physique chimie college lycee



spring scale vs balance



Description: In physics, mass and weight cover different concepts.

To measure a MASS, we use a BALANCE.
We compare a known mass with a mass we want to know, and we know it at equilibrium.
Here, this bag of coffee has a mass of 200 grams.
The device is a BALANCE. The official unit is kilogram.
It is the unity of the international system.
I draw your attention to the fact that the symbol here is written in lower case: kg.
You can do this experiment everywhere, you will always find 200 grams.

Regardless of the place, and even the planet where you are going to take this measurement, you will always have to put 200 grams on this tray to achieve balance with this bag of coffee.

In physics, weight is a STRENGTH.
Definition: It is the attractive action exerted by a planet on nearby objects.
It is an action which is exerted at a distance, like a magnetic force or an electric force, but there, it is what one calls a GRAVITATIONAL INTERACTION.
It's gravity. The device which measures the value of this action is the dynamometer.
We see one here, and the unit of measure is NEWTON.
So, when we talk about unity, we do not capitalize, but to pay homage to Mr. Newton, Isaac Newton, we will capitalize the symbol.
All symbols that come from a last name begin with a capital letter.

If I measure on Earth, I find about 2 newtons.
This is what the dynamometer says.
The spring extends to the value 2.
So, I write: W = 2 Newtons, with uppercase N.

On the Moon, you see that the spring extends less. The Moon is less attractive to the coffee bag.
The weight is 6 times lower on the Moon than on Earth. And it will be six times lower for all objects.
So, you retain "dynamometer", you retain "newton" and you retain that the weight, unlike the mass, depends on the planet.
In fact, the weight depends on where the measurement is taken.

πŸ‘‰ On Earth, it also depends on LATITUDE. That is to say that we will find a lower weight at the equator than at the poles, for example.
There are two reasons for this:

πŸ‘‰ It is because the Earth is a little crushed at the poles. So, we are not at the same distance from the center of the Earth depending on whether we are at the poles or at the equator.

πŸ‘‰ And as the Earth rotates, being at the equator causes the weight to be compensated by another force which tends to push the object away.

πŸ‘‰ Finally, the weight depends on the ALTITUDE.

If the measurement is made at altitude, it is a little as if we were moving away from the planet.
We will therefore find a slightly lower weight. It's tiny, but it's measurable.

This is the difference between mass and weight.


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