Identify heat generated by friction as the usual explanation for apparent violations of the law. Try to get students to understand heat and temperature at a molecular level. Explain that energy lost to friction is really transforming kinetic energy at the macroscopic level to kinetic energy at the atomic level. We saw earlier that mechanical energy can be either potential or kinetic. In this section we will see how energy is transformed from one of these forms to the other.
We will also see that, in a closed system, the sum of these forms of energy remains constant. Quite a bit of potential energy is gained by a roller coaster car and its passengers when they are raised to the top of the first hill.
Remember that the potential part of the term means that energy has been stored and can be used at another time. You will see that this stored energy can either be used to do work or can be transformed into kinetic energy. For example, when an object that has gravitational potential energy falls, its energy is converted to kinetic energy. Remember that both work and energy are expressed in joules. Refer back to Figure 9.
The amount of work required to raise the TV from point A to point B is equal to the amount of gravitational potential energy the TV gains from its height above the ground. This is generally true for any object raised above the ground.
However, note that because of the work done by friction, these energy—work transformations are never perfect. Friction causes the loss of some useful energy. In the discussions to follow, we will use the approximation that transformations are frictionless.
Work was done on the roller coaster to get it to the top of the first rise; at this point, the roller coaster has gravitational potential energy. It is moving slowly, so it also has a small amount of kinetic energy. As the car descends the first slope, its PE is converted to KE.
At the low point much of the original PE has been transformed to KE , and speed is at a maximum. As the car moves up the next slope, some of the KE is transformed back into PE and the car slows down. Help them make the logical leap that, if energy is the ability to do work, it makes sense that it is expressed by the same unit of measurement.
Ask students to name all the forms of energy they can. Ask if this helps them get a feel for the nature of energy. Ask if they have a problem seeing how some forms of energy, such as sunlight, can do work. Relate this to the origin of a coordinate grid. It is assumed that the speed is constant. Any KE due to increases in delivery speed will be lost when motion stops. Explain that the word potential means that the energy is available but it does not mean that it has to be used or will be used.
This simulation shows how kinetic and potential energy are related, in a scenario similar to the roller coaster. Observe the changes in KE and PE by clicking on the bar graph boxes. Also try the three differently shaped skate parks. Drag the skater to the track to start the animation.
This animation shows the transformations between KE and PE and how speed varies in the process. Later we can refer back to the animation to see how friction converts some of the mechanical energy into heat and how total energy is conserved.
On an actual roller coaster, there are many ups and downs, and each of these is accompanied by transitions between kinetic and potential energy. Assume that no energy is lost to friction. At any point in the ride, the total mechanical energy is the same, and it is equal to the energy the car had at the top of the first rise. This is a result of the law of conservation of energy , which says that, in a closed system, total energy is conserved—that is, it is constant. Using subscripts 1 and 2 to represent initial and final energy, this law is expressed as.
Either side equals the total mechanical energy. A baseball pitcher, for example, does positive work on the ball, but the catcher does negative work on it. Work can be zero even when there is a force. The centripetal force in uniform circular motion, for example, does zero work because the kinetic energy of the moving object doesn't change.
Likewise, when a book sits on a table, the table does no work on the book, because no energy is transferred into or out of the book.
Pour some water into a styrofoam cup. Determine the mass of the water and the initial temperature of the water. Add the lead to the cooler water and measure the final temperature of the two water and lead. You should now be able to calculate the specific heat of the lead. Falling For the next part, you are going to use a tube with lead shot in it. The goal is the let the shot fall a certain distance and measure the change in temperature. Compare the change in thermal energy to the change in gravitational potential energy.
Measure the starting temperature of the lead shot. Make sure the temperature is stable. Unplug the thermometer and turn over the tube 75 times. Be sure the hold the bottom of the tube so that the lead shot does not spill out.
Measure the final temperature of the lead shot.
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