Episode #125 of the Stack Overflow podcast is here. We talk Tilde Club and mechanical keyboards. Listen now

Hot answers tagged

69

The easy explanation is that the tennis ball is hollow. When you merely drop the objects, they are subjected to the same acceleration - the aceleration due to gravity - and nothing else. Conservation of energy then says that their gravitational potential energy should be completely transformed into kinetic energy at the ground: $$mg\Delta h=\frac{1}{2}mv^2\...


47

Is it inertia that a rotating object will rotate forever without external force? Someone told me that this is not inertia [...] Well, sort of - it’s somewhat correct to say it is inertia, and somewhat correct to say it isn’t. One has to be precise with language! But there is some truth to what you were told. “Inertia” generally refers to the tendency of ...


44

Assuming for a moment that your bones are proportionately stronger... (because you are asking about motion, not strength: but see for example this question about scaling in nature) That still leaves us with some physics that "doesn't scale well". First, there is the issue of muscle mass: assuming your muscles are made of the same fibers, their strength (...


43

To make it clear that it is not obvious it is better to stop using the word "mass" in both cases. So it is better to say that it is not obvious that the inertial resistance, meaning the property that scales how different objects accelerate under the same given force, is the same as the "gravitational charge", meaning the property that scales the ...


38

That's not true, Newtons's laws do not say that. What's important here is conservation of momentum. Inside the phone, there is an oscillating mass. While the mass inside has a momentum and thus a velocity in one direction, the (friction-free) phone has to have the same momentum in the opposite direction. It "vibrates". Homework: Get on a skateboard (best ...


26

Scenario I: I have a white ball and a black ball. In the system of units I've adopted, I discover that: Gravitational mass of white ball = 2 Inertial mass of white ball = 3 Gravitational mass of black ball = 10 Inertial mass of black ball = 15 But I want each ball's gravitational mass to equal its inertial mass. So I fix the constant $G$, ...


25

Yes! In fact, this is very common. For example, the mass of a proton is much greater than the sum of the masses of the constituent quarks. Much of the extra mass comes from the gluons that bind the quarks together; each gluon is massless, but collectively they contribute to the inertia. The point is that the mass of a system is not the same as the sum of ...


21

From here: Higgs is an atheist, and is displeased that the Higgs particle is nicknamed the "God particle", because the term "might offend people who are religious".Usually this inappropriate nickname for the Higgs boson is attributed to Leon Lederman, the author of the book The God Particle: If the Universe Is the Answer, What Is the Question?, but the ...


19

Yes, you would have 27x the muscle mass. However, muscle mass is not actually the factor which defines the strength of a muscle. The strength of a muscle increases based on its cross sectional area, not its volume. For an intuitive image, consider a rubber band. How hard is it to stretch it apart? Now think of a much longer rubber band with the same ...


18

At its most basic, an object will rotate forever for the simple reason that there is no preferred direction in space. Emmy Noether's theorem of 1918 explains how various conservation laws arise from from differentiable symmetries. It is a mathematical theorem, not a physics theory. Because of this mathematical certainty, it is one of the most important ...


16

When the cosmonaut sneezed they would start moving, and rotating, in the opposite direction, but when the sneeze hit their faceplate (ugh!) this would stop the motion. The net result is that the velocity of the cosmonaut would not have changed, but their position and angle would have. According to Wikipedia a typical breath is 500cm$^3$ and a sneeze ...


16

At least one mobile phone I've heard about uses an unbalanced spinning weight. As the weight moves in one direction, the phone moves in the other, in accordance with Newton's Third Law of Motion.


15

Momentum: The resistance of an object to a change in its state of motion. That sounds like a fishy definition of momentum to me. A slightly better definition, at least at your level, is that momentum represents the "amount of motion" an object has. Granted, "amount of motion" is a very vague term, but it stands to reason that if "amount of motion" were to ...


15

On the whole, static friction is higher than dynamic friction. This means that if you can brake without your wheels skidding, you will come to a halt more quickly. So let's assume that the truck brakes without skidding, and see where that gets us. Let's assume that your truck has weight $W = Mg$ with a haystack with additional weight $w = mg$ on top. ...


14

I haven't tried this experiment but the first two factors that spring to mind are: Rolling Friction The bowling ball is hard and smooth while the tennis ball is fuzzy and softer. This would lead to a larger coefficient of rolling friction for the tennis ball. Distribution of Mass The tennis ball is hollow while the bowling ball is solid. This means that ...


13

Not to detract from Floris' answer, but I think this is an instance where it is nice to think in terms of limits. If the hay is tied down, you're stopping an object with mass (truck + hay). If the hay isn't tied down, but on a sufficiently sticky surface such that it doesn't move, it should be the same as stopping it if it were fixed, since the outcome is ...


13

If the asteroid is in parallel to the orbit of the earth and at rest it will feel the gravitational attraction and will fall with velocity growing as $g\cdot t^2.$ This force will be there whatever the angle and velocity of the asteroid, centrifugal forces may make it miss the earth in a parabolic orbit, or be caught in an elliptical as the path of the ...


13

By adding flywheel makes engine to take more power to spin the flywheel because of its huge mass. Efficiency of the engine drops very low. I can only see more burden than smoothness. The burden is temporary in order to get the flywheel going. Once it gets going the stored rotational kinetic energy reduces the energy required by the engine. Refer to the ...


12

It would help if you gave some context. Is there any evidence, or even theoretical work, that suggests neutrinos are not affected by gravity? I suppose you could argue that the similar arrival times of photons and neutrinos from SN 1987A was evidence that neutrinos and photons are following the same path through spacetime and both being "gravitationally ...


12

In contrast to the other answers, I am going to go a different direction. This is not a question of mathematics, where we can just say "oh yeah just redefine your variables." This is a physics question. Going further, The OP is assuming $m$ stands for the mass of an object, not some arbitrary proportionality constant that ends up being the mass. Therefore, ...


11

Inertial mass describes an object's resistance to change in velocity. The more inertial mass something has, the harder it will be to change its velocity. Gravitational mass describes an object's ability to attract other matter (and under GR, to curve spacetime). The more gravitational mass something has, the more attracted to it other things will be. When ...


10

The term "God Particle" is used only by journalists. It's a wholly inappropriate term and I'd be very surprised if any physicist used it (outside of the lower end popular science TV programmes). General Relativity tells us that inertial and gravitation mass is the same thing. The Standard Model isn't going to say anything directly about gravitational mass ...


10

Mass is one of fundamental attributes of a particle. These fundamental attributes are defined based on their interactions we observe in nature. There's no other way for us to assign a valued attribute to a particle. For example, charge is defined based on electromagnetic interaction. We observe the motion of particles under electromagnetic interaction and ...


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Similar questions are: "why does electric charge happen?" and "why does gravity happen?" etc. The "art" of physics is in the identification of the fundamental "stuff", stuff for which the question "why" is actually misguided. You see, if there are fundamental "things" then, by the definition of "fundamental", these are the givens that we accept without ...


9

Inertia is an intrinsic characteristic of the object related to its mass. Inertia tells you how much force it will take to cause a particular acceleration on the object. Momentum is a function of an object's mass and velocity. Momentum is a measure of the kinetic energy of the object. A massive object can have any momentum (at least as long as its velocity ...


9

Let's make your fan really big - say, as big as the moon. You probably know the rotation of the moon is tidally locked to the earth - that is why we always see the same "face" of the moon. The tidal friction is a real effect - it depends on the size of the object and the distance to the source of gravitational attraction, but it will produce a small ...


8

The answer depends on the identity of the dark matter. In the most widely believed scenario, dark matter is composed of "weakly interacting massive particles" ("WIMP"). The adjective "weak" really means that the particles interact via the weak nuclear force. This pretty much guarantees that they interact with the Higgs boson, too: the WIMPs carry the ...


8

Like Wikipedia says: "Moment is a combination of a physical quantity and a distance." This 'physical quantity' could be various things. To take the examples you mention: Moment of momentum (commonly known as angular momentum) is expressed as $\vec{L}=\vec{r}\times m\vec{v}$, and is a measure for the rotational momentum of an object around some axis. Moment ...


8

The law of inertia can be seen as the result of the translation invariance of the laws of physics. Of course it's a matter of taste whether you think translation invariance makes a more intuitively appealing axiom than the law of inertia itself.


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