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Introduction to Physics Concepts
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| Cosmonauts train in a weightless environment provided by an airplane flying on a parabolic path. |
MASS is a fixed property of an object and does not depend on anything outside the object. It defines how much there is of the object and is related to its inertia.
WEIGHT, on the other hand, is the force required to support an object against the pull of gravity. A common illustration of this is when the same physics teacher who weighs 160 pounds on Earth weighs less on the Moon (same mass with less gravity equals less weight).
Weight is related to both mass and gravity.
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| A person's weight on the Moon is 1/6 of his or her weight on Earth. |
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| m | = | F |
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Once the mass is known, the weight can be calculated by using the expression weight = mg, where g is the acceleration of gravity (9.8 m/s2). Let's try some examples.
What is the weight of a 25 kg child?
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| Because this triple-beam balance scale measures the mass (not the weight) of an object, it will read the same on any planet. |
What is the mass of a 1 N apple? It may seem strange that the expression for weight has an acceleration in it, since objects are normally weighed at rest. Remember that the force of gravity acting on an object of mass "m" is given by F = mg, whether the object is moving or not. If the object happens to be in free fall (under the influence of gravity only), then Newton's second law says that F = ma. Comparing the two expressions for force, it is clear that a = g. Thus, the gravitational constant is the free-fall acceleration.
Weight a Minute
What if the mass of an object is unknown? Can its weight be measured? Sure — with a scale. Inside the scale is a spring that is stretched or compressed when a force acts on it. This distortion is indicated by a pointer attached to the spring. When a person stands on the scale, the tension in the spring mechanism matches his or her weight, and the pointer indicates this tension (the weight). Just remember that a scale measures weight, not mass.Here's an example to illustrate.
What does this all mean, really? All of Newton's laws, including universal gravitation (which will be addressed later) relate force, acceleration, and mass. Weight, wherever and in whatever units it is measured, is a force that affects anything with mass. So the force really is with you!
