# Tag Info

### What counts as the Earth's mass? At which point would it increase or decrease?

A great tool for answering these types of conceptual questions is an order of magnitude estimate, or a Fermi estimate. Let's assume the Curiosity Rover has about the same mass as a car. A quick ...
• 10.1k

### What is the relation between rope length, object weight and force needed?

The human body is a terrible tool with which to understand Physics. It contains countless complications which turn simple equations into complex detailed patterns -- all in the name of being more ...
• 50.6k

### "Kilogram" confusion: If you weigh a stationary object in "kilograms" and divide the result by about $9.8$, does that tell you the mass of the object?

Scales are usually tuned to tell you the mass, not the weight. The weight would be measured in newtons. Note that some types of scale, i.e. spring scales, often do measure in newtons (weight). To ...
• 1,502
Accepted

### Rate of change of surface area is acting weird

You are constantly adding 4kg of mass and the density remains the same. The extra volume added stays constant. Imagine adding the mass as an extra shell on the outside of the sphere with an inner and ...
• 140

### How total mass of universe is calculated?

This is a cute observation, but is essentially just dimensional analysis. It follows from doing a rough order of magnitude estimate where you only keep track of the Hubble constant plus fundamental ...
• 52.2k

### "Kilogram" confusion: If you weigh a stationary object in "kilograms" and divide the result by about $9.8$, does that tell you the mass of the object?

Manufacturer of devices which measure "mass" do differentiate between balances and scales. A balance consists of a pivoted beam with the object whose mass is to be found on one side and a ...
• 98.6k

### What counts as the Earth's mass? At which point would it increase or decrease?

The real answer is "if you have to ask what counts as Earth-mass, it's a poor unit for the question at hand." We only use this for approximations, giving a sense of scale to otherwise ...
• 50.6k

### If a massive sphere spins faster and faster, will a black hole appear?

Wikipedia gives the total gravitational mass-energy sum of a Kerr black hole as: $$M_{total} = \sqrt{M_{\text{irr}}^2 + \frac{J^2c^2}{4M_{\text{irr}}^2G^2}},$$ where $M_{total}$ is the equivalent ...
• 7,441

### "Kilogram" confusion: If you weigh a stationary object in "kilograms" and divide the result by about $9.8$, does that tell you the mass of the object?

Scales measure mass. This should be clear if you first weigh an object on a spring (or digital) scale, and then on on a balance. Both give the same result, but with the balance gravity acts on both ...
• 11.1k
1 vote
Accepted

### Effect of a spring’s mass on acceleration in a mass-spring system

Assume that the $x$-axis starts at the point mass $M$ and extends down the spring, whose length is taken to be $L$. Draw a free-body diagram of an infinitesimal portion of the spring, and sum the ...
1 vote

### How total mass of universe is calculated?

If the universe is flat (which our universe seems to be within the margins of error) or hyperbolic the total mass is infinite, since finite density times infinite volume equals infinite mass. If the ...
• 12.5k
1 vote

### What counts as the Earth's mass? At which point would it increase or decrease?

In addition to the previous answer: the mass of the Earth is not constant. You have examples of objects leaving the Earth, but the mass can also increase when meteorites (big or small) hit the Earth. ...
• 46.9k
1 vote
Accepted

### "Kilogram" confusion: If you weigh a stationary object in "kilograms" and divide the result by about $9.8$, does that tell you the mass of the object?

There are several wrong answers here, and a few that are worded in a misleading way. A scale always measures force. The measurement is a force, which can be thought of in units of Newtons or pounds (...
• 50.6k
1 vote

### "Kilogram" confusion: If you weigh a stationary object in "kilograms" and divide the result by about $9.8$, does that tell you the mass of the object?

"Kilogram" confusion: If you weigh a stationary object in "kilograms" and divide the result by about 9.8 , does that tell you the mass of the object? No. If the scales are ...
• 7,441
1 vote

### "Kilogram" confusion: If you weigh a stationary object in "kilograms" and divide the result by about $9.8$, does that tell you the mass of the object?

If the units on the scale are kilograms, then it's measuring kilograms. It is rare but possible to find a scale that directly measures Newtons. For such a scale you could divide the reading by 9.8 ...
• 41.9k
1 vote

### Anthropogenic relocation of Earth's mass

When a large artificial reservoir in Canada began to fill with water some years ago, the redistribution of that mass was sufficient to alter the earth's rotation by an extremely tiny, but measurable ...
• 96.6k
1 vote

### Why can't we measure inertia? What are the applications of inertia?

The way I've heard it explained is that inertia is a quality, whereas mass is a quantity. Inertia is a qualitative property that makes objects prefer their current state of motion. Mass is like the ...
• 37

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