# Need help with force and moment?

1. Nov 28, 2013

### cracktheegg

1. The problem statement, all variables and given/known data

http://s1345.photobucket.com/user/Duk_Bato/media/Untitled2_zps52bc2429.png.html

2. Relevant equations

M= F * d

3. The attempt at a solution

I can't solve the last part the iii)

My problem is roller only got one force and how to resolve it?

It may sound excuses but, it will be best it someone can explain and even provide me with the working, since i can't understand really well

2. Nov 28, 2013

### cracktheegg

Please clike the link, photobucket got error so i could not get the image link

3. Nov 28, 2013

### cracktheegg

4. Nov 28, 2013

### SteamKing

Staff Emeritus
We can't work out the problem for you; that is against the rules.

However, for the reaction force at D, assume that it has two components, Dx and Dy. You can still write equations of static equilibrium using Dx and Dy and solve for them.

5. Nov 28, 2013

Dx= Fcos52
Dy= Fsin 52

?

6. Dec 1, 2013

### cracktheegg

Can anyone briefly tell me what need to be done to solve this question, I have been trying but still wrong

7. Dec 1, 2013

### cracktheegg

If I take A as the point where moment taken place and I seperate D into force x and y and use the total moment= 0 can i solve?

8. Dec 1, 2013

### cracktheegg

If I take A as the point where moment taken place and I seperate D into force x and y and use the total moment= 0 can i solve?

9. Dec 1, 2013

### cracktheegg

My moment equilibrium equation
Take A as moment:
5cos20(6)-5cos70(1)+2(cos20*8) - Fsin15(0.53) +F cos15 (0.55)=0

sin20*1.6=0.55
cos 2-*1.6=0.53

10. Dec 1, 2013

### haruspex

A few problems there...
Check the signs. Draw the '5cos70' force. Which way does that act around A?
Resolving F first into horizontal and vertical and then into the 20 degree/70 degree system may be confusing. Perhaps simpler to skip the intermediate step and use 35/55 angles.
The parentheses are wrong in '2(cos20*8)'.
Haven't checked the numbers exactly, but I can see that can't be right. It makes cos 20 slightly less than sin 20; cos 20 must be significantly the larger.

11. Dec 1, 2013

### cracktheegg

thanks, T T but I really can't understand the force of roller

what is the angle i should use?and the distance?

Last edited: Dec 1, 2013
12. Dec 1, 2013

### haruspex

If you resolve the normal force, F, at the roller into a component along AD and a component perpendicular to it then only one of those will have a moment about A.
What angle does AD make to the slope?
So what then is the angle F makes to AD?

13. Dec 2, 2013

### cracktheegg

What angle does AD make to the slope? 35?
So what then is the angle F makes to AD? 20?

14. Dec 2, 2013

### cracktheegg

isit Fsin35(12)?

15. Dec 2, 2013

### haruspex

Yes.
F is at 90 degrees to the slope and AD is at 35 degrees to the slope. So what angle does F make to AD?
Close, in a sense. Try that again.