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Physics Bridging Course
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Workshop 8.1
Topic: Conservation of Energy
Preparation:
• GR&R: Read Sections 6.1 – 6.3
• WebAssign: Do the assignment called: BC: Wk 8 – Problems on
Work Done by a Constant Force
Purpose:
To investigate how energy is transferred between gravitational
potential energy and kinetic energy.
Equipment:
1. A sturdy rod stand
2. Two three-prong clamps
3. The tube from the lacrosse ball cannon. Remove the plunger
assembly by turning the tube upside down and allowing the
plunger assembly to gently fall out.
4. An ultrasonic motion detector
5. Two balls, one 60 g and one 40 g.
6. A computer with DataStudio installed on it.
Introduction:
One of the most fundamental ideas in physics is that all energy is
conserved. You can waste energy, you can lose energy (we will be
investigating this in our next workshop), but cannot destroy it. You
also cannot create it. All the energy in the universe has been here
since the Big Bang, and will always be here; we are simply recycling
energy over and over when we use electricity, heat or batteries.
Work is the method that we use to transfer energy from one object
to another. When you lift an object through a distance, against a
gravitational field, you are doing work on the object, and giving it
energy. This is called gravitational potential energy, and its
symbol is U. The amount of gravitational potential energy that you
give an object is equal to the amount of work you do in lifting it:
!
U !=!mgh
Keep this.
1
(Note that both force and displacement are vectors, but this kind of
product is a "dot" product. The answer to a "dot product" is always
scalar. Energy is a scalar quantity.)
So,
! !
U !=! F !!"y
We know that the force that is necessary to lift an object is greater
than or equal to its weight. We will take the limiting case and assume
that the force is equal to the weight of the object.
!
!
!
F!=!W =!mg
So, after substituting:
! !
U !=! F !!"y
! !
U !=!mg ! "y
The more familiar form of this equation is:
!
U !=!mgh
Kinetic Energy is given by the formula:
1 2
K !=! mv
2
Predict:
Discuss conservation of energy with your partners. Imagine a freely
falling object. At the start of its fall, when vo = 0, all of the object's
energy is gravitational potential energy.
1. As the object falls, what happens to its potential energy? Why?
Explain.
2. As the object falls, what happens to its velocity? What happens
to the kinetic energy?
3. If you added up U and K at each point in its fall, how would you
expect the sums to change at each point?
4. If you do the experiment with an object with different mass,
Keep this.
2
what do you expect to happen U, K and the sum U+K?
Method:
Clamp the tube in the the two pronged clamps so that the end with
the collar is down. Leave enough room under the tube for the
ultrasonic motion detector.
Tube
Clamp
Stand
Motion
detector
It is important that the tube be vertical. Use a plumb bob to make
sure that the tube is exactly vertical.
Set the motion detector under the collar end of the tube. Set
DataStudio to measure position and velocity. Set it up to give you a
graph and a table. (Put position and velocity on the same table.)
Start collecting data, and drop the 60-gram ball into the tube. Look
at the velocity graph; if the line is excessively spikey, discard that
data and drop the ball again. You want the smoothest, straightest
linear graph that you can get.
When you have suitable data, use the "select and
zoom" button on DataStudio to select the portion of
the position-time graph that shows the position of
the ball as it falls. Copy only these points from the
table and insert them into an Excel spreadsheet. Do
the same with the velocity data.
For each of your selected points, calculate the gravitational
Keep this.
3
potential energy (U) of the ball. Calculate the kinetic energy of the
ball (K) for each point.
You should have a spreadsheet now that looks like this:
Position ( m ) Time ( s ) Velocity ( m/s )
1.15
4.03
-0.74
1.14
4.05
-0.6
1.12
4.07
-0.7
1.11
4.09
-0.79
1.09
4.11
-0.7
U
4.4
4.4
4.3
4.3
4.2
K
0.3
0.2
0.2
0.3
0.2
(You will have many more data points; this is just a sample.)
Select the columns for Potential Energy and for Kinetic Energy. Click
on the Chart Wizard. When you choose the kind of graph you want,
click on the tab labeled "Custom Types." Choose the "Stack of
Colors" type. Properly label and name your graph.
Repeat with the 40 gram ball.
Now answer the four questions again, after considering your data
and graphs.
1. As the object falls, what happens to its potential energy? Why?
Explain.
2. As the object falls, what happens to its velocity? What happens
to the kinetic energy?
3. If you added up U and K at each point in its fall, how would you
expect the sums to change at each point?
If you do the experiment with an object with different mass, what
do you expect to happen U, K and the sum U+K?
Keep this.
4
Name:
Team members:
WS 8.1: Forces in Equilibium
Neatness counts! If I can't easily follow your work, I won't grade it.
Show your data tables and graphs here.
1. As the object falls, what happens to its potential energy? Why?
Explain.
2. As the object falls, what happens to its velocity? What happens
to the kinetic energy?
3. If you added up U and K at each point in its fall, how would you
expect the sums to change at each point?
If you do the experiment with an object with different mass, what
do you expect to happen U, K and the sum U+K?
Turn this in.
5