Scales measure weight, but to calculate body density we need mass. Some scales check out off mass, such as the electronic range in the picture listed below, even though they actually meacertain weight as questioned in the previous chapter.

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Mass can be determined from a weight bereason weight is simply the pressure of gravity on the body and pressure of gravity depends on mass in a recognized means. On the surface of the Planet, the pressure gravity on a things is related to its mass by the equation:

(1)

The acceleration because of gravity on Earth, typically abbreviated to*g*, has actually a value of 9.8 **m/s****2** and doesn’t readjust much over the entire surconfront of the Planet. Thus we (and also scales) can measure weight and then usage equation (1) above to calculate mass. Understanding why the continuous *g* is called the *acceleration due to gravity* requires presenting acceleration, which we will execute in a later unit, so for currently we recognize it as a constant worth that relates mass and weight for objects on the surchallenge of Earth.

Force is a vector, so we need to specify a direction for the gravitational pressure, which is always dvery own toward Earth’s center. We deserve to summarize the previous equation in symbol form:

(2)

### Reinforcement Exercises: Helen Maroulis

Body Weight and also Mass on the Moon

The value of*g*just holds consistent near the surconfront of the Earth, and therefore scales that use equation (1) to calculate mass from measured weight will certainly check out incorrect outcomes. For instance, your mass doesn’t readjust simply because you go to the moon (there isn’t all of a sudden less issue inside you), yet your weight does adjust. In truth if youstood on a scale on the moon it would meacertain a weight about 1/6 of what it would check out on Planet. The scale wouldn’t know you were on the moon rather of the Earth, so if the scale then tried to calculate your mass from weight, it would check out a mass that is 1/6 the actual value. Of course you didn’t shed 5/6 of yourself on the means there, so that would not be correct.

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When you carry out want to calculate the force of gravity and you are not close to the surconfront of the Earth then use the Universal Law of Gravitation**.**

The Universal Law of Gravitation says that the gravitational pressure in between two objects counts on the mass of each object (

and also ) and the distance in between their centers, (). To calculate the gravitational force we have to multiply the two masses together, divide by the distance in between them squared, and also finally multiply by the global gravitational consistent , which constantly has the same worth of . Written in equation form the universal regulation of gravitation is: