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Physical Science: Session 5

A Closer Look: What Are the "Units of Pull"?

Do our "push meter" and "pull meter" measure different things?

push meter
Science Studio push meter.

When the children in the Science Studio in Session 5 pushed the Styrofoam under the water, the scale read about 30 pounds. When they used the "pull meter" to pull the Styrofoam under the water, the instrument read 11 “somethings.” It was not clear what the unit was. In fact, both meters measure the same thing — force. Force is sometimes defined as "any kind of push or pull on an object." The push meter measured force in pounds from the English system of measurement, while the pull meter measured force in Newtons from the metric system. One pound of force is equal to 4.4. Newtons. In general, when talking about force, it is equally correct to use Newtons and/or pounds as units.

Weight and mass in different units: “But I thought 2.2 pounds was a kilogram?”

If pounds are a unit of force in the English system and Newtons are the unit of force in the metric system, where do kilograms fit in? This is sometimes a confusing issue in introductory science. The bottom line can be put into one sentence: In the United States, we (incorrectly) use a unit of force, the pound, when we are really talking about an object’s mass.

Recall that mass is a measure of the amount of matter in an object. Another equally good definition is “the measure of an object’s resistance to motion” (i.e., a measure of its inertia). This amount of matter in an object is not defined in terms of any forces at all, and so there are different units for mass. In the metric system, the unit of mass is the gram; in the English system, the unit of mass is the slug.

Why is weight a force?

We know that forces are defined as pushes or pulls. If weight is a force, where does the pull come from? The answer is gravity. Every piece of matter in the universe pulls on every other piece of matter with a gravitational force. As you sit reading this sentence, every other piece of matter in the universe is pulling you toward it: your coffee cup, your neighbor’s lawnmower, the Empire State Building, the Sun, and even a galaxy halfway across the universe. However, by far the largest gravitational force that acts on you is the pull toward the Earth, because it is so big and so close. The other forces add very little to the total gravitational force you feel. It is the gravitational force from the Earth that we call “weight.”

Let’s look at an example. When astronauts visited the moon, their mass (i.e., the amount of matter in their bodies) did not change. However, because the Moon has less mass than the Earth, it generates less gravitational force. As a result, the astronauts on the Moon had less force acting on them and, therefore, their weight on the Moon would have been less than their weight on the Earth. Weight is a measurement of the amount of force, i.e., the pull being exerted on an object, and mass is a measurement of the amount of matter in an object.

It would be more precise if we replaced the word “weight” with the phrase “gravitational force due to Earth” but, in everyday language, we rarely need to make our meaning so exact. Language in science, however, is designed to be more precise than everyday language. Certain words carry specific meaning, and we must be aware of those meanings when we look to communicate in a scientific way.

In summary, here is a table showing the relation between weight and mass in the two systems of units we’ve been examining:

Unit System/Quantity Mass Force (weight = gravitational force due to Earth)
Metric (SI) Gram Newton
English Slug Pound
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