Download Investigation 7.4: Total Energy of a Toy Car

Investigation 7.4: Total Energy of a Toy Car
Preamble:
The Law of Conservation of Energy was arrived at by experiment. One interaction after another was studied;
energies were measured and totaled. When the total energy was not constant, invariably a new form of energy
was discovered to account for the difference. A big step forward was the acceptance of heat as a form of
energy. The force of friction, which slows objects down, also
generates heat. Kinetic energy is being changed to heat energy.
This investigation involves a strobe photograph of a toy car
moving down a curved ramp. Measurements of the distance traveled
between dots will enable you to calculate the speed and the kinetic
energy of the car at various times. Measurements of the car’s height
above the desk at these same times will enable you to calculate the
corresponding gravitational potential energy. Adding these results
together will give the total mechanical energy, unless some of it is
transformed into another kind of energy, such as heat.
Problem:
Is the total mechanical energy of a toy car on a ramp constant?
Materials:
accurate metric scale
Procedure:
1. Number images of the car.
3. Measure the height above the zero line of each square and determine the scale of the photograph (that’s a
meter stick in the photograph).
The scale of the photograph:
The meter stick is ____ mm long in the photograph. So, 1 meter = ____ mm.
4. Set up a table like the one below in your notebook and begin to record your observations.
Interval
1
2
3
h
h
∆d
∆d
v
Eg
Ek
Et = Eg + Ek
(mm)
(m)
(mm)
(m)
(m/s)
(J)
(J)
(J)
We will be using the Dot-Before, Dot-After method to calculate velocity!
5. Measure the distance travelled between the two dots on either side of each
numbered dot, and correct for the scale of the photograph.
To minimize error, measure from the top right of each square!
(Unfortunately, the diagram doesn’t show this…)
6. Divide each distance traveled by the time interval. The time between images
is 0.0415 s. The time interval here is 2 x 0.0415 s, or 0.083 s. This gives you
the average speed for each numbered interval.
7. Calculate the kinetic energy, the potential energy, and the total energy for
each numbered interval. The mass of the car is 100 g.
8. On a single set of axes, draw graphs of each type of energy plotted against
time.
Assuming that we are
measuring for square #4…
Questions:
1. What should the graph of total mechanical energy against time look like if the sum of the kinetic energy and
potential energy is constant? Consider the Law of Conservation of Energy!
2. What percentage of the sum of the original kinetic energy and potential energy remains at the end of the
trip?
3. What other energy transformations could have taken place as the car went down the ramp?
4. Focusing on the (seemingly) horizontal part of the track, what was the efficiency of the car as it rolled along
the level portion of the track?