Physics patches

[Buggi]

Ah yes, particle physics. Lets add the ability to calculate the effect the steam clouds have on the first trains in the game, or how about bus slipstreams!!

*explodes*

[Archonix]

Those pages lost me. Pounds of thrust? I was taught in metric at school, so I might be mistaken, but I thought a pound was a unit of mass.

It'sa unit of weight, not mass, so can be used to measure the amount of pressure something exerts against something else, and which has lead to the rather interesting measurement of pounds per square metre that was occasionaly used by NASA in the early 90s...

[richk67]

Those pages lost me. Pounds of thrust? I was taught in metric at school, so I might be mistaken, but I thought a pound was a unit of mass.

It'sa unit of weight, not mass, so can be used to measure the amount of pressure something exerts against something else, and which has lead to the rather interesting measurement of pounds per square metre that was occasionaly used by NASA in the early 90s...

LOL... just remember NASA are the ones who read m(iles) for m(etres) in an early shuttle mission and bounced a laser back to a 10,000 mile high mountain top!

[Brianetta]

It's a unit of weight, not mass

Out of interest, what is the imperial measure of mass?

[gkirilov]

mass == weight, isn't it?

[bobingabout]

IIRC mass is how heavy something is, weight is the dawnwards force applied due to gravity.

mass is measured in newtons is it?

highschool phisics.

[Teren]

IIRC mass is how heavy something is, weight is the dawnwards force applied due to gravity.

mass is measured in newtons is it?

highschool phisics.

mass - kg
weight - newtons

[bobingabout]

it was 7 or 8 years ago, what do you expect?

[hertogjan]

In classical mechanics, mass is the amount of inertia an object has. That is, the ratio between the amount of force applied to that object and its acceleration (assuming that this ratio is constant). So if I push an object with, say 10 N, and it accelerates with 5 m/s^2, then the mass is 2 kg. This definition is sometimes called "inertial mass", since there is also a notion of "gravitational mass", which is determined by the amount of gravitational force the object has on other masses, or to state it diferently, how strong the gravitational field generated by the object is.

Weight is something totally different. It is the amount of force exerts on a mass by its surroundings (other than gravitational forces, probably). This may be equal to the gravitational force, but this is not always the case. If I am in free fall, then I am weightless. But still there is a gravitational force.
If I put a mass of 1 kg on a table, then the gravitational force pulls it with 9.8 N and the table pushes back with 9.8 N, since the net force most be zero (we assume that the mass is at rest). The latter 9.8 N is the weight. If I drop a mass of 1 kg, then the gravitational force on it is still 9.8 N. But there are no forces pushing back. Hence it has no weight. Until it comes down to the ground.

I agree, that in everyday life we may assume that weight and mass are equivalent things, since they are related by a constant ratio. But things can get confusing if you go, for instance, away from the earth. For example: You say that you "weigh" 120 kg on earth and since the gravitational force of the moon is 6 times smaller than earth's, you say that you weigh 20 kg on the moon. But this is not true: You are 120 kg on earth and also 120 kg on the moon. But what you mean is that your weight on the moon is equivalent to the weight of 20 kg on earth, i.e. approximately 200 N.
This example shows that you have to be careful with the differences between mass and weight. Not in all situations, but in some situations incorrect use of these notions may be confusing.

[DaleStan]

Out of interest, what is the imperial measure of mass?

http://scienceworld.wolfram.com/physics/Pound.html and http://scienceworld.wolfram.com/physics/Slug.html

In the foot-pound-second system of units, the pound is defined as the weight of a pound-mass at a point on the Earth's surface where the acceleration due to gravity is 32.174 ft/s^2, while the slug is a derived unit of mass equal to 1/32.174 lbm, or 14.5939 kg.

The unit of mass in the foot-pound-second (British engineering) system, equal to the mass that will require an acceleration of 1 ft/s when subjected to a force of 1 pound. It is therefore equal to 1 lb s^2/ft, 32.1740 pound-mass or 14.5939 kg

[Buggi]

I've forgotten what this thread is supposed to be about.

And here I thought matter couldn't be destroyed, apparently that doesn't apply to brain cells.

[DaleStan]

The brain-cells aren't destroyed; they just go on an extended vacation.

This thread has a topic?
*poke*prod*
Oh, so it does. Well, at least we're still talking about physics.

[Brianetta]

OK, I have been reading up on the Imperial system of weights and measures. I genuinely cannot understand why anybody on the planet clings to such a complex, irregular system. Such oddities as the number of feet in a mile (5280), or the number of ounces in a pound (16) compared to pounds in a stone (14) ... then the fact that the USA uses different values for all of these base units!

Three little letters popped into my head: W, T and F.

There's no way I'd be comfortable doing stuff in my head with numbers like those.

[hertogjan]

There is not even an unambiguous definition of most imperial units. For weight units, there are multiple systems (troy and avoirdupois), which are different. For volumes, it is even worse: A gallon of a liquid does not have the same volume as a gallon of dry substance. To complicate things even further: British imperial units are often different from American imperial units.

If this is still not enough reason to get rid of the old-fashioned* imperial system...

*Before SI units were used, every country had its own system of units. There were even differences between cities in the definition of units. This must have been causing mass confusion . Luckily, in some parts of the world this is history.

---

Now a little update on the patch. I removed some errors.
-The most serious error was that the maximum speed for newgrf road vehicles was not converted correctly, causing the speeds of the vehicles to be too low. This was caused by a misinterpretation of the newgrf specifications I had read. This also forced me to bring the road speed limit up to 512 km/h, which is close to the highest maximum speed newgrf can assign to a vehicle.
-Bridge speed limits now hold on the ramps as well.
-I had removed a piece of code that should not have been removed. In the end, it may cause crashes, but I never saw that actually happen. The code is put back into place now.
-A revision of the trunk caused the engine table to be broken. This is fixed.

[Archonix]

To continue off topic for a moment, imperial units aren't as arbitrary as you'd think. They're based on the sizes of things, and set up to allow quick divison in to multiple smaller units without fraction. A foot can be divided in to more whole units than a metre, and is scaled to produce manageable sizes inmeasurement to boot. The smallest imperial measurement is the barleycorn, taken from... a corn of barley, oddly enough, three of which are about the same width as your thumb knuckle, or an inch. And it goes from there.

Yes, metric is great for accuracy, and yes, it should be used whenever possible because of that, but don't go knocking imperial simply because you don't understand it. For instance, when butter was sold in 1lb portions it was easy to get a useable amount for cooking, typically 4 ounces, just by slicing ioff a wuarter of the pat. The equivalent is around 120 grammes, but these days butter is sold in portions of between 540g and 500g, somewhat less than a pound, which makes it difficult to gauge the amount. How do you get 120 from 500 without using fractions?

Aaaanyway...

Good job with the patch. I'm going to try out the new version now and see how it works.

[Prof. Frink]

but these days butter is sold in portions of between 540g and 500g, somewhat less than a pound,

500 > 454

And you use the markings on the pack to estimate - it tends to say

|<-----2oz----->|<-----2oz----->|<-----2oz----->|<-----2oz----->|

[Archonix]

but these days butter is sold in portions of between 540g and 500g, somewhat less than a pound,

500 > 454

a) I suck at metric or,
b) typo.

Take your pick.

I'll be quiet now because I don't want to drag this topic off topic again, but I still reckon that imperial isn't getting its fair hearing here.

[Basje]

If I drop a mass of 1 kg, then the gravitational force on it is still 9.8 N. But there are no forces pushing back. Hence it has no weight. Until it comes down to the ground.

As a 'Master in Physics' I have to disagree with this statement. This is only true if you travel along with the stone, or to put it in physics language: if you are in the same inertial frame (http://en.wikipedia.org/wiki/Inertial_f ... _reference, for those who are interested: this inertial frame of reference is a very important part of the 'common relativity theory').

So let's assume it is a big rock that is falling and you are the unlucky guy standing on top of it while it falls (so you are in the same inertial frame of reference). Then both you and the rock appear to you as weightless. However, for a bystander that sees you falling down together with the rock while he stands on solid ground, both you and the rock have a weight defined by mass (kg) * 9.81 (m/s^2).

The force that usually pushes back, as you mention, is called the normal force and is equal to your weight if you are not de-/accelerating in same direction as the normal force points.

[hertogjan]

I hate it when topics "derail", but I still wish to say something about this issue.

I do agree that forces depend on the inertial frame* you look from, i.e., force vectors are not invariant under Lorentz transformations (since momentum vectors aren't).
But in this example of classical mechanics, we assume that the earth is a rigid body which does not accelerate. If a mass is in free-fall, then it does not exert forces on any other object**. Hence it has no weight. This is seen from the reference frame of the mass. From this frame, you (we assume "you" and "the mass" to be the same object) can only feel the gravitational pull when other objects are pushing back. This is what you feel as weight. If you are in free-fall, for instance in a space ship with the engines off, or in an airplane in a parabolic flight (use to do experiments in weightless environment), you feel no weight at all. On the other hand, if you are in a gravity free environment in an accelerating spaceship, you would feel weight, without the presence of a gravitational field.
The weight is however not well-defined in other reference frames that the rest frame of the object itself. The force may have a value in any reference frame, but this value has no meaning. In other words: the definition of weight already assumes that we look from the perspective of the mass itself.



*I think you do not completely understand what an inertial frame is. In your example, the rock is accelerating with respect to the observer on earth. Since inertial frames are by assumption reference frames that do not accelerate with respect to one another, at least one of the frames (probably both) are not inertial.
**It does have a little gravitational pull on the earth, but we assume that the earth is not affected by this force. Instead, we can also remove the earth from this experiment and just assume that there is a gravitational force field.

[Basje]

*I think you do not completely understand what an inertial frame is. In your example, the rock is accelerating with respect to the observer on earth. Since inertial frames are by assumption reference frames that do not accelerate with respect to one another, at least one of the frames (probably both) are not inertial.

I do know what I am talking about (I recently got my Masters in Applied Physics). However, I did asume (and that is the mother of all f*ckups) that the rock was no longer accelerating and thus could be seen as an inertial frame. But as I can see from your replies, you also have a lot of Physics knowledge, so let's rail the topic again.

I really like the idea of this patch, so keep up the work and perhaps try to get it in the Integrated Nightly Build (see the general OpenTTD forum). Perhaps they are willing to replace their realistic acceleration patch.