Mass and Weight Lab - University of Michigan



Mass and WeightUM Physics Demo Lab 07/2013Pre-Lab QuestionAre weight and mass the same thing? Explain your reasoning.EXPLORATIONExploration Materials200 gram Spring Scale (Ohaus 8262 M)4 – ? inch cut steel washers (standard weights)Clear plastic RulerHang the 200g Ohaus 8262 M spring scale vertically (fingers work fine). Use the aluminum tab at the top of the scale to slide the faceplate up and down until the pointer reads 0 grams. Now hang one washer on the scale and record the scale reading, keeping in mind that the smallest division mark on the scale corresponds to 2 grams. Repeat the measurement for two, three and four washers and record your data in table below: # WashersScale Reading (grams)1234Now plot your data on the grid below: Number of Washers 0 Scale Deflection (“grams”)Using a clear ruler, draw the best straight line you can draw through your data. Do your data points reasonably represent a straight line?Note that we labeled the horizontal axis of the graph with a unit of “grams”? What physical property of the spring inside the scale did you actually observe as you changed the number of washers?What force is proportional to the number of washers?What intrinsic property of the washers determines the magnitude of this force? How is this force related to this intrinsic property and g, the acceleration of gravity near the surface of the Earth? Discuss both the magnitude (size) of the force and its direction.Hooke’s Law states that force required to stretch an ideal spring is proportional to the amount the spring stretches. Does your scale obey Hooke’s Law? Explain your reasoning.A Free Body Diagram (FBD) is a cartoon depiction of an object used to analyze the forces acting on the object. Trace around a washer and indicate the forces acting on a washer which is hanging from a scale. Represent the forces as arrows, with the length of the arrow corresponding to the magnitude of the force and the direction of the arrow indicating the direction that the force acts. Take the bottom of the page to be the direction downward toward the floor. Think carefully about the length of the arrows relative to each other.Everyday ApplicationsAPPLICATIONSpring scales are still commonly used in many everyday applications. Give two examples of spring scales that you have used or have seen in use.Explain how the “Pelouze Model P1” pan-type spring scale differs in its operation when compared to the 200g Ohaus 8262 M scale. Specifically, compare the operation of the spring in the two types of scales.Does Hooke’s Law apply to springs that are compressed as well as stretched? Explain your reasoning.Summary:Mass, also called inertia, is an intrinsic property of matter and does not change due to the objects proximity to gravitating bodies. Mass is a scalar, that is, a number that has a magnitude, but no direction associated with it.An objects weight is the product of its mass and the acceleration of gravity near the Earth’s (or any other gravitating body’s) surface. Weight is a vector which points toward the center of the Earth (the definition of “down”). Weight is not intrinsic because it depends on the local acceleration of gravity. On the Moon the acceleration of gravity is approximately 1/6 that at the Earth’s surface. On the surface of the Moon your weight is 1/6 of your “Earth weight” but your mass is unchanged.Simple scales can be built from springs. Springs obey Hooke’s Law which states that the force required to stretch a spring is proportional to the amount the spring stretches. Equivalently, the amount a spring stretches is also proportional to the applied force. If the applied force is the weight of an object, then the amount the spring stretches is proportional to the weight of the object which is in turn proportional to the mass of the object.Spring scales measure a force, the object’s weight. Because weight is proportional to the objects mass (assuming we stay on Earth), scales are sometimes calibrated in units of mass. Scales calibrated in English units almost always use force units (pounds or ounces) whereas metric scales are typically calibrated in mass units (grams or kilograms). The metric unit of force is the Newton (N). ................
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