## A physics question.

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McClueless
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Joined: Thu Apr 08, 2010 2:54 am UTC

### A physics question.

here is a simple (not for me..) physics question. Before you say i am a stupid douche bag, i have no prior experience with physics except for 'Rates of Change' stuff that i did last year in maths.-i am currently trying the learn the laws of motion, hence the question.

here is question;

"An object is positioned on a frictionless surface inclined at 20degrees to the horizontal, given that the acceleration due to gravity is 9.81 (meters per second, per second), what is the component of gravitational acceleration down the plane?"

~we were given the answer which is: 3.36 (meters per second, per second).

I honestly don's have the slightest clue about how to do this. If you could give me a step by step including the formula you use that would be great .

edgey
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Joined: Sat Oct 10, 2009 6:34 pm UTC

### Re: A physics question.

I am too lazy to make a diagram right now, but as a thought exercise, you're walking up a hill.
The hill in this sense is your "plane". When it's at zero degrees, gravity's force is completely perpendicular to the plane, 9.81m/s^2.
As the hill gets steeper, you're still walking on the plane, and therefore less gravity affects you perpendicular to the plane, and more parallel to it.
Imagine 10 degrees, (completely made up numbers) you might have 1m/s^2 parallel, 9 m/s^2 perpendicular (it's still not very steep)
You can feel that it's harder just to make a "horizontal" (parallel to the plane) movement, whereas before it was fine. When you get to 45 degrees, it's getting pretty hard just to move, at that point you have equal amounts of gravity acting on you perpendicular and parallel to the plane.

The formula to use is simply 9.81cos20 / 9.81sin20 (depending on if you want parallel or perpendicular). You should be able to figure out in this case which is correct. Try drawing out the problem and fiddling with triangles, you should be able to figure out why.

Tass
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### Re: A physics question.

McClueless wrote:Before you say i am a stupid douche bag.

The stupid douche bags that would say a thing like that have been banned long ago. There's no shame in being ignorant when you are working to change it.

Fume Troll
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### Re: A physics question.

no-genius
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### Re: A physics question.

Tass wrote:The stupid douche bags that would say a thing like that have been banned long ago.

Yeah, but they keep coming back!

A better way of doing it might be taking the 20° plane as your x-axis (so you rotate everything by 20°) and seeing where the acceleration comes out to your new x-axis.
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squareroot1
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### Re: A physics question.

Basically, you want to break the gravitational force (a vector straight down) apart into a force perpendicular to and a force parallel to the inclined plane. In order to do so, use trig.

An aside, by "Rates of Change", do you mean derivatives? Did you learn anything about integrals? The two are tied into the laws of motion (especially when acceleration dosen't have to be constant.)

That problem has much less to do with the laws of (Newtonian) motion than the ability to resolve a vector under a new basis.

There is no need to put units in parentheses or write them longhand, feel free to just write m/s^2.

thicknavyrain
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### Re: A physics question.

HEY, this might help, I tried drawing it out to explain it but I think I might have managed the impossible of being horribly patronising without explaining anything whatsoever, but see if it helps you anyway, if not, there are lots of online resources.

Spoilered for length:
Spoiler:

Sorry for the shittiness but oh well...

EDIT: When it says "to the horizontal considered can be" I mean "To the horizontal can be considered". Sorry, I guess all this talk about the force made me go a little Yoda.
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McClueless
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Joined: Thu Apr 08, 2010 2:54 am UTC

### Re: A physics question.

thankyou everybody for you help. The diagram certainly made things easier to picture ( ), i enjoy your bunny killing magician.

squareroot1 wrote:
An aside, by "Rates of Change", do you mean derivatives? Did you learn anything about integrals? The two are tied into the laws of motion (especially when acceleration dosen't have to be constant.)

yes, derivatives. we did not learn anything about integrals, and the differentiation stuff we did was really watered down, so i cant say i gained anything much from it. but thanks, now i have something to go off and read about