Introduction

Energy Graphs Without Magnets

In this activity we will frequently make approximate sketches of the energy of the toy car. As a first example, consider what happens when you push the car along the track with no magnets near the track or on the car. Try it now by giving the car a gentle push.

Use your observation of the motion and sketch a graph of speed vs. distance. Your graph should be just a rough sketch. We do not measure the values of the speed, so we can only approximate the shape of the graph.

Now, sketch a rough graph of the kinetic energy vs. distance.
HINT: The kinetic energy is proportional to speed².

Describe the graphs you sketched below

Again, we have not measured the kinetic energy, so we cannot put values on the vertical axis, but we can learn a lot from the shape of the curve. For example, your curve probably shows that the kinetic energy decreases with distance. This result tells us that the car's energy decreases as it moves along the track. That is not a surprise, because the car has internal friction as well as friction with the track. Your graph gives you an idea about the rate at which the friction changes the speed.

The decrease in kinetic energy is caused by energy going into overcoming friction. If friction were not present, the sketch of kinetic energy vs. distance would be a straight line as shown in Figure 2.

Figure 2: A graph of kinetic energy vs. distance for a car which loses none of its energy to friction.

This graph represents a situation in which the kinetic energy never changes. Many times during this activity we will try to imagine how other graphs would appear if friction were not present.

Consider another example - the graph of the total energy of the car when it is moving on a level track. Because we ignore the effect of friction, the total energy (total energy = potential energy + kinetic energy) will not change. Thus, the graph will look like Figure 3.

Figure 3: The graph of total energy vs. distance for a car experiencing no friction and traveling on a level track.

We can draw this graph because the total energy of the car at every point along the track is equal to its kinetic energy when no interactions are present. So far you constructed the energy diagram of the car, when no magnets are present.