Mass and Weight
Pre-Lab Question
UM Physics Demo Lab 07/2013
Are weight and mass the same thing? Explain your reasoning.
EXPLORATION
Exploration Materials
200 gram Spring Scale (Ohaus 8262 M)
4 – ½ inch cut steel washers (standard weights)
Clear plastic Ruler
1. Hang 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:
# Washers
Scale Reading
(grams)
1
2
3
4
2. Now plot your data on the grid below:
Number of Washers
0
Scale Deflection (“grams”)
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Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109
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3. Using a clear ruler, draw the best straight line you can draw through your data. Do
your data points reasonably represent a straight line?
4. 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?
5. What force is proportional to the number of washers?
6. 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.
7. 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.
8. 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.
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Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109
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Everyday Applications
APPLICATION
1. Spring scales are still commonly used in many everyday applications. Give two
examples of spring scales that you have used or have seen in use.
2. 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.
3. Does Hooke’s Law apply to springs that are compressed as well as stretched?
Explain your reasoning.
Summary:
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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).
Property of LS&A Physics Department Demonstration Lab
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109
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