GS 104 EV Lab Report: Kinda Pisa
I) Force and Mass :
While forces are really familiar to us in our daily lives we still harbor
incomplete understandings and ideas about forces. In this lab we will try to
bring some of those incomplete ideas to the surface and explore them. The
goal is for you to establish the conceptual relationship between force and
mass that underlies Newton's Laws which collectively form one of the
pillars of classical physics.
The experiment itself is fairly simple. You will place one of your lab
partners in a rolling computer chair. Using a spring scale, which measures
the amount of force being applied, you will pull them down the hallway in
different ways while keeping track of the things like speed (velocity) and
acceleration.
We will begin with a picture. As we (hopefully) discussed in class pictures
are a critical part of the science communication process. This idea applies
equally in math as affirmed by the following excerpt of a communication
with Galileo Galilei who was a physicist and mathematician.
[A] mathematician, however great, without the help of a good
drawing, is not only half a mathematician, but also a man without
eyes.
Lodovico Cigoli to Galileo Galilei, 1611
In physics a picture which describes the forces we think are involved in
some problem is called a freebody diagram. We will refer to it as a force
picture for now. Make a rough sketch of the object you are interested in and
indicate the direction of each force acting on the object and give it a clear
label.
As you approach the task of doing the force picture (freebody diagram)
consider the non-contact and contact forces which might be acting on the
person and the chair collectively. For each force you put on your drawing
list the two partners which "produce" the force. (If you ignore air resistance
you will probably find 4 forces)
II) Data: You will be performing two different experiments. In each case
you need to repeat the experiment with three different people sitting in the
chair so that you can generate enough data to make a plot later. You may
well find that you will have to repeat each part of the experiment to get
consistent results. Keep track of all of your data as you do so.
Experiment I: You have some idea what it means to move with constant
speed. What I want you to determine is what force do you need to apply
(with the spring) to the person in the chair to KEEP them moving at a
constant speed down the hall (give them a little push to get them going
first)? Does this depend on the mass of the person in the chair?
Experiment II: The second experiment involves applying a constant force
which is large enough to make the person in the chair speed up. From the
data in your first experiment choose a force larger than that required to keep
the "heaviest person" moving at a constant speed. Now the difficult part,
apply that CONSTANT force to each of the different people in the chair and
record how long it takes to speed up to a "fast walk". You will need to define
what a fast walk means and try to reproduce it in each experiment. Again,
record all your data for future use.
IIIa) Plotting:
Experiment I: Plot the force required to maintain constant speed as a
function of the mass of the person and the chair. The force will be in
Newton's and the mass will be in kg. Some unit conversion may be
necessary
Experiment II: Plot the time required to reach your target speed as a
function of the mass of the person and the chair. The time will be in seconds
and the mass will be in kg.
IIIb) Sloping:
Experiment I: From your plot determine the rate of change of the force with
respect to mass. Show all of your calculations and be clear how it all relates
to the data you took and plotted.
Experiment II: From your plot determine the rate of change of the
acceleration time with respect to mass. Show all of your calculations and be
clear how it all relates to the data you took and plotted.
IV) Analysis (What might it mean?):
The core process of science is to take the data and then sit down and look at
it and see "what it says". You have some data and some relevant calculations
about how people in chairs behave when pulled by forces. What does your
data have to say about the following questions. Be careful, I'm not asking
what you think the answer should be or what other experiences you have had
suggest but specifically what the data you took in this lab has to say about
the situation.
I) Does the force you needed to keep the person moving at constant speed
depend on their mass? How strongly (you will need to figure out some why
of expressing what you think this means)?
II) Does the time you need to get the person "up to speed" depend on their
mass? How strongly?
III) Does your data suggest that the mass plays a bigger role in experiment I
or II? How do you know this from the data?