# View all Ask-a-Physicist

In general relativity, gravity is described as a distortion of space time. Most vulgarized books use the simplified image of a 2D plane being bent downwards by a mass, so that any matter traveling in the area would have to follow the bending of the plane, which would then explain why things are attracted to one another.

Keeping the same simplified metaphor, could we imagine something that would bend the plane upwards, thus causing objects to be repelled? Would such a thing be considered to have negative mass? Is the concept theoretically possible?

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If you had a light in a room which was entirely sealed with mirrors, then switched the light off, would the room stay lit?

Would the light just keep reflecting off the mirrors?

Would it slow down? Perhaps to the point where it is no longer visible?

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I understand what happens and why, when I drop a tennis ball that is resting on top of a basketball. What I am trying to figure out is, why do I not get the same results with a ping pong ball on top of a basket ball. Any ideas?

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It looks like they are looking for positive mass particles at the LHC at CERN. Isn't it logical that there are negative-mass "anti-particles"? Does the LHC have the capability of measuring negative mass?

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If a person stepped on a home bathroom scale, and weighed 170 lbs, he would weigh a different weight on this same scale when on other planets due to the force of gravity.

If, however, a person stepped on a professional medical scale — the kind with weights attached that you slide (as opposed to a bathroom scale) and weighed 170 pounds on Earth, would the weight of the person be the same on the other planets as it is on Earth (170 lbs)? This has become a bone of contention in an otherwise ideal marriage.

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Is there a particular range of frequencies at which parts of the human body (or the
human body overall) normally vibrates? Because in an episode of the sci-fi TV series
*Fringe*, they claimed that they could figure out if one character was from this universe
or a parallel universe by measuring the frequency at which he was vibrating.

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If air is heated and its starts to go towards the space, what is the reason for this? Do you think the air molecules will get accelerated and because of this they will resist Earth’s gravitational force?

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My 9 year old wants to know if a single atom exists in a certain state of matter — or must it be associated with other atoms in order to define a state?

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With the latest telescopes, we can look at some galaxy's some 13 billion light years away, correct?

And that is much older than the earth is thought to be. I also heard that we have seen what the universe looked like only a short time from its birth. So how can all this be true? If nothing can travel faster than light and the light from that moment in time would have passed the spot the earths current location long ago, how can we see that light? What am I missing? I can not fathom how we beat the light from the early universe to this spot. Something doesn't add up for me, so can someone break it down for me?

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I've read that orbiting objects like the space station stay in orbit because they are falling at the same rate the Earth is curving away underneath them.

What I don’t understand is their downward velocity should be increasing because it is caused by gravity / acceleration due to gravity. Shouldn’t it be “falling” with a greater velocity the longer it travels? Does it reach a terminal velocity like that of a skydiver? If so Why?

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Under acceleration, a helium-filled balloon inside of a car will jump forward in the direction of acceleration. I have been searching for the reason why with no definitive results. This is the dilemma: An acquaintance of mine, who has a degree from Johns Hopkins is attempting to argue for some magical force that drives the balloon forward, also that it has something to do with gravity.

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Hello, I am trying to figure out the practice behind the experiment I've heard about in a National Geographic documentary, so please help me a bit.

How do they generate entangled particles in the lab (photons)? Also, how do they make photons to interact with each other and what kind of interaction do they make?

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I’m not a physics guy, but with all the discussion around the disaster in Oklahoma recently it had me pondering a terminal velocity question. From what I understand, any object with mass would possess a terminal velocity.

Since air has mass, what would be the terminal velocity of air? Specifically, how fast can a tornado/hurricane/natural disaster hurl air before it reaches a terminal velocity?

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How does one measure the kinetic energy of the ejected electrons resulting from the photoelectric effect?

My understanding is that the KE of the ejected electrons was dependent on the frequency of the incident light, not the intensity. A higher intensity light, however, would [have] ejected MORE electrons; provided that the light's frequency is high enough to overcome the work function of the metal.

Thus, it seems to me that an ammeter would show a higher current either way. A higher frequency light would produce faster electrons and a higher intensity light would produce more electrons. Given that current is a rate of charge (I = dq/dt), how did Einstein know that the frequency was the primary factor for the KE and not the intensity?

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My 7 year old son Ben (who is sitting next to me now) has recently become interested in how small things can be. He is not convinced that nothing can be measurably smaller than a Planck length because whatever is Planck sized can always be divided into something smaller.

He thinks perhaps Planck lengths can be divided into energy beams that then become infinite.

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A friend of mine claims to notice that when he takes a hot frying pan (with a warm, insulating handle), and pours cold water into it, the handle seems to get hotter. He thinks it is because the handle is less insulating than the air, thus the path of least resistance is through the handle.

This would make sense to me when you take the pan off of the heat, but he claims that it isn't until the water hits it that the handle gets significantly hotter. If this is true, what does the water have to do with it? If anything, wouldn't the water become the path of least resistance, and heat would move from the metal to the water, and from the handle to the air?

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In figure skating, if you are doing a spin on ice and you leave your arms spread out it creates resistance. If you pull the arms in toward the body, you create less resistance so you spin more easily. How do I express this with mathematical equations in physics?

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Does Einstein's relativity of simultaneity mean that two events cannot be simultaneous or that we cannot prove that two events occurred simultaneously?

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If somehow there was a tunnel straight through the center of the earth and we dropped a capsule through it, what will happen to the capsule? Would the tunnel shoot the capsule into space?

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Assume two photons are moving in opposite directions from each other from a common light source. How fast would they be traveling relative to each other? Twice the speed of light? If the speed of light is the ultimate speed limit in the universe, how can something travel twice that speed?

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I have a question about waves and particles. Since waves and particles seem to be somewhat interchangeable at the subatomic level, at what wavelength does the wave / particle duality stop?

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In the proton-proton chain reactions which happen, for instance, in our Sun, two protons collide and form a proton and a neutron. However, this just blows my mind.

What is the mechanism by which a proton simply loses its charge, becomes slightly more massive, and turns into a neutron?

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Can electricity (static or not) be used to move very large objects? Could you somehow control a sufficiently strong electrical current through the air or across a surface to lift a log or a car? Are there certain objects it could lift and certain ones it couldn't?

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A ninth grader wants to know what it's like to be a physicist. Here is one physicist's response.

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If the recent result of the tests of the speed of the neutrino are validated by other research groups what are its implications for the standard model of particle physics as it has been described to date?

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There seems to be a lot of debate about how accurately CERN measured the departure of their neutrinos. Could they not aim them at some other neutrino detector in the world and just use the difference in distance and time from the readings in Italy as this would eliminate the error in not knowing exactly when the neutrinos left CERN?

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When carrying out the double slit experiment using electrons or buckyball molecules, do the particles have to be traveling at near light speed velocities to produce an interference pattern?

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When we talk about an object on an incline, the force of gravity has two components, one vertical and one parallel to the incline, as you know, while the normal force is matched in magnitude by the vertical component of gravity. When we talk about a banked road, however, the normal force becomes the force whose both components are considered, while gravity becomes the force that is matched by the vertical component of normal force. In a way, they almost reverse their roles. Why is that?

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If you shake the sun, how long would it take before it had an effect on the position of the earth?

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Since the earthquake in Japan the country has been having trouble with their nuclear energy plants and possible meltdown. The question is why aren’t they using lead to absorb the radiation?

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Does an ice cube have heat energy (kinetic energy)? If so, does a glass of water with ice have more heat energy than a glass of water without ice?

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If a helicopter hovers in a fixed position for 24 hours will the earth rotate around it?

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I built a sand art box for performing sand art but I have a problem. The sand I use keeps bouncing around on the glass as opposed to being pencil thin. How or what kind of sand can I use to remedy that?

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Can you explain why a penny under a glass beaker of water disappears? If you add some water to top of the penny and return it to under the beaker it is visible.

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What are the scientific reasons for spin or curve on a soccer ball? What forces are affecting this?

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Why is it said that increasing heights of building affect the revolution time of earth?

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