Physics I - Simple Form Truss Lab
Introduction:
Trusses are structural units made of straight bars that form stable, rigid forms. The first trusses
were used in 2500 BCE in primitive lake houses and they also saw extensive use in the Roman
Empire. Today, we interact with trusses almost everywhere we go, they can be found in the
bridges and buildings we travel across and live or work inside. Without an understanding of how
trusses work we would be hard-pressed to build the large bridges that connect us to those around
us.
Materials:
500 g mass, protractor, simple truss form, 20-N digital scale, ring stand, c-clamp
Purpose:
Learn the relations between the locations of the mass and scale as well as the effect of the scale’s
angle.
Procedure:
We began the lab by setting up our truss form on the ring stand and clamping it to the table. We
then hung the mass and scale at 40cm and found the force required to keep the truss form level at
30°, 45°, and 60°. Next the mass was hung at 25cm and 10cm, with three tests using the same
angles. The scale was then hung at 25cm and the mass at 40, 25, and 10cm with the same three
angles. Finally, we hung the scale at 10cm and once again hung the mass from 40, 25, and 10cm
using the same three angles. For the last part of our lab, we found the length of string we would
need using the position and angle of the scale.
Data:
Test #
Digital
Scale
Position
Mass
Position
Boom
Angle
Digital
Scale
Angle
30o
Calculated
String
Length
(cm)
46.19
Digital
Scale
Force
(N)
11.70
1
40 cm
40 cm
0o
2
40 cm
40 cm
0o
45o
70.71
8.45
3
40 cm
40 cm
0o
60o
80
6.56
4
40 cm
25 cm
0o
30o
46.19
8.14
5
40 cm
25 cm
0o
45o
70.71
5.66
6
40 cm
25 cm
0o
60o
80
4.02
7
40 cm
10 cm
0o
30o
46.19
3.23
8
40 cm
10 cm
0o
45o
70.71
2.42
1
9
40 cm
10 cm
0o
60o
80
1.90
10
25 cm
40 cm
0o
30o
28.87
18.09
40 cm
o
o
35.36
13.05
o
11
25 cm
0
o
45
12
25 cm
40 cm
0
60
50
10.01
13
25 cm
25 cm
0o
30o
28.87
11.42
14
25 cm
25 cm
0o
45o
35.36
7.84
15
25 cm
25 cm
0o
60o
50
6.81
Test #
Digital
Scale
Position
Mass
Position
Boom
Angle
Digital
Scale
Angle
16
25 cm
10 cm
0o
30o
Calculated
String
Length
(cm)
28.87
Digital
Scale
Force
(N)
5.27
17
25 cm
10 cm
0o
45o
35.36
3.63
18
25 cm
10 cm
0o
60o
50
2.99
19
10 cm
40 cm
0o
30o
11.55
>20
20
10 cm
40 cm
0o
45o
14.14
>20
21
10 cm
40 cm
0o
60o
20
>20
22
10 cm
25 cm
0o
30o
11.55
>20
23
10 cm
25 cm
0o
45o
14.14
17.62
24
10 cm
25 cm
0o
60o
20
14.75
25
10 cm
10 cm
0o
30o
11.55
9.23
26
10 cm
10 cm
0o
45o
14.14
7.19
27
10 cm
10 cm
0o
60o
20
6.39
2
Questions:
1) Which boom setup completed the objectives in the best possible manner (sign is
farthest from wall without breaking the cable and uses the smallest amount of
cable)?
Test Number 6
2) In order to provide the strongest support, where is the best place to hang the mass
in relation to the scale attachment?
The locations for test number 9. The mass at 10cm and the scale at 40 cm.
3) What is the relationship between the angle of the scale and the tension in the string?
They are inversely proportional, the greater the angle the lower the force (up to 90°).
4) Compare the force measurements for when the scale and mass are attached at the
same position (trials 1-3, 13-15, & 25-27)?
When the mass and the scale are at the same position the forces for any given angle will be the
same.
5) Which boom below would produce the most tension in the string? Why? Assume the
mass in each figure is the same.
Boom c. would create the greatest tension in the string. This is because the mass and scale are
hung at the same locations in each so the distance does not matter just the angle.
Conclusion:
In this lab I learned that the smaller the angle the greater the force required to balance the truss.
This is because the closer the angle is to parallel the greater the horizontal component of the
force and the smaller the vertical component. During our testing we most likely did not have the
truss perfectly level or the scale at the exact angle we were testing which could cause our force
values to be greater of smaller than they should have been.
References:
Lab handout, Google
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