Static Equilibrium Lab

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Pasadena City College *

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8A

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Mechanical Engineering

Date

Apr 3, 2024

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pdf

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4

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Static Equilibrium _________________________________________________________________________ Introduction In this lab, you will use your understanding of static equilibrium and center of mass to predict the mass of two objects, a 2-meter stick and an irregularly-shaped block. Your grade will be based on the accuracy of your predicted masses (see table below). Theory For ages, people have used balances to weigh objects. An object of unknown mass m o is hung some distance L o from the fulcrum (balance point), and a known mass m w is hung some distance L w . If the object and mass balance each other out, then the system is in static equilibrium and the net torque on the balance must be zero: N•m τ 𝑛𝑒𝑡 = 0 Furthermore, we can find the torque exerted by both the object and the mass using the formula , τ = 𝑟 𝐹 recalling that r is the perpendicular lever arm and F is the force exerted by the object. Remember that a torque causing counterclockwise motion is regarded as being positive, and clockwise is negative. In this lab, you will be determining the mass of the unknown object as well as the mass of the 2-meter stick that you will be using as a balance. To find the mass of the 2-meter stick, you will need to place the fulcrum somewhere other than the balance point! Then, you can use a similar technique to balance the system with a known mass, keeping in mind that the force of gravity on the 2-meter stick acts on its center of gravity. For both, you will want to perform multiple trials to arrive at precise values, since your grade for the lab will depend on the accuracy of your results. The table at the bottom of the lab shows how precision correlates with final grade.
Equipment (1) 2-meter stick (2) unknown mass object (3) fulcrum & string (4) various known weights (6) precise digital scale Procedure I. To find the mass of the unknown object, set up the system as follows: Set up the symbolic equation for static equilibrium and solve for the mass of the unknown object, m o . Show your derivation in the Analysis section. Perform 5 independent measurements using this setup, carefully measuring and recording L o , L w , and m w in Table 1. Use these measurements to derive a value for m o for each trial as well as an average value for m o . Remember that this is the value you will be giving to your instructor as your prediction, so if there are any trials that seem problematic, best to do them over! II. To find the mass of the 2-meter stick, set up the system as follows: Draw a force diagram showing the relevant forces in this system and use it to set up and derive an equation for the mass of the 2-meter stick, m s . Show your diagram and derivation in the Analysis section. Carefully determine the location of the center of mass of the meter stick and record this location in the Analysis section. Perform 5 independent measurements using this new setup, recording values in Table 2. Use these values to find the average value for m s, th .
Analysis Derivation of equation used for finding m o, th : Table 1 L o (cm) L w (cm) m w (g) m o, th (g) Average value for m o, th : ______________________ Derivation of equation used for finding m s, the : Table 2 L s (cm) L w (cm) m w (g) m s, th (g) Average value for m s, th : ______________________
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Average Percent Error Grade Average mass of 2-meter stick and block accurate to ≤ 0.1% 100 Average mass of 2-meter stick and block accurate to ≤ 0.5% 95 Average mass of 2-meter stick and block accurate to ≤ 1% 85 Average mass of 2-meter stick and block accurate to > 1% Repeat with a new 2-meter stick and block, lose one grade point