern match.
• Wooden or paper sticks are coated with flammable chemicals and then “struck” against a rough surface. The friction generated by the striking action produces enough heat to ignite the chemical coating, which results in a flame.

Note: It is interesting that after all of this time, the fire started with common matches like the one on this slide is still caused by the heat produced by friction as it was hundreds of thousands of years ago!

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FRICTION-2-5

• This slide illustrates that friction is not limited to our planet Earth. One of Saturn’s moons, Enceladus, has the interesting property of sending large plumes of water vapor into space.
• As the model in the lower left of this slide suggests, this plume of water vapor may be the result of heat generated by friction.
• The hypothesis is that there are large faults in the ice crust of the moon that, due to tidal forces (caused by the gravitational pull from the giant Saturn), slide against one another and generate heat through friction. The thought is that this heat then acts to vaporize ice and water in the faults, forming the magnificent plumes.

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FRICTION-2-6

• The final example of friction at work involves rock climbing.
• Notice, in the large photo, that in the absence of good finger and toeholds, the friction of the sole of the climber’s shoes, fingertips, and the rock face is the only force working against gravity.
• Imagine spraying this rock front with oil to reduce friction. What impact do you think this would have on the climber?
• The insert photo shows a situation where a climber is taking advantage of a crevasse to scale a rock wall. This technique may be used even when there are no finger or toeholds. Under these circumstances, it is only the force of friction that suspends the climber perhaps hundreds of meters above the ground.

Note: You may be familiar with this arrangement of forces if you have ever climbed up in a doorway or narrow hallway simply by “jamming” yourself between the two vertical surfaces and crawling upward.

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FRICTION-2-7

• This slide was first presented in Investigation 1.
• Use it to review the calculation of velocity where v = d/t.

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FRICTION-2-8

• This final slide shows the experimental setup for Trials 1 through 4 of Investigation 2.