Help, with resolving forces and energy !

Hey! :P In my last lesson i learnt fully about F= Ma etc etc

then we got given a homework, and i cant even do the first question, i know the answers and the step through of the answers, if needed ill give them, but could some one give the question ago and say how they got their answer? Hopefully some ones will match the correct one so i can see how it is done. Thanks.

'' A particularly macho mountain biker sets out to prove something. He attacks a 20% hill. The mass of the cyclist is 100kg.''

Calculate the size of the retarding force due to gravity, acting along the slope.

The only thign i got correct was 9.81 * 100, = 981 newtons.

The steps after that, i have no clue, could some one try ? Thanks.

Reply 1

lilpenguin

Any one?

its hard! :P

Reply 2

teachercol

mgsin (theta) - resolve his weight parallel to the slope.

Reply 3

lilpenguin

I havnt learn how to do these types, if some one could explain i would be amazingly grateful :smile:

Reply 4

Jasmine*

you work with vectors
then you choose a spatial base (such as cartesian base for this problem)
and you get 3 equations on every direction (you just need to project you vectors on these directions, that's why a theta angle invite himself in the equations
this is a basic exercise, you really need to learn how to do it with your lesson or at school...)
Good luck.

Reply 5

lilpenguin

I know i need to learn it, thats why i need some one to explain it to me because i dont know it..... I can resolve vectors, i know the force downwards is 981 taking gravity at 9.81, i know that its 20%, so dividing it by 5, gives 196.2, which is the correct answer.

I can do that logically, what i cant do, is the way stated, with Sin, thats what i need help with...

Reply 6

teachercol

A 20% slope means you go up 20m for every 100m along the slope.
That means that sin theta is 0.2

Reply 7

Jasmine*

All right my friend,
let's start from the beginning
there is a math tool that you need :
the scalar product : see also http://en.wikipedia.org/wiki/Scalar_product
when you have a vector, you can write this vector as a sum of other vectors: you just need to do scalar products of your vector with the vectors of the base in which you want to express your vector
ok?
excuse my English, if i could make you a draw....
Ask your teacher, really, dont be afraid, this is the best way to understand once for ever !

Reply 8

teachercol

Scalar products are a little beyond lil if she's only just learned F=ma!

Reply 9

lilpenguin

Yeah, would i learn F=ma in the AS course? Im going to drop physics after 1 year, i just want a high B or low A :P

Reply 10

lilpenguin

Not would i learn F=ma in the AS course, i meant

Would i learn about scalar products in the AS or A2 courses would you imagine? or is it Degree + :biggrin:

Reply 11

Henerz

OK, resolving forces in two dimentions. You are going to have to do it sooner or later, so why not now:

We are only going to consider 2 dimentions; it makes it a lot easier to visulize, and draw on paper - 2D is also only as far as A-level Physics goes. So, the first important concept is that vertical motion and horizontal motion are independant. You can try justifying this to yourself if you like.

So, a force (or velocity or any vector) can be resolved into two components at 90 degrees to each other. now, for your question you need to resolve the force which is the weight. For these questions its always easier to draw a diagram. So, draw the slope and a block (to represent the cyclist) with the weight force acting vertically downwards. The slope is 1 in 5, so what does this equate to in terms of degrees?

Now, on diagram draw two further forces, perpendicular to one another - parrallel and perpendicular to one another. These two forces are the components into which we'll resolve the weght. i hope yo can see how a right-angled triangle is set up with the weight as the hypotenuse and the two components as the other two sides. You can then work out the magnitudes of the two perpendicular sides using trigonometry. Hence, for a general slope angled

\theta

with a mass

mg

on it the Force downslope is

mg\sin\theta

and the force perpendicular to the slope is

mg\cos\theta

This is resolving forces. It can also be applied to projectiles (generally resolving velocities)

Reply 12

Jasmine*

lil you gonna be a great physicist soon
everyone making his own effort to help you :wink:

Reply 13

tillhm

This is what you are looking for:
http://en.wikipedia.org/wiki/Inclined_plane
sin(&#8706 :wink:

=0.2
The force you are looking for is given by F*sin(&#8706 :wink:

=m*g*sin(&#8706 :wink:

=0.2*m*g
You can just contact me via ICQ if you have any more questions.

Reply 14

teachercol

lilpenguin

Not would i learn F=ma in the AS course, i meant

Would i learn about scalar products in the AS or A2 courses would you imagine? or is it Degree + :biggrin:

Dot and cross products arent in any A level Physics courses although of course we use them implicitly.eg
Work - F.d
Moment = F x D

. Ist term university physics.
They are in some Maths/ Fmaths courses at A level.

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