101
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How big would my telescope have to be if I wanted to see the Mars rover from my backyard?
Telescope resolution is all about apparent angles. From the sounds of it, the lowest resolution you'd settle for would be something capable of resolving about $1 \operatorname{cm}$ objects, right? ...
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82
votes
Force of photons from the Sun hitting a football field = weight of 1 dime?
Photons are massless so their weight is 0. However, photons do have momentum so they can exert force. This force is due to their momentum and would occur even in the absence of gravity, so it is not a ...
- 94.5k
78
votes
If there is friction and everything stops on earth why does earth not stop due to friction?
For friction to occur there must be a medium which can exert the friction forces. For example if a raindrop falls through air there is friction against the air and it reaches some terminal velocity ...
- 11k
71
votes
When a balloon pops and lets a brick fall, where does the energy come from?
Your estimate that the brick would lose "a bit" of its energy to air resistance is incorrect. It would lose most of its energy. The terminal velocity of a brick (http://physicsbuzz.physicscentral.com/...
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63
votes
Why is the ideal gas law only valid for hydrogen?
The short answer is ideal gas behavior is NOT only valid for hydrogen. The statement you were given in school is wrong. If anything, helium acts more like an ideal gas than any other real gas.
There ...
- 69k
63
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Can a gym be built to supply electricity to homes?
The maximum continuous power that can be generated for an hour by a fairly fit person on an efficient machine like an exercise bike or rowing machine is $\sim 200$ W (olympic-standard track cyclists ...
- 128k
60
votes
Accepted
Why do these two ways of understanding constant acceleration give different results?
Your mistake is in assuming that within those one second intervals the velocity is constant. The velocity as a function of time $v(t)$ for motion under constant acceleration $a$ is given by $$v(t)=at+...
- 55.8k
58
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Does friction always oppose motion?
Friction opposes relative motion between two bodies.
Note that might mean that friction can create motion relative to i.e. you. For example, dropping an item on a moving belt. Friction opposes and ...
- 14.3k
52
votes
Are the units of energy the same in higher dimensions?
Suppose for a moment that we're specifically interested in the kinetic energy of a single, non-relativistic particle, so that $E=\frac{1}{2}m\vec{v}^2$. I include the vector notation for the velocity ...
- 35.3k
52
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Why is my hand not burned by the air in an oven at 200 °C?
my first conjecture was: It’s the nature of the air (i.e., a gas) that its molecules are more disperse than those of a solid.
Yes, but you can go a few steps further. The sparseness of molecules has ...
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52
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Oh-my-god particle: How can it get through Milky way in 10 seconds?
Key point in your quote is: "from protons reference frame". In the reference frame, travelling at a relativistic speed, length contraction is experienced. All the lengths in the direction of travel of ...
- 1,171
48
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When a balloon pops and lets a brick fall, where does the energy come from?
The kinetic energy comes from gravitational potential energy
The gravitational potential energy comes from the buoyant energy (force * elevation) lifting the balloon and brick into the air
The ...
- 597
46
votes
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Why do my "steel ice cubes" have water in them?
Water thaws at 0°C. The latent heat of fusion of water is $L_i = 344000$ J/kg, which means that to thaw the frozen water inside your cubes requires much, much more energy than you're accounting for. ...
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46
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Why do these two ways of understanding constant acceleration give different results?
I think that a good way to highlight the error in your "concept of acceleration" answer is to draw a velocity against time graph and to remember that the area under such a graph is the displacement.
...
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43
votes
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Why is the period of rotation the same for two stars orbiting the same centre?
The centre of mass of the binary system cannot move because there are no external forces acting.
The line joining the two stars must always pass through the centre of mass, because by definition the ...
- 351k
42
votes
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How can a simple physical situation give rise to two different possible outcomes?
The two results that you got are absolutely acceptable both mathematically and experimentally and either of them can happen if the experiment is done (but it's not random) practically with all ...
- 8,006
41
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Why can't a nucleus become a black hole?
Let's take the carbon nucleus as a convenient example. Its mass is $1.99 \times 10^{-26}$ kg and its radius is about $2.7 \times 10^{-15}$ m, so the density is about $2.4 \times 10^{17}$ kg/m$^3$. ...
- 351k
40
votes
Accepted
Why is the ideal gas law only valid for hydrogen?
The school question is wrong. What were they thinking? (My guess is that it was a simple slip-up and they meant helium.)
The ideal gas equation of state works for any gas in the limit of low density. ...
- 56.6k
39
votes
Accepted
Would a uranium 235 fuel pellet the size of Earth explode?
It is worth noting that there is no such thing as a critical mass: there is a critical condition where there is enough mass, neutron reflection and other shape properties to make a chain reaction run ...
- 30.9k
37
votes
Accepted
Estimate the Sun's temperature, based on the duration of sunrise
The sunrise time gives you an estimate for the angular size of the Sun, roughly it's diameter divided by the distance to Earth.
This comes from dividing the sunrise time by 24 hours, assuming this is ...
- 128k
37
votes
Accepted
Calculating equivalent resistance
I am used to smoothing out badly shaped circuits by pulling the wires:
Then I get a better circuit by cutting the extra wires:
So there are three resistors in parallel, indicating that the current ...
- 4,560
36
votes
How can a simple physical situation give rise to two different possible outcomes?
What a lovely conundrum!
I'm afraid that the solution is a rather simple. You didn't specify in your maths whether the 4 kg mass was ejected in the direction of the initial velocity of the 5 kg mass ...
- 34.8k
36
votes
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How are beats formed when frequencies combine?
Beats can be thought of as the next level of complication from constructive destructive interference. To demonstrate this best, we should visualize what actually happens when we sum two sine waves of ...
- 45.6k
34
votes
Can the temperature of a body become lower than the temperature of the surrounding air?
If conductive/convective heat transfer is low, then radiative heat loss can cool an object below the temperature of the surrounding air. As discussed in the answers to What is the temperature of the ...
- 8,415
34
votes
Accepted
Is quantum mechanics applicable to only small things?
The relationship between quantum and classical descriptions is somewhat tricky, unlike the relationship between the relativity and the classical mechanics. Classical mechanics can be simply thought of ...
- 57.5k
34
votes
Accepted
Why doesn't a spinning object in the air fall?
We have the ball orbiting at a distance $R$ from the centre of rotation and the string inclined at angle $\theta$ with respect to the horizontal.
Two main forces act on the ball: gravity $mg$ ($m$ is ...
- 35.1k
32
votes
How are beats formed when frequencies combine?
As an alternative explanation, this beat interference is similar to the Moiré pattern which you get for instance if you overlay two sets of line gratings with different spacing.
https://upload....
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