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### Course: Physics library>Unit 4

Lesson 3: Newton's law of gravitation

# Mass and weight clarification

Difference between mass and weight. Created by Sal Khan.

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• Sal's mass is 70 Kg. How does he know what his mass is? If he steps on a scale, isn't that a measurement of the force of gravity? Is Kg. a measure of weight and mass?
• Scales divide the weight by g=9.80665 m/s^2.
• If gravity pulls too objects together instead of one object to the other why did the moon and earth not collide yet?
• Around , Sal gives us the formula for gravitational pull between 2 objects (F=G*m1*m2/r^2)
Let's say I have 2 hands, and I put them very close to each other. The distance between each of their centers of mass is very small. Would they not, therefore, both impose a strong force on each other and accelerate towards each other?
Furthermore, what happens when you take each of the singular atoms of your hands, which are extremely close together. Is a similar effect experienced?
• After thinking about it, I realize that the masses of my 2 hands are much smaller than the masses of the earth and myself, so even though they are a lot closer to each other, they are also a lot less massive, whch would explain why my hands don't impose a strong force on each other, even when they are touching.

However, I am still puzzled by the second case I gave. I am thinking it probably takes into consideration a whole bunch of other forces at that level, including the strong force, the weak force, and electromagnetism, which might cancel out gravity. But if it did, then how is my hand keeping the structure of my hand?

EDIT

The reason my hand does not fall apart is because each of the atoms in my hand are in some sort of bond (covalent or ionic) with each other, increasing greatly the force of electromagnetism between them, which allows each atom to have a fairly functional defined relationship to the rest of the atoms in my hand, thus retaining the structure. If this disappeared (for example, if I was completely made out of water) then the answer would be obvious, even common sense: I would not hold my structure.
• Does a common weighing scale measure mass or weight? For example, if the reading on the scale is 55kg, is my mass 55 kg or is it ~5 kg?
• It measures weight, but it is made in such a way that is shows your mass:
When you are on the surface on the earth, and have a mass of 55 kg, the force you exert on the scale will be:
m*g = 55 * 9.8 = 539 N
But the scale has the gravitational acceleration at the surface of the earth "built in" so that it will display your mass of 55 kg.
But if you took the scale to a place where the gravitational acceleration (g) is different that the one that is was calibrated for, and stand on it, it will show a different mass, on the moon for example, g is only 1.6. so the force that you exert on the sale will be:
55 * 1.6 = 88 N
And the scale will probably read something like 10 kg.
But your mass has not changed, it is just that the scale measures weight, and is calibrated to be used on the surface of the earth. Where it shows the corresponding mass.
• What exactly is BigG?
• It's just a universal constant, like the speed of light or the mass of an electron. It is found by very carefully measuring the way very accurately known masses interact when placed a very accurately known distance from each other. Whether the masses are large or small, close or far, they always have this factor involved.
http://en.wikipedia.org/wiki/Cavendish_experiment
• So, weight is the gravitational attraction(g) * mass right ?
Then, We must weight more when we are near the equator, and we must weight less when we are near the poles Right ? Is n't this the same feeling that happens on the moon ? We are feeling weightless on the moon. I have a silly question :)
If we buy some gold from some place very near to equator just imagine that I bought 1 kg of gold, and so the Weight must be equal to g*m = 9.8*1 = 9.8 N. Then my house is near the poles, when I bought it home it should feel more lighter right ?
• Actually you weigh more near the poles and less at the equator for two reasons.
1. The Earth is a flattened spheroid from its rotation so it bulges out at the equator and is flattened at the poles. This means the distance between you and the center of the Earth is more at the equator so the gravity is going to be a bit weaker there.
2. At the equator, you are spinning faster around the axis of rotation of the Earth than at the poles, so you will experience a larger centrifugal force. That centrifugal force has the effect of reducing your weight since it is acting in the opposite direction of gravity. From these two effects you will weigh 0.5% more at the pole than at the equator.
• My Physical Science teacher said that an objects weight had nothing to do with how quickly it fell,instead she said that objects fall at different speeds because of air resistance. If it's weight is gravity's force on it then wouldn't it change it's falling speed.
• how can we prove that weight in moon is 1/6 of the weight in the earth?
• If we can calculate radius and mass of moon we get it.
• What is the difference between Mass and Weight ?
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