Joy Paul
Energy Lab
In an ideal situation, mechanical energy is conserved. The amount of potential energy
(PE) in a system, plus the amount of kinetic energy (KE) in the same system, should
equal a constant number. As a ball bounces up and down, the sums of PE and KE should
remain constant if no energy is lost.
Pre-lab questions:
1. If a ball and a point mass are on the floor, how much PE does each have? Neither
has any PE. Since they are on the floor, they do not have any height needed to
gain PE.
2. The graph for the sum of PE and KE for a superball and putty should be as drawn
below. A superball loses very little of its mechanical energy with each bounce,
where as putty has very little “bounce” losing most of its energy right away.
3. The prediction for the KE of the ball as a function of time is drawn below. After
each bounce, the amount of KE declines. That is because energy is “lost” to sound
(the noise the ball makes when bouncing), heat (the friction of the ball against air
molecules and the floor), and the bending of the elastic material the ball is made
of.
The attached graphs show the PE, KE and total E of the bouncing ball. As the three are
plotted together, shown on graph 4, interesting features emerged. Per the conservation of
energy, the sum of PE + KE should equal a constant, the total E of graph 3. A constant E
is shown as a straight line on the graph. Energy was conserved between each bounce.
From 0-apx. .75 seconds, a straight line is seen on the energy graph at about y=1.9. Again
from about .9 seconds until approximately 1.25 seconds, y=1.25. The third straight line is
from about 1.4-1.6 seconds, where y=.8. If you were to sum up the totals of PE + KE
from each separate graph during these times, the sums are about 1.9, 1.45 and 1
respectively. They are very close to the total energy sums listed above. The reason for
differences are most likely due to the fact that the sonic ranger does not measure the exact
same place on the ball for each bounce. PE is determined by mgh. If the “h” measured by
the ranger is at a different point each bounce as the ball moves, results will contain
inaccuracies. The Law of conservation of energy states that the total mechanical energy
of the bouncing ball should always be the same. This occurs only in an ideal situation.
Mechanical energy is lost during each bounce on the floor to noise, heat and elastic
bending of the ball. This is why the total energy decreases after each bounce.
Looking at the PE and KE lines, graphs 1 and 2, it is easy to see that as the PE of the ball
is at its maximum, which is the highest point from the floor, the KE is 0. As the ball
heads toward the floor, the PE decreases and the KE increases. Just as the ball bounces on
the floor, the KE reaches its maximum value, and the PE is at its lowest. The spikes of
one graph coincide with the valleys of the other.
The loss of PE as the ball bounces should be represented by a curve. The ball should be
losing the same amounts of PE to sound, heat and elastic with each bounce. When
graphing the height of the each bounce for our experiment, this did not happen as seen by
the attached graph #5. This was most likely caused by the ranger reading different parts
of the round ball with each bounce.
Despite the variable of where the ranger recorded “h” from, our energy graph does show
signs of conserved energy with straight lines. The amount of total mechanical energy
does decline as energy changes to noise, heat and elastic forces. The loss of PE is not
uniform, most likely due to inaccurate readings by the ranger.