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136 votes

Do the weights of two liquids not add when mixed?

Of course, by common sense, if you put together two objects with masses $m_1$ and $m_2$, and nothing comes out, then you end up with mass $m_1 + m_2$. Weights are a little more complicated because ...
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  • 95.5k
132 votes
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Why don't we use weights to store energy?

You can use dead weights, but you need a huge amount of weight. For example the biggest pumped hydroelectric system in the world (the Gianelli Hydroelectric Plant in California, USA) uses water ...
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  • 9,842
112 votes
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Will helium in the tires of a bike make it lighter?

It will make it lighter, but the effect will be very small. The volume of the tube is probably less than a liter. One mol of an ideal gas is 23 liters at atmospheric pressure. So you have about 0.2 ...
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95 votes

Why don't we use weights to store energy?

Let's spin some numbers to further illustrate the poor energy density of gravity-based storage systems. Assume that you have a 100 kilogram lead weight that you can lower into a 10 meter deep hole in ...
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  • 2,238
85 votes
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Why do gases have weight?

Imagine a gas molecule in a closed box bouncing vertically between the top and bottom of the box. Let's suppose the mass of the gas molecule is $m$ and its speed at the top of the box is $v_t$. When ...
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84 votes
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Will a bathtub full of water weigh more if I add something that floats in the water?

Mass is mass. If you add something which has mass to the bucket, the bucket now has more mass. It doesn't matter if it was more dense or less dense. If you add 1kg to a bucket, you add 1kg. Now ...
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  • 43.1k
65 votes

Is weight a scalar or a vector?

On earth, weight of a body is defined as the force by which the body is attracted by the earth towards its center. Weight can thus be considered the same as the gravitational force exerted by the ...
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64 votes

Why don't we use weights to store energy?

We do use mass in a gravitational field to store energy and have done for hundreds of years! Grandfather Clocks Grandfather Clocks have used powering weights since the 1660s. This was when they ...
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  • 741
53 votes

Is weight a scalar or a vector?

We can change the definition of things whenever it is useful. Definitions serve us. If a definition isn't useful, individuals and communities change it, sometimes on the fly, sometimes in context, ...
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  • 3,809
44 votes

Why don't we use weights to store energy?

Another way to store energy in mass is the use of flywheels. You simply take a massive wheel and spin it up to store energy, use an electrical generator as a break to take energy out. Their main ...
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  • 19.8k
42 votes

How can there be weight without movement?

It's not $ \vec{F} = m \vec{a} $, but $\sum \vec{F} = m \vec{a}$. When you are standing motionless there are two forces acting upon you, the gravitational one, and the normal. The normal force is ...
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39 votes
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Doesn't a box holding a vacuum weigh the same as a box full of air?

The buoyant force on a body immersed in a fluid is equal to the weight of the fluid it displaces. In other words, $$ F_B = -\rho_{\text{fluid}} V_{\text{body}} ~g $$ The force of gravity on the body ...
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35 votes

What impact does air have on our weight?

A human at sea level on Earth will experience a buoyant force equal to the volume of their body times the density of air, pointing upward against gravity. This works out to about 75 grams for an ...
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34 votes

Would a gas "weigh" less than a liquid if they have the same mass?

First of all, it is impossible to have $1L$ of liquid water in vapor form in a $1L$ container. It is difficult for liquid form and the gaseous form to occupy the same volume. The gas molecules would ...
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  • 6,963
33 votes

Would a gas "weigh" less than a liquid if they have the same mass?

The force the box exerts on the scale will be given by the difference between the force that the gas does downwards on the bottom of the box and the force that it applies upwards to the top of the box....
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  • 4,525
32 votes

Do the weights of two liquids not add when mixed?

@knzhou supplied a good answer. I’m going to offer a couple of other interpretations. The first has nothing to do with the fact that you’re mixing liquids—it’s just that there are difficulties in ...
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  • 9,221
30 votes

Where on Earth does the mass of 1 kg actually produce a 1 kg reading on a digital scale?

EDIT: Though most of the comments have now been deleted, I state the following for completeness: The kg is a unit of mass. Weight is a force, and like other forces it is measured (in the SI system) ...
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30 votes

Will helium in the tires of a bike make it lighter?

Helium has been used in racing bicycle tires for indoor track (velodrome) events. The helium will decrease the overall weight of bike and rider only slightly, and it will somewhat reduce the angular ...
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  • 531
26 votes
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How do astronauts measure weight/mass on other planets?

First of all, the astronauts would be measuring mass. This is the property of a thing that determines how hard it is to accelerate the thing. It does not change with the local gravity. or lack of ...
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  • 10.4k
24 votes

Why do gases have weight?

Think of the atmosphere as if it were an ocean. You might not think water has weight if you were diving underwater, but obviously when you fill up your cup with water you feel its weight increase. The ...
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  • 341
24 votes

Will helium in the tires of a bike make it lighter?

The most common wheel size for mountain bikes is a torus with R = 307mm and r = 27 mm. Assuming about 7mm of rubber on the ...
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23 votes
Accepted

Can a ship float in a (big) bathtub?

Yes a ship can float in a big bath tub with very little water. No you do not need as much water as the weight of the ship. In theory, you can use less than a cupful! Image from B5-2. How much water ...
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  • 26.5k
22 votes

Why don't we use weights to store energy?

We do. Just the weights we use are made not from lead, but from water. Many water reservoirs are also used to store energy by pumping water up when you have energy surplus, and letting it come down ...
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  • 619
21 votes
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Why do we use kilograms instead of newtons to measure weight in everyday life?

The problem is that while mass is the same everywhere on earth, weight is not - it can vary as much as 0.7% from the North Pole (heavy) to the mountains of Peru (light). This is in part caused by the ...
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  • 117k
21 votes

Why do gases have weight?

Here is a short answer: Imagine you would have an empty box (i.e. vacuum), that you would put on a weighing scale. It would have some weight. Now, if you would insert some gas into it, the measured ...
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  • 211
21 votes

Which weighs more in atmosphere, $1\,{\rm kg}$ of steel or $1\,{\rm kg}$ of feathers?

Feathers are made from keratin, with a density of about $1.3\ \mathrm{g/cm^3}$. The net volume displaced by a kilogram of feathers is then $751\ \mathrm{cm^3}$. Steel has a density of $7.86\ \mathrm{g/...
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20 votes

Will a bathtub full of water weigh more if I add something that floats in the water?

Not only does adding something that floats cause the system to weight more, hanging something that doesn't float from a string and lowering it into the water without letting it touch the bottom or ...
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20 votes

Is weight a scalar or a vector?

If he is a PhD and you are a Major in physics there is no reason to take his word above your own in this case. Physicists don't get a rerun of "basic concepts in mechanics" after majoring. Most likely ...
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