# Homework Help: Minimum force needed

1. Oct 7, 2015

### werson tan

1. The problem statement, all variables and given/known data
is part C lack of information ? can someone give me a hint how to start ?

for part a ,
-(1.35-0.95)(200cos60)-2(200sin60)=-386N(anticlockwise)

for part b , -386.4/2 = 193N (in upward direction)

2. Relevant equations

3. The attempt at a solution

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2. Oct 8, 2015

### haruspex

There is enough information.
Suppose you resolve the tension into a force parallel to AB and a force perpendicular to it. What can you say about how these components contribute to the moment about A?

3. Oct 10, 2015

### werson tan

If i resolve the tension into a force parallel to AB and a force perpendicular to it , then I need to take the perpendicular distance multiply the force to get the moment . But , the angle between the force and x , y -axis is unknown , how to determine the force ? how to know it's minimum ?

4. Oct 10, 2015

### haruspex

What is the moment about A of the component parallel to AB?
There is enough geometrical information to find the angle ABF.

5. Oct 10, 2015

### werson tan

why angle ABF is required ?

6. Oct 10, 2015

### haruspex

You are asked to find the smallest force that could be applied at B in order to produce the same momemt about A that the force along BF has. I suggested resolving the force along BF into a component parallel to AB and a force perpendicular to AB. In order to do that you need to find the angle which BF makes to AB.

7. Oct 12, 2015

### werson tan

well, i still dont knw what do you mean here, btw , i managed to get the angle of ABF = arc(tan 0.4/2) = 108.7

8. Oct 12, 2015

### insightful

What direction must the minimum force at B have in relation to line AB?

9. Oct 12, 2015

### werson tan

Sorry, I am nt sure

10. Oct 12, 2015

### insightful

Exercise: Take a 1m bar and apply a 10N force to one end. In what direction will the force produce the maximum moment at the other end of the bar?

11. Oct 12, 2015

### haruspex

Ok, so now you know how to resolve the applied force F into components parallel to AB (call this FAB) and perpendicular to AB (FP). What moment does FAB have about A?

12. Oct 13, 2015

### werson tan

max moment will produce when the force is applied exactly 1m from the point .

13. Oct 13, 2015

### werson tan

this looks weird, this is the first time i resolve the force with the angle more than 90 degree

#### Attached Files:

• ###### 2.png
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14. Oct 13, 2015

### haruspex

That's the right diagram, but you did not answer my question. What is the moment of the FAB component about A?

15. Oct 13, 2015

### insightful

That is correct, but I was asking about the direction of the force at that point. For example, in this drawing, which direction of the 10N force (A, B, or C) produces the maximum moment at point O?:

16. Oct 13, 2015

### werson tan

B

17. Oct 13, 2015

### insightful

Correct! So, what angle to any moment arm would give the minimum force to produce a given moment?

18. Oct 13, 2015

### werson tan

90 degree is to produce max force , how to determine the min force ?

19. Oct 13, 2015

### haruspex

No, 90 degrees produces the maximum moment for a given magnitude of force; which is the same as saying it requires the minimum magnitude of force for a given moment.

20. Oct 13, 2015

### werson tan

ok , i understand it . but , how to solve the question ?

21. Oct 14, 2015

### haruspex

Use the diagram you posted at #13.
What I was trying to get you to see was that the component of F parallel to AB has no moment about A, so you might as well set it to zero. That leaves only the component perpendicular to AB. You know what moment this needs to have about A, and you know the distance AB, so what is the magnitude of this force?

22. Oct 14, 2015

### werson tan

so , i have found out that the AB = 2.3m , this is due to sin11.3= 0.45/AB
the angle between the line AB and the horizontal line = 11.3 degree becoz i take 180 degree - 60 degree-108.7 degree , so i gt 11.3 degree

so i have found out the force = 386.4/ 2.3 = 168 N ... is it correct ?

23. Oct 14, 2015

### werson tan

why the distance AB is required ?

24. Oct 14, 2015

### haruspex

The 0.45 is wrong, leading to the wrong value for the minimum force.

25. Oct 14, 2015

### werson tan

why the distance AB is required ? if i substituted 0.45 with 0.4 , then my ans would be correct ?