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General Physics 1 Lab - PHY 2048L
Name _______________________
Lab 1: Simple Harmonic Oscillator / Pendulum – Physics Motion PhET Lab Date ________________________
Author: Harsh Jain / PhET
Source:
Part 1: Simple Harmonic Oscillator http://phet.colorado.edu/en/simulation/mass-spring-lab
Part 2: The Pendulum – Introduction to Harmonic Motion http://phet.colorado.edu/en/simulation/pendulum-lab
Part 1: Simple Harmonic Oscillator
Introduction
The University of Colorado PhET group has produced a nice on-line simulation that lets you explore Newton’s 2nd law
of motion and the period of oscillation for a mass hung from a spring as well as how the mechanical energy is
distributed. In this problem, you will explore some of the aspects of the mass-on-a-spring system. This lab is available
online.
First explore the interface. It is shown below:
Notice that you can:



Click and drag a mass to the bottom of a spring and it will hook onto the spring.
Click and drag the horizontal (dotted) line wherever you like, click and drag the vertical ruler anywhere you
like, slow down the pace at which things happen by choosing a time fraction in the "time box" in the green
control panel, turn on a stopwatch by clicking on the stopwatch box in the control panel, turn on a display of
the components of the energy of one (and only one) of the mass/spring systems.
When adding and removing the masses, you might find it useful to pause the simulation by clicking on the
"pause" radio button in the time box. You can start it up again by clicking on one of the "time" buttons.
Procedure:
A. The sliders for the friction and spring stiffness have no numbers marked on them. Determine what they mean by
changing them and seeing how the motion of a hanging mass changes. Does the "0" marker (the leftmost mark on the
slider) correspond to the value "0" for friction? for spring stiffness (k)? Explain how you decided.
B. Set the slider for "stiffness of spring 3" in the middle (at the "5" mark). When it is set this way, do all three springs
have the same spring constant? Explain how you decided. Can you find the spring constants of each of the springs? If
you can, find them. If you can't, explain why you can't. When you change the slide for "stiffness of the spring 3" to a
different value, do all the springs change together? Explain how you decided.
C. There are three masses on the right of the mass collection colored red, gold, and green. These are not labeled as to
their mass. Can you find their masses?
D. Turn on the display of the energy of 3. Determine where on the screen for what configurations of the mass and
spring it defines the zero of the gravitational potential energy and for what configurations of the mass and spring it
defines the zero of the elastic (spring) potential energy.
E. For this part of the problem, you are to watch the pattern of the energies change as the mass and spring oscillates.
First, set a mass on one of the springs. Turn on maximum friction and let the mass come to rest. Move the dotted line
to show where the equilibrium point is. Now turn off friction and move the mass to another spring at just the
equilibrium point so it remains almost at rest. (This will restart the energy calculation.) Turn on the view of energy for
your system. You should have some elastic PE and some gravitational PE.
Predict what you think will happen to the two potential energies and the total energy if you grab the mass and pull it
down slowly. Then do it and see what happens. If it agrees with your prediction, explain the basis of your prediction.
If it disagrees, explain what went wrong.
Now set the timing so that the mass is oscillating slowly and you can watch what happens to the energies. Pull the
mass down and release it. Use the stopwatch to find an approximate time between the maximum values of each of the
kinetic and the two potential energies. Is time between two maxima of the KE the same as between the two potential
energies? If so, explain why. If not, explain why not.
Part 2: The Pendulum – Introduction to Harmonic Motion
Introduction:
Old grandfather clocks have large pendulums that swing back and forth to
keep time. A Foucault pendulum is a huge pendulum that swings in two
axes as the earth rotates to also keep time. The time a pendulum takes to
swing back and forth (one cycle) is referred to as one period. The period of
a pendulum is measured in seconds and is given by the formula shown
below. The inverse of period is frequency, the number of complete cycles each
second. The equilibrium position is the point below the pivot, at a neutral position. The
amplitude of the pendulum’s swing is the displacement from the equilibrium. The top of
each swing is referred to as maximum displacement or maximum amplitude.
Important Formulas: T  2
l

g
f 1
T
Procedure: PhET Simulations  Play with the Sims  Motion  Pendulum Lab
1. Spend some time learning about pendulums. The simulated pendulum is frictionless, so it will attain the same
amplitude in every swing. That is, it will lose no energy to friction (heat).
2. Using a 1.00 kg pendulum, for each trial, adjust the length of the pendulum and determine the period.
(In this lab, you may use the photogate timer to determine the period, but in the next lab, the spring lab you will
not have this luxury.)
3. Complete the table below.
Mass (kg) Length (m) Period (s) gravity
1.00 kg
1.00 kg
1.00 kg
1.00 kg
Earth, 9.8
m/s2
Earth, 9.8
m/s2
Earth, 9.8
m/s2
Earth, 9.8
m/s2
4. Repeat the investigation but, for each trial, adjust only the mass of the pendulum, leaving all other variables
constant.
Mass (kg) Length (m) Period (s) gravity
5. Repeat the experiment, but, for each trial, adjust the gravity (location) leaving all other variables constant.
Mass (kg) Length (m) Period (s) gravity
Velocity and Acceleration Vectors
6. Turn on the velocity and acceleration vectors.
7. Observe the magnitudes and directions of the vectors as the pendulum moves.
8. The green vector represents _____________ and the yellow vector
____________.
Pre-Lab Questions:
½-point each
1. What force (or acceleration) causes the pendulum to speed up on the way down and slow down on the way up?
___________
2. As pendulum length increases, the period of harmonic motion increases / decreases / remains the same.
3. As pendulum mass increases, the period of harmonic motion increases / decreases / remains the same.
4. As gravity (Jupiter) on the pendulum increases, the period of harmonic motion increases / decreases / remains the
same.
5. A pendulum attains maximum velocity at the equilibrium position / at maximum amplitude.
6. A pendulum attains minimum velocity at the equilibrium position / at maximum amplitude.
7. A pendulum attains maximum acceleration at the equilibrium position / at maximum amplitude.
Click on Show
Energy
More about PE
and KE next
unit!
8. A pendulum attains minimum acceleration at the equilibrium position / at maximum amplitude.
9. A pendulum attains maximum PE (potential energy) at the equilibrium position / at maximum amplitude.
10. A pendulum attains minimum KE (kinetic energy) at the equilibrium position / at maximum amplitude.
11. Consider a playground swing set. Is it possible for a kid to swing over the middle bar? ____________
12. Why / Why
not?________________________________________________________________________________
13. In real devices that use pendulums (clocks, Foucault pendulums in museums) a force must be added to counteract
friction. When should that force be applied? Constantly / at the same period as the pendulum / it doesn’t matter.
14. A pendulum that completes a cycle in 4 seconds has a period of ____________seconds.
15. That same pendulum has a frequency of ____________cycles per second (Hz)
16. If a pendulum completes 25 cycles in a minute, its period is ____________seconds.
17. …and its frequency is ____________Hz.
Use the period formula on the front page for the following. Answer in decimal form.
18. What is the period (on earth) of a .25 kg pendulum with a length of .45 m? (use the formula) ____________
19. What is the period (on earth) of a 7.5 kg pendulum with a length of .45 m? (use the formula) ____________
20. In order to swing with a period of exactly 2.0 s, a grandfather clock’s 1.5 kg pendulum must have a length of
______m.