IGCSE/GCSE/O & A Level/IB/University Student Forum

Qualification => Subject Doubts => GCE AS & A2 Level => Sciences => Topic started by: Lillymon on October 10, 2009, 10:31:33 pm

Title: Finding resultant of a system of forces ...
Post by: Lillymon on October 10, 2009, 10:31:33 pm

... by using rectangular components.

Hello everyone! First post in this forum I hope I'll find what I'm looking for.
I'm home-studying Physics for the upcoming AS Level exams. Right now, I'm sort of stuck with understanding resultant forces. I've read about it in several books but none really explains what happens when adding the x and y components. Sometimes I solve the problems correctly and some other times, I just get stuck there not knowing what am I supposed to do next =|

I understand the steps of calculating but somewhere around calculating the horizontal and vertical components things start to get messed up  :(
I don't think I fully understand how to calculate those because I don't get when to use sin or cos. Could someone please explain to me when to use the sin/cos or why sometimes it's in negative and sometimes it's positive ? Also at the very end when I find the direction of the resultant force, how can I tell if it's above the horizontal or below it or whatever it is supposed to be ?

Here is a problem I'm currently working on http://i34.tinypic.com/5upjzs.jpg

Thanks alot in advance for any help  :)

Title: Re: Finding resultant of a system of forces ...
Post by: astarmathsandphysics on October 10, 2009, 10:44:28 pm

I will modify a page on my website to explain it now

Title: Re: Finding resultant of a system of forces ...
Post by: astarmathsandphysics on October 10, 2009, 11:15:58 pm

http://www.astarmathsandphysics.com/a_level_maths_notes/M1/a_level_maths_notes_m1_finding_resultants_of_forces.html

Try this

Title: Re: Finding resultant of a system of forces ...
Post by: astarmathsandphysics on October 10, 2009, 11:54:18 pm

I am such a nutter, working on my website at every opportunity. I will do your prb first thing to morrow, and illustrate the answer on my website with the link posted.

Title: Re: Finding resultant of a system of forces ...
Post by: Lillymon on October 11, 2009, 10:29:16 am

Thank you very much for the prompt reply and your help sir. I've read the notes and understood what's written there. I also got the part about assigning negative or positive signs.
There are still some confusions but I'm hoping it'll be cleared out when I read your next reply.

Once again, thank you very much.
 :)

Title: Re: Finding resultant of a system of forces ...
Post by: astarmathsandphysics on October 11, 2009, 12:01:04 pm

I answered you question at the link I posted above.

Title: Re: Finding resultant of a system of forces ...
Post by: aiyla on October 11, 2009, 06:09:47 pm

as far as i know .. doing the horizontal components we have to use  ucos
while doing vertical ones we have to use usin

you did like this


Resolving Vertically: 40+60cos 70 -50sin30 = 35.52N

Resolving Horizontally: 80-60sin 70-50cos 30 = -19.68N


so how is that ?  this part i want to know how can cos and sin be together in verticall or horizontal part ?

Title: Re: Finding resultant of a system of forces ...
Post by: astarmathsandphysics on October 11, 2009, 09:41:07 pm

You can use cos only for horizontal forces ONLY if the angles are between the force and the horizontal. That is not the case with your example.