Teacher Portal
Friction: Investigation 3 –
Concept Day
ZERO-IN
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ASK WHY
Remind students that friction plays the role of both hero and villain in our world. Activities like walking and eating would be impossible without it. Nails would slip from boards, knots would come untied, and hair clips would fall out if it weren’t for friction On the other hand, skinned knees, scuff marks on floors, and wear and tear of moving parts are caused by friction. Learn about this force then decide if you think friction is a hero or villain.
BRANCH OUT
Explain to students that naval architects design ships from aircraft carriers to submarines, from sailboats to luxury yachts. To be successful, they need a working knowledge of fluid dynamics and ship resistance. One type of resistance they take into account when constructing ships is frictional resistance due to the motion of the hull through the water.
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FRICTION-3-1
- Inform students that the main focus of the Lab for this Investigation is to determine the effect of mass and surface area on frictional force.
Note: We will end with the experimental setup for the Lab, however, we will begin by presenting a few slides aimed at getting students to appreciate the relationship between friction and heat. More generally, we wish to have students begin to think in terms of the energy transfer involved in friction and its relationship to the Law of Conservation of Energy.
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FRICTION-3-2
- Explain that friction is essentially a force that acts against motion.
- Remind students that, when we think of motion, we should think immediately of kinetic energy. Thus, friction is a force that opposes kinetic energy.
- The Law of Conservation of Energy tells us that energy can neither be created nor destroyed; therefore, any loss of kinetic energy as a result of the opposition of motion by friction must be transferred in some way or another.
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FRICTION-3-3
Note: This is a slide students have seen before. It is used here to reiterate the relationship between motion, kinetic energy, heat, and temperature.
- Explain that we must always be able to think of what is happening at the molecular level.
- Tell students that, if energy is transferred to a substance, its molecules begin to move faster and its temperature increases.
- Explain that, when friction opposes motion, it opposes the kinetic energy of the moving object. This energy must be transferred somewhere.
Note: Over the next few slides, we will look at examples that students should be familiar with in which friction transfers kinetic energy from one substance to another or one form to another.
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FRICTION-3-4
Note: The first example of friction transferring kinetic energy into heat is shown on this slide. Everyone has rubbed their hands rapidly back and forth together to warm them.
Note: Another typical human response to cold is to rapidly rub our upper arms with our hands, something like shown in the picture on the right. Interestingly, the woman in this photo is not actually cold but demonstrating the sign language to indicate that she is cold when SCUBA diving and unable to talk to diving partners.
- Read the slide to the students.
- Explain that the movement of hands together or of hands agains the upper arm, results in frictional forces that generate heat. As a result, the two surfaces in contact rise in temperature, it is not an illusion!
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FRICTION-3-5
Note: Next we will examine how friction is responsible for stopping a bouncing ball from bouncing forever.
- Ask students, “Why doesn’t the ball keep bouncing?”
Note: This is an interesting photograph. It is set up by shining a strobe light in an absolutely dark room. The background is set up so as not to reflect the strobe light. The strobe light switches from very bright light to no light many times per second. A camera is then set up and its shutter is kept open. Therefore when the basketball is bounced, the camera captures its position only when the strobe is flashed on. The result of this technique is that one can see the motion of the bouncing object very well over time.
- Emphasize that the second bounce does not result in as high an arc as the first bounce. A third bounce would be less high and so on until the ball stops.
- Explain that the friction of the ball hitting the floor is largely responsible for the loss of kinetic energy by the ball with each bounce.
Note: This is explored further on the following slide.
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FRICTION-3-6
- Inform students that this slide represents a rubber ball on impact with the floor.
- Draw students’ attention to the deformation (somewhat exaggerated) of the ball on impact in the model.
- Explain that when the ball hits the floor, its kinetic energy is transferred as shown.
- Explain that when the ball strikes the floor, the floor is slightly deformed as some of the ball’s kinetic energy is transferred to its surface molecules.
- Tell students that as the floor molecules begin to move faster in response to the energy transfer, a small amount of heat is also generated.
- Explain that, if the floor is hard, it will not be deformed much and therefore not much of the ball’s kinetic energy will be transferred to it.
- Ask students to imagine bouncing a basketball on a dry sandy beach. The soft surface of the sand would be greatly deformed and a large amount of the ball’s kinetic energy would be transferred to it on the very first bounce. The ball certainly wouldn’t bounce too often, if at all, on the sand!
- Explain that, when the ball hits the floor, we can hear it make a sound. This is because some of the kinetic energy of the moving ball is transferred to nearby air (gas) molecules, causing them to vibrate. These vibrations travel through the air as sound waves to our eardrums, which are induced to vibrate as well. As indicated, the vibrations of air molecules also cause a certain amount of heat.
- Ask students to look at the shape of the ball.
- Explain that the obvious compression of the rubber material reflects the movement and rearrangement of molecules within the ball. The movement and increased vibration of the rubber molecules also leads to a slight increase in the heat of the ball.
- Tell students that the energy from all of the molecular interactions, vibrations, sound and heat noted above come directly from the kinetic energy of motion of the bouncing ball as it hits the floor. T
- Explain that this transformed/transferred energy is thus no longer available for the movement of the ball. After a few bounces, all of the balls kinetic energy is transferred through these interactions until it is depleted. Without kinetic energy, the ball no longer moves.
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FRICTION-3-7
- Tell students that wind is the result of solar energy.
- Explain that as air is heated by the sunlit surfaces of the Earth, it expands and its density is reduced. It begins to rise and is replaced by cooler air masses. The moving and mixing of air masses results in wind.
- Tell students that wind is a mass of air molecules in motion with kinetic energy. The kinetic energy of wind can be converted to electrical energy by windmills. Consequently, electricity produced by windmills is a form of solar energy.
- Explain that when wind hits a tree, as shown in this slide, some of its kinetic energy is transferred to the tree itself. As a result of this transfer of energy, we see the leaves rustle and the branches bend. We hear sound waves that result from the vibration of nearby air molecules that are in contact with the rustling leaves.
- Explain that in high winds, the millions of leaves, twigs, and branches act like a sail or kite and transfer an enormous amount of kinetic energy to the thick tree trunk, which may be seen to bend as well.
- Ask students to imagine the amount of force and kinetic energy carried by the wind as it bends and eventually snaps a one-meter diameter thick trunk of an oak tree! Wind is an awesome force.
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FRICTION-3-8
- Inform students that this slide illustrates the experimental setup of the first Trial of the Lab for Investigation 3. In this Trial, the influence of mass on frictional force is examined.
- Tell students that, up to this point, we have been discussing friction as a force that opposes motion. The question here is “If frictional force opposes motion, will the frictional force opposing the motion of a heavy object be more than a light object?” This experiment will answer that question.
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FRICTION-3-9
- Inform students that this final slide illustrates the experimental setup of the second Trial of the Lab for Investigation 3.
- It shows an object, the woodblock, which will be dragged across the table in three different orientations. Each orientation places a different amount of surface area in contact with the table and therefore susceptible to frictional force when it is moving.
- Emphasize that, while the surface area varies, the mass is held constant in the experiment.
- The question here is “If frictional force opposes motion, will the frictional force opposing the motion of an object with a fixed mass be dependent on its surface area?”
- Tell students that this experiment will answer that question.