Physics
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Boundless Physics
The Laws of Motion
Other Examples of Forces
Physics Textbooks Boundless Physics The Laws of Motion Other Examples of Forces
Physics Textbooks Boundless Physics The Laws of Motion
Physics Textbooks Boundless Physics
Physics Textbooks
Physics
Concept Version 7
Created by Boundless

Weight

Weight is taken as the force on an object due to gravity, and is different than the mass of an object.

Learning Objective

  • Infer what factors other than gravity will contribute to the apparent weight of an object


Key Points

    • Weight is taken to be the force on an object due to gravity.
    • Weight and mass are not the same thing!
    • The weight of a given mass will be different when the acceleration due to gravity is different.
    • /Apparent weight can change because of the effect of buoyancy.
    • The strength of gravity is almost the same everywhere on the surface of the Earth.

Term

  • Gravitational acceleration

    Gravitational acceleration is the acceleration that an object undergoes due solely to gravity


Example

    • If you find a rock that is 10 kg, its weight would be approximately $10 kg * 9.8 \frac{m}{s^2} = 98 N$.

Full Text

In physics, it is important to differentiate the weight of an object from its mass. The mass of an object is an intrinsic quantity, independent of the location of the object. On the other hand, the weight of an object is an extrinsic quantity. It is considered as the force on an object due to gravity. Since gravitational acceleration changes depending on the location in the universe, weight does as well.

Mathematically, the weight of an object (W) can be found by multiplying its mass (m) by the acceleration due to gravity (g): $W=M\cdot g$. The strength of gravity varies very little over the surface of the Earth. In fact, the greatest percent difference in the value of the acceleration due to gravity on Earth is 0.5%.

For most calculations involving the weight of an object on Earth, it is sufficient to assume that $g= 9.8 \frac{m}{s^2}$.

The weight of an object has the same SI unit as force—the Newton ($1 N= 1 kg\cdot \frac{m}{s^2}$).

In US customary units, the weight of an object can be expressed in pounds. Keep in mind that in US units the pound is either a unit of force or of mass. If one must find the weight (as opposed to the mass) of an object in US units, it can be calculated in terms of pounds of force.

It is important to note that the apparent weight of an object (i.e., the weight of an object determined by a scale) will vary if forces other than gravity are acting upon the object . For example, if you weigh a given mass underwater you will find a different result than if you weigh that mass in air. In this case, the weight of the object varies due to the force of buoyancy. While the mass is in the water it displaces fluid, resulting in an upward force upon it. This upward force affects the net force that the mass exerts on the scale, and thus alters its "apparent" weight.

Spring Scale

A spring scale measures weight by finding the extent to which a spring is compressed.This is proportional to the force that a mass exerts on the scale due to its weight.

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