Hey there Sign in to join this conversationNew here? Join for free

M1 resolving forces

Announcements Posted on
    • Thread Starter
    • 0 followers
    Offline

    ReputationRep:
    I hate resolving forces so much and when I thought I sussed it something comes up so can someone please help me out.

    Two forces P and Q act on a particle at a point O. The force P has magnitude 15 N and the force Q has magnitude X newtons. The angle between P and Q is 150°, as shown in
    Figure 1. The resultant of P and Q is R.

    Given that the angle between R and Q is 50°, find
    (a) the magnitude of R

    I know that I have to resolve vertically but for resolving forces I was under the impression that the forces that point up equal those that point down. Like kind of the forces in different directions equal each other.

    So I thought it would be 15sin30 + Rsin30 = 0

    However it's actually 15sin30 = Rsin30

    ohhh confusion, please some help! I attached a good forces diagram to illustrate the information.

    thanks
    Attached Thumbnails
    Click image for larger version. 

Name:	mechanics.jpg 
Views:	246 
Size:	8.4 KB 
ID:	70257  
    • 10 followers
    Offline

    ReputationRep:
    It's easiest to do this by a triangle of forces. Draw the force P of "length" 15 horizontal, then the force Q of length X at 150 degrees to the horizontal, so the angle between P and Q is 30. Then you can draw the force R from O to the end of Q; the angle between R and Q being 50 degrees. Now one can use the sine rule to find R.
    • Thread Starter
    • 0 followers
    Offline

    ReputationRep:
    I am generally fine with resolving it the standard way but I just don't get this concept of when to add or minus the forces.

    I think for this one I am getting confused between whether the particle is at equilibrium or not. I just realised it doesn't say equilibrium. So I am I right to think if it's in equilibrium all the forces towards teh right = forces towards the left. However in the case of no equilibrium would it be X(unknown) - every other parallel force?
    • 0 followers
    Offline

    ReputationRep:
    is the answer 9.79? if so i can help.
    Ok just checked, i used the sin rule for my solution. To answer your question i dont think it matters which sign you use because the mag will always be positive. (i think you meant Rsin50 not R sin30*)
    • Thread Starter
    • 0 followers
    Offline

    ReputationRep:
    (Original post by T.P.D-L)
    is the answer 9.79? if so i can help
    Yeah it is.

    I know I have to resolve vertically and I can get all of the horizontal forces however I just don't understand whether we add the two horizontal forces together ot minus them or make them equal to each other. I know the mark scheme makes them equal to each other but why?
    • Thread Starter
    • 0 followers
    Offline

    ReputationRep:
    (Original post by T.P.D-L)
    is the answer 9.79? if so i can help.
    Ok just checked, i used the sin rule for my solution. To answer your question i dont think it matters which sign you use because the mag will always be positive. (i think you meant Rsin50 not R sin30*)
    Yeah sorry my bad

Reply

Submit reply

Register

Thanks for posting! You just need to create an account in order to submit the post
  1. this can't be left blank
    that username has been taken, please choose another Forgotten your password?

    this is what you'll be called on TSR

  2. this can't be left blank
    this email is already registered. Forgotten your password?

    never shared and never spammed

  3. this can't be left blank

    6 characters or longer with both numbers and letters is safer

  4. this can't be left empty
    your full birthday is required
  1. By joining you agree to our Ts and Cs, privacy policy and site rules

  2. Slide the button to the right to create your account

    Slide to join now Processing…

Updated: May 21, 2009
New on TSR

Talk about SQA results day

Join the chat ahead of grades coming out on Tuesday

Article updates
Reputation gems:
You get these gems as you gain rep from other members for making good contributions and giving helpful advice.