Physics Project: Mobiles (Designed by Nancy J. Easterly/Modified by Tambra Hamilton)
Description: Students will demonstrate an understanding of torque and equilibrium by designing and building
a balanced mobile.
Grading: The mobile project will count for 2 lab grades
Due Date: _________________________
Grading Rubric:
1. The mobile will have at least 3 suspension bars.
2. At least one of the suspension bars must be balanced at a point other than its center.
3. The mobile will have a minimum of 4 objects suspended from the three bars.
4. The entire mobile must be suspended and balance from a single string. Crossbars and objects cannot touch
each other.
5. The mobile will be thematic and have a title (birds, space ships, etc.)
6. Weight limit: 1.00 kg, (2.21 lb)
7. Size limit: 50 cm wide by 50 cm tall (from the top of string to the bottom of the mobile).
8. Coming to class prepared to work and working throughout class period.
9. Make a drawing of the mobile, with weights and torque arms recorded on it. Don’t forget to include the
weight of the bars themselves!
10. An equation that describes the rotational equilibrium for each suspension bar will be included on the
drawing.
11. Answers to problems. Work must be shown—units, equations, complete FBD.
NOTE: No soft drink cans, plastic bottles, cds, or dvds as your suspended objects. These items may be used to
build other items to hang. The mobile must be in good taste, abiding by the school district rules concerning
alcoholic beverages, tobacco, drugs, etc. Any violation of such rules will result in a zero grade for the project.
Each group is responsible for providing a plastic bag to hold all items until the project is complete. Projects
may NOT be left hanging at the end of the day as this sets off motion detectors in the building. All mobiles
must be stored at the end of each work period.
Problems:
1. The bar of a barbell has a mass of 10.0 kg and is 1.70 m long. A weight lifter places a 50.0 kg mass 25.0 cm
from one end and a 40.0 kg mass 25.0 cm from the other end.
(A) With respect to the midpoint of the bar, where is the center of gravity of the loaded barbell?
(B) If the lifter places his hands .30 m on either side of the midpoint of the bar, how much force must each
hand exert to support the loaded barbell?
2. (A) At what point should a uniform board 3.00 m long and mass 1.50 kg be supported so that it balances a
1.00 kg mass placed at one end, a 6.00 kg mass on the other end and a 4.00 kg mass 1.00 m from the 1.00 kg
mass?
(B) What is the magnitude of the equilibrating force?
3. A mobile has an initial suspension bar of length 40.0 cm. From one end a 1.35 N bauble is suspended 5.00
cm from the other end a second bar of 25.0 cm is suspended with a .300 N bauble from one end and a .500 N
bauble form the other end. Find where the support rope for each bar must be placed to balance the mobile.
Assume both bars have negligible weight.
Checklist for Mobile Assessment
Title: ___________________________________________ Team: ________________________
I. Mobile (25%)
2.5 points: Neat construction and assembly, well balanced, more than three levels
2 points: Good construction and assembly, well balanced, three levels
1.5 points: Poor construction and assembly and/or poor balance and three levels
II. Balance and Objects (25%)
2.5 points : more than one bar is balanced other than at center, more than four objects
2 points: one bar balanced at other than center, four objects suspended
1.5 points: one bar balanced at other than center, less than four objects suspended OR bars/objects touch
1 point: all bars balanced at center
III. Physical Properties (15%)
1.5 points: mass and volume requirements met and theme clear
1 points: mass and volume requirements violated OR no theme/title
IV. Drawing/Blueprint (30%)
3 points: Neat, complete, all torque arms/forces labeled.
2 points: Blueprint is neat, mostly labeled, and to scale.
1 points: Blueprint is incomplete.
V. Participation (5%)
0.5 points: All materials everyday, always on task, works entire class period, all materials stowed away at end of
day, present each day or made up work session after school.
0.25 points: Not always on task, materials missing OR did not work entire class period. Absent one or more days
and did not make up missed time.
0.15 points: inconsistent effort, often off-task but present every day
Comments:
Mobile Total points: _________________ x 10 = -------- /100 (two lab grades)
Problems graded separately—one per student. 25 points. Graded Problem Set—Do not give or receive help
on these problems.—Test/Quiz grade.
Late---5 points each day late.