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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 (... 44 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 ... 31 Very good question! The point is that when the elevator begins to move (either upwards or downwards), it's accelerating, while the drone -- having no force acting on it directly -- is still moving at a constant velocity. Imagine you had a drone in a moving car or airplane, and it was "hovering" next to you. This means that its velocity with respect ... 27 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 ... 25 This is a deep question. There are (at least) two definitions of mass: gravitational mass is how something is influenced by gravity, which is the m in F = Gm_1m_2/r^2, and is more-or-less 'how much stuff there is'; inertial mass is how resistant to acceleration something is, and it's the m in F = ma. If we call these two versions of mass m_G ... 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 ... 19 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 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 ... 14 Physicists distinguish gravitational mass from inertial mass. In practice we find that gravitational mass is equal to inertial mass, but the distinction is important because conceptually they need not be the same. A measurement of weight is, in effect, a measurement of gravitational mass. That is to say, the amount of gravitational force acting on a body as ... 13 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 ... 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 ... 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 ... 11 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 ... 11 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 If you were a rigid metal pole welded to the floor of the bus, there would be no bending / lag of the upper section of the pole moving forward as the bus moves forward; it would all accelerate together with the bus. So this clearly has something to do with the "floppiness" of people. Therefore let's consider an extreme case. Imagine an ice block on the ... 11 A drone does not always fly with respects to the air in a moving car/airplane. It does so only if they are not accelerating. The elevator is accelerating in your video. 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 ... 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 ... 9 As Newton stated with his 1st law, an object without a force acting on it will keep moving with the same speed and direction. This is also known as the law of inertia. Inertia is the tendency of an object to resist acceleration. This is because no force is acting on it to affect acceleration. For rotational motion, the version of this is the moment of ... 9 So does this mean that weight is measure for inertia rather than mass being the unit to measure inertia. No. Inertia is resistance to change in velocity (acceleration, a). From Newton's second law$$a=\frac{F_{net}}{m} where $F_{net}$ is the net force acting on the mass $m$.. It's true that a mass $m$ will be harder to accelerate upward in opposition to ...