Posts Tagged ‘acceleration’
gravitational mysteries – part one, maybe
what happens when you fall for gravity…
I don’t understand gravity, and I doubt that memorising equations will be of much help.
Gravity, I’m told, is a killer. If I fall from a high cliff, or a multi-storey building, onto hard ground below, I’ll most certainly die, due to gravity (and carelessness, because I know what falling onto hard ground, even just from a standing position, can do to a person). So gravity should be treated with gravity.
But then, gravity has benefits. It keeps us on the ground, prevents us from flying away. In fact, gravity has essentially formed our bodily structure. We have muscular legs which with some small effort we can lift from the ground and plonk down in another place in a tiny ongoing battle with gravity, which we’ll eventually lose.
So I suppose it could be said that gravity is a given. An essential element in the development of all living things that creep over the earth and even fly in the sky just above it. We just have to deal with it.
And yet, I hear things about gravity that don’t make much sense to me. I hear that gravity pins humans to the Earth, but also pins our planet to the Sun, and pins the Moon to our planet. And yet it doesn’t. The Moon hasn’t fallen to the Earth in the way that my body would fall to Earth from a tall building. It circles the Earth. In fact it is spiralling slowly away from the Earth. Something else must be happening, surely?
So what do I do when I don’t know? I consult people who claim to know. And what do they say? Well, in terms of the Moon’s spiral, it’s about velocity. Here’s an explanation designed for children, or children at heart like me:
From Earth, it might look like the moon is stationary, meaning it is not moving, but in reality, each year the moon gets 3 cm [further] away from Earth. Without having the force of Gravity from earth [the] moon would have just floated away from us. The moon’s velocity and distance from Earth allow it to make a perfect balance between fall and escape.
In case the velocity of rotation of the moon was a little bit faster, it would have escaped the Earth’s Gravity. On the other hand, if it’s a little bit slower, it would have fallen on Earth. That’s why the moon doesn’t fall on Earth.
So that’s a good start, but why is the Moon revolving around the Earth at just such a speed that it keeps at (almost) the same distance? Isn’t that just too convenient? I also hear that the Moon is ‘tidally locked’ to the Earth, keeping the same ‘face’ to us all the time. That means it rotates on its axis over the same time-frame as a single orbit around Earth. Or nearly so, because the Moon’s orbit isn’t perfectly circular, which seems to be the case with every other orbit we know of. I suppose a precisely circular orbit would be a wonder, but then again…
Anyway, our Earth isn’t precisely globular either, and I’m betting it’s the same for the Moon, and every other planet and moon out there. I’m beginning to sense a pattern in this lack of a pattern. Or this approximation of a geometric pattern which doesn’t quite get there with the purity of mathematics.
Not that this is a bad thing. I’ve written previously about Milankovic cycles, variations in the eccentricity and tilt of Earth’s orbit around the Sun, which add spice to our planet’s climate. It’s like we use mathematics to understand the universe’s endless play with mathematics.
But getting back to that cliff fall. I’ve more than once heard the tale that Einstein’s ‘happiest thought’ was of such a scenario. Nothing to do with sadism or masochism, nothing to do with the landing. It occurred to him that, though the falling fellow might feel the force of the air swishing by him, he would not feel any ‘force’ of gravity. In a vacuum he wouldn’t feel any force at all. He might as well be stationary. Gravity, according to my good mate Wiki,
… is most accurately described by the general theory of relativity, proposed by Albert Einstein in 1915, which describes gravity not as a force, but as the curvature of spacetime, caused by the uneven distribution of mass, and causing masses to move along geodesic lines.
Which all sounds pretty radical, especially for 1915, when Fokkers had only just become a thing. So I get that mass is very unevenly distributed. At night we see clumps of stars here and there, with lots of apparently blank space in between. And though we can see for miles and miles and miles, this messy distribution of matter and space extends way beyond what we can see, perhaps even with our most inventive gadgetry. But ‘curvature of space-time’ still smacks of science fiction after all these decades.
Einstein had of course come up with this marriage of space and time 10 years earlier with his very special theory of relativity. So there are three dimensions of space and one of time. But are there? What exactly is dimensionality? Is it more than a human invention? In looking this up I’ve come up immediately with an essay ‘The invention of dimension’, on the naturephysics website. So that answers that question. Or does it? Here’s a quote from the start of the essay:
The modern concept of dimension started in 1863 with Maxwell, who synthesized earlier formulations by Fourier, Weber and Gauss. In doing so he added a nuance that we acknowledge today whenever we refer to the dimensions of, say, g (≈ 9.81 m s−2) as distance over time squared, rather than just the dimensional exponents (1, −2). By referring to the dimensions of a quantity, Maxwell seemed to imply that real things have natural dimensions. In the same spirit he designated units of mass, length and time as ‘fundamental units’.
Distance over time squared is a formula for constant acceleration, which again takes me back to gravity. When we fall from a cliff or a plane we constantly accelerate (leaving aside prevailing winds etc) until we hit the ground, but until that moment we’re not feeling any force upon us, according to Einstein. So acceleration isn’t a force? Apparently not. Is it the result of a force – the effect of a causal force? Well it can’t be an effect of gravity, because gravity isn’t a force.
So our acceleration in the above example is caused by a distortion of space-time which in turn is caused by the mass of planet Earth. But if we had fallen not from a plane but from a spacecraft much much further away, say the distance of the Moon from Earth, what would happen? Would we fall at all? We have satellites and a space station up there (I’m not exactly sure where), so would we just go into orbit like they do? Or are they carefully put into orbit by exquisitely precise mathematical calculations?
But, returning to Einstein’s not-so-happily falling fellow. The only thing he has to worry about is the landing. But the landing, and the force of the landing, is caused by the Earth’s mass. Presumably if we lived on a life-sustaining planet with the mass of Jupiter, which Dr Google tells me is over 300 times that of Earth, we’d be falling, or accelerating at a much faster rate (I’m tempted to say 300 times faster, but the mathematics is always more complicated). But then we couldn’t even live on Jupiter because our weight would be 300 times greater than that on Earth, just as the twelve men who walked on the Moon weighed, for a few days, only one sixth of what they weighed at home. So for life to have evolved on a planet like Jupiter (mass-wise) it would have to be made from very different stuff, molecularly. None of those heavy bones and dense tissues, like brains. An elephant’s brain weighs about 6 kilograms, and on Jupiter it would weigh 1800 kilos. So I suppose it’s important to think about planetary or lunar mass when we’re looking for extraterrestrial life, or alternatively, to think about different building blocks….
Anyway, it’s fascinating to note where thinking about gravity can take you, even when you know virtually eff all about the science. But I do want to learn more, and I’ll keep plugging away at it….
References
https://www.vedantu.com/physics/why-doesnt-the-moon-fall-into-the-earth#
https://en.wikipedia.org/wiki/Tidal_locking
aspects of climate change – Milankovic cycles
an interminable conversation 10: more basic physics – integrals
that’s for sure
Jacinta: So I watched episode 3 of the crash course physics videos, on integrals, and found it so overwhelming I had to immediately go and take a nap. I’m surely too old and thick for this stuff, but I must soldier on.
Canto: We can do battle together – so this episode is about integrals, the inverse of derivatives. I’m not sure we gleaned much from the episode about derivatives, but if we combine this with exercises from Brilliant and some other practical application-type videos and websites we might make some more progress before we die.
Jacinta: Okay so equations, at least some of them, can be plotted on graphs with x-y axes, and the integral of the equation is the area between the curve and the horizontal x axis. Dr Somara is going to teach us some shortcuts for calculating these integrals, which sounds ominous.
Canto: I didn’t really understand the stuff about derivatives, but I’ll keep going, hoping for a light-bulb moment. So integrals help us to understand how things move, she says, which in itself sounds weird. And then she mentions the displacement curve, which I’ve forgotten.
Jacinta: Looking elsewhere, I’ve found a simple video showing displacement-time graphs. Displacement, which is simply movement from one position to another, is shown on the y (vertical) axis, and time on the x axis. A graph showing a straight horizontal line would mean no displacement, therefore zero velocity. A graph showing a vertical straight line would mean displacement in zero time, which would indicate something impossible – infinite velocity? Anyway, a straight line between the horizontal and the vertical would indicate a fixed velocity – neither accelerating nor decelerating. A curve would indicate positive or negative shifts in velocity. I think. Sorry – the terms used are constant velocity and variable velocity. That’s much neater. Oh and there’s also negative velocity, but that’s a weird one.
Canto: Thanks, that’s useful. Need to point out though that ‘curve’ is just the term for representing the data on a graph – in that sense the curve could be a straight line, or whatever. So Dr Somara starts with a gravity problem. You want to know the distance between your bedroom window and the ground, in a multi-storey building. You have a ball, a stop-watch and a knowledge of gravity. The ball will fall at g, 9.81 m/sec². What is the distance? According to the Doctor, discussion so far about motion has involved three aspects, position, velocity and acceleration, and has focussed on velocity as the derivative of position, and acceleration as the derivative of velocity. The connection has to be reversed to work out the distance problem. So velocity is the integral of acceleration.
Jacinta: And of course position is the integral of velocity. And this is important – velocity is the area under the acceleration curve, and position the area under the velocity curve. Which might be difficult to calculate. Areas within polygons, without too many sides, is easy enough, sort of, but under complex curvy stuff, not so much. And when we talk about ‘under’ here, it’s the area, often, to the axis, which represents some sort of zero condition, I think. Anyway, one method of calculating this area is to treat it as a series of rectangles, growing more or less infinitely smaller. Imagine dividing a circle into squares to determine its area – a big one extending from four equidistant points on the circumference, which would account for most of the area, and then progressively smaller squares in the interstices. You get the idea?
Canto: Yes, with that method you’d get infinitely closer to the precise area… Anyway, Dr Somara shows us a curve which is apparently the graphic representation of a formula, x⁴- 3x² + 1, and shows us how to find the integral, or at least shows us how we can divide the curve and its connection to the x axis by dividing it into rectangles. But what’s a more practical way of doing it? Well I’ll follow her precisely here. ‘If you know that your velocity is equal to twice time (v = 2t), then you know this is the derivative of position. So to find the equation for position, you have to look for an equation whose derivative is 2t, as for example, x = t². So x = 2t is the integral of v = t²
Jacinta: Yeah I can barely follow that. But the good doctor assures us that integral calculation is a bit messy. But apparently we can use the ‘power rule’ which we used for derivatives, and reverse it, in some instances. To quote: ‘Basically, you add one to the exponent, then divide the variable by that number’. Here’s an example: with v = 2t, x = 2/2t¹+¹, so x = 2/2t², so x = t². x = t² is the integral of v = 2t. She shows another more complex example, but I can’t do the notation for it with my limited keyboard skills. It involves some division. Anyway, with these mathematical methods we can look at trigonometric derivatives and do them backwards, e.g. the integral of cos(x) is sin(x).
Canto: We need to look at a variety of explanations of all this to bed it down methinks. I can only say I know a little more than I did, and that’s progress. And next we get onto constants. So what’s a constant (c)? It’s a number, and can literally be any number, positive, negative, fractional, whatever. It can be a placeholder, as for gravity, g (here on Earth), or presumably the speed of light, or ye olde cosmological constant, which is apparently still alive and well. Anyway, the derivative of a constant is always 0. That’s because a derivative’s a rate of change, and constants don’t change, by definition.
Jacinta: The derivative of t² is 2t, which presumably works by the power rule. Add any number and the derivative will always be 2t. That’s to say, the derivative of t² +/- (any number) is 2t. So, ‘if you’re looking for the integral of an equation like x = 2t, you have infinite choices, all of which are equally correct’. It could be t² or perhaps t² -18 or t² + 0.456. But I’m not clear on what this has to do with constants.
Canto: We’re flying blind, but it’s not too dangerous. The idea seems to be that the integral of x = 2t is t² + c. And here, if not back there, is where it gets tricky. With a bit of practice, we might know what the graph of the integral would look like, but not so much where it will lie vis-a-vis the vertical axis. For that we need to know more about the constant, ‘in order to know where to start drawing its integral. Whatever the constant is equal to, that’s where the curve will intersect with the vertical axis’. If it’s just t², it will intersect at zero, if it’s t² – 10, it’ll intersect at minus 10, etc. To avoid this infinity of integrals, the practice is to add c at the end of the integral, to stand for all the possible constants. So, saying that the integral of x = 2t is t² + c covers all the infinite options for c.
Jacinta: Well, Dr Somara next talks about the ‘initial value’, which you can apparently use to work out ‘where your integral is supposed to be on the y-axis’ without knowing the value for c – I think. For a graph of position, the initial value would be your starting position – where it intersects the vertical axis. This is the c value.
Canto: So returning to the bedroom window and the ball, the ball is dropped from the window sill at the same time the stopwatch starts. It hits the ground at 1.7 secs. So we know the time and the acceleration, 9.81m/sec². We need an equation for the ball’s position. We do this by finding its velocity, working out the integral of its acceleration. If you have a graph with the y-axis representing acceleration, which in this case is constant, and the x-axis representing time, the uniformly accelerating ball would be represented as a flat line, making the area under the ‘curve’ – between it and the x-axis – fairly easy to calculate. The area would be rectangular, and would be calculated by base x height. The base is t, the amount of time the ball took from release to hitting the ground, and the height is a, the acceleration, so it’s just a matter of a x t. The integral is at plus c, the constant. We need this constant, according to Dr Somara, because we can see that the velocity graph will be diagonal, a line ‘slanted in such a way that, every second, it rises by an amount equal to the acceleration’.
Jacinta: But, where to put it the line on the vertical axis? We’re looking for the integral of the acceleration, so we may use the power rule, which I still don’t get. So I’ll quote the doctor, for safety: ‘The acceleration, a, is a constant, but we could also say that it’s a x tº (t to the power zero)’. And anything ‘to the power zero’ is always 1. So, according to this mysterious power rule, the integral of acceleration (i.e the velocity) would be equal to the acceleration multiplied by the time – plus c. The c is added because we didn’t know where to place it on the y axis when time, on the x axis, is zero.
Canto: Yeah right. Let’s continue to quote the doctor – ‘Now here’s where the initial value [??] comes in. The velocity graph tells you what the velocity is for each moment in time. But we had to add the c, because we didn’t know…’ the initial value, being the velocity at time zero. ‘So the integral of the acceleration could have just been a x t, or at’. Or at plus or minus whatever. The c in the integral represents these options. But if we can work out the velocity at time zero (v0) we won’t need c. So, according to our Doctor, ‘if we write our equation with that v0 in it, as a placeholder for the velocity when time equals zero, we end up with the full equation for velocity, v = at + v0′. This is the kinematic equation, the definition of velocity. So the equation tells us that the final velocity of out tennis ball is at, that’s to say 9.8m/sec x 1.7 secs, that’s to say, 16.7m/sec (down, towards the Earth’s centre of gravity).
Jacinta: All of which seems to complicate something not quite so complicated. Anyhow, the pain isn’t over yet – we need to link acceleration with position, and this requires further integration, apparently. So, according to the power rule, which we should have learned, the integral of at is half at squared, and to get the integral of v0 you multiply it by t. Get it?
Canto: No. Let me quote from a highlighted comment: ‘i cant imagine how the avg viewer with no prior knowledge of calculus would actually understand calculus just by watching this video’.
Jacinta: Hmmm. Maybe we’ll try Khan Academy next. Anyway, if you put these integrals together you’ll get something that looks like the kinematic equation, the displacement curve. So for our example, we can work out the height from which the ball was dropped, or the distance the ball has travelled, using the initial position and time (zero and zero) plus half at squared. a was 9.81 m/sec², and t was 1.7 secs. t² is 2.89. Multiplying them makes 28.35, which, halved, is 14.175. metres. At least I got the calculation right, but as to the why….
References
https://sites.google.com/a/vistausd.org/physicsgraphicalanalysis/displacement-position-vs-time-graph
towards James Clerk Maxwell 4: a detour into dimensional analysis and Newton’s laws
Canto: Getting back to J C Maxwell, I’m trying to learn some basic physics, which may or may not be relevant to electromagnetism, but which may help me to get in the zone, so to speak.
Jacinta: Yes, we’re both trying to brush up on physics terms and calculations. For example, acceleration is change in velocity over time, which is hard to put in notation form in a blog post, but I can steal it from elsewhere
in which the triangle represents ‘change in’. Now velocity is a vector quantity, therefore so is acceleration – it’s a particular magnitude in a particular direction. So imagine a car that goes from stationary to, say 50 kms/hour in 5 seconds, what’s the acceleration? According to the formula, it’s 50 – 0 kph/5 seconds, or 10kph/sec, which we can write out as a change of velocity of ten kilometres per hour per second.
Canto: So every second, the velocity of the car is increasing by 10 kilometres per hour. I’m trying to picture that. It’s quite hard.
Jacinta: Okay while you’re doing that, let’s introduce dimensional analysis, so that we reduce everything to the same dimension, sort of. I mean, we have hours and seconds here, so let’s take it all to seconds. I won’t be able to do this properly without an equation-writing plug-in, which I can’t work out how to get without paying. Anyhow..
10 kms/hour.second.1/3600 hour/second. Cancelling out the hours, you get 10 kms/3600 seconds squared, or 1/360 km/s2
Canto: I wonder if there’s a way of hand-writing equations in the blog, that’d be more fun and easy. So can you briefly explain dimensional analysis?
Jacinta: Well physical quantities are often measured in different units – for example, quantities of time – time is called the base quantity – are measured in seconds, hours, days etc. So, it’s just a matter of getting such measurements to be commensurate, so that an equation can be simplified – all in seconds, or all in metres, when they can be. Though actually it’s more complicated than that, and I’ve probably got it wrong.
Canto: So talking of brushing up on stuff, or actually knowing about stuff for the first time, I thought it might be good to go back to Newton, his three laws of motion, in written and mathematical form.
Jacinta: Go ahead.
Canto: Well, the first law, which really comes from Galileo, is often called the law of inertia. Newton formulated it this way, in the Principia (translated from Latin):
Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed.
And as Sal Khan and others point out, Newton is talking about an unbalanced force, one that isn’t matched by an equal and opposite force (which would be a balanced force – see Newton’s third law). This law doesn’t come with a mathematical formula.
The second law, which I filched from The Physics Classroom, can be stated thus:
The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.
It’s famous formula is this:
Fnet = m • a
It can be written different ways, for example simply F =m.a, or with the vector sign (an arrow) above force (F) and acceleration (a), showing the same direction, but it’s certainly important to explain net force here. It’s essentially the sum of all the forces acting on the mass, in vector or directional terms. It’s this net force that produces the acceleration.
So to the third law, and this is how Newton presented it, again translated from Latin:
To every action there is always an equal and opposite reaction: or the forces of two bodies are always equal and are directed in opposite directions.
It’s often stated in this ‘wise proverb’ sort of way: ‘for every action there’s an equal and opposite reaction’.
Jacinta: What goes around comes around.
Canto: That’s more of a wise-guy thing. Anyway, the best formula for the third law is:
FA = −FB
where force A is the action and force B the reaction. This law is sort of counter-intuitive and also sort of obvious at the same time! I think it’s the most brilliant law. Sal Khan gives a nice extra-terrestrial example of how you might utilise it. Imagine you’re in outer space and you’ve been cut off from your spaceship and are accelerating away from it. To save yourself, take something massive, if you can, something on your suit or a tool you’re carrying, and push it hard away from you in the opposite direction to the ship, and this should send you accelerating back to the ship. But make sure your aim is true!
Jacinta: Okay, so this seems to have taken us absolutely no closer to Maxwell’s equations.
Canto: Well, yes and no. It makes us think of forces and energy, albeit of a different kind, and it makes us think in a logical, semi-mathematical way. but we’ve certainly got a long way to go…
References
https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion
https://www.physicsclassroom.com/class/newtlaws/Lesson-1/Newton-s-First-Law
https://www.livescience.com/46558-laws-of-motion.html
https://www.physicsclassroom.com/class/newtlaws/Lesson-3/Newton-s-Second-Law