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Home Concept Builders Newton's Laws Mass and Weight Video Video Transcript

Video Transcript: Mass and Weight

We provide the transcript below to those who for whatever reason would find the written words to be preferred over in addition to the actual video.

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Introduction

Two often confused terms are mass and weight. What exactly is mass and weight? How are they different? And how are they related? I'm Mr. H and I have some answers for you.

Mass

Mass refers to the amount of stuff (more precisely, matter) that is present in an object. Matter consists of atoms; it's the stuff that gives objects mass. So the mass of objects depends on how many atoms and what type of atoms are in it. The standard unit of mass is the kilogram, abbreviated kg. You can feel how much mass an object has by lifting it. But you can also feel mass by shaking it or giving it a horizontal push. The more mass an object has, the more resistance it offers to the shaking and the pushing.

Weight

Weight is the force of gravity that acts on an object. Being a force, the amount is expressed in units of force like the Newton (abbreviated N) or the non-metric unit pound. You can determine an objects weight by ... weighing it. That is. put an object on a scale and read the scale reading. Gravity pulls the object down. The scale pushes back up to balance gravity and outputs the upward force. It's equal to the down force of gravity; we call this the weight.

Mass, Weight, and Location

Now be careful with this. Mass, the amount of stuff in an object, does NOT depend on location. If you take your stuff to the moon, it would be the same amount of stuff ... assuming you got it all there. Your mass would be the same. Weight, the force of gravity acting on your stuff, does depend on the location. On the moon, you would weigh less than on Earth. That's because how much an object weighs depends on the gravitational environment. Gravity forces on the moon are one-sixth of Earth's. A 90-kg person on Earth would weigh about 900 N. But on the moon, the same 90-kg person would weigh about 150-N. Weight depends on location. Mass does not.

Relation Between Mass and Weight

More massive objects weigh more than less massive objects. Obviously there's a connection between mass and weight. Here it is. The weight or force of gravity acting on an object is equal to the object's mass multiplied by g. 

Weight = Fgrav= m • g

g is the gravitational field constant. On Earth, it's value is 9.8 N/kg. On the moon, g is ~1.7 N/kg. On Mars, g is ~3.8 N/kg. 

The value of g depends on the gravitational environment of the location where the object is at. Because g depends on location, the force of gravity or weight depends on location. The real constant in this equation is the mass. It does not vary with location and is dependent only upon the amount of stuff present in the object. 

Calculating Weight

The weight of an object can be calculated from its mass using this equation. 

Weight = Fgrav= m • g

It was mentioned earlier that a 90-kg object weighs about 900 N on Earth. To be exact, it weighs 882 N.  On the moon, where the value of g is 1.7 N/kg, it weighs about 150 N. The weight difference between Earth and moon is due to the value of g ... not due to mass.

Calculating Mass

Let's try this one: What mass does an object need to weigh 900 N on Mars, where g is 3.8 N/kg? First we need to perform some algebra on our equation. (Pause) There we go. Now we can calculate the mass by substitution. It is ~240 kg (rounded from 236.84 ... kg).

A Final Caution

The equation Fgrav= m • g can cause troubles if you're not careful. Avoid referring to g as gravity. And avoid referring to Fgravas gravity. The moment you do, you introduce confusion that doesn't need to be there. Fgravis the force of gravity and g is the gravitational field constant. Keeping the language straight helps keep the concepts straight in your brain.

Conclusion

When learning a Physics concept, it's important to do something to make the learning stick. To help, there's some pretty awesome interactive exercises on our website. You'll find links to them in the Description section of this video. Give one a try and make sure you got this! I'm Mr. H. Thanks for watching!

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