# Turning effect of forces

1. Mar 17, 2008

### tyana21

to find the moment of force X about A,is the working
20x1.4=0.8X
X=35Nm clockwise.

and what does it mean when the question asks "what is the minimum force X that must be applied in order to stop the door from turning?"

does that mean it has to be in equilibrum?

2. Mar 17, 2008

### Staff: Mentor

X is the force; the moment of the force is just the force times the moment arm (distance to pivot).

The force F exerts a moment tending to turn the door; force X must exert an opposite moment to cancel it.

Yes, rotational equilibrium.

(FYI: It's always a good idea to state the problem exactly as given.)

3. Mar 17, 2008

### tyana21

okay the question is "what is the minimum force X that must be applied in order to stop the door from turning?"

(p.s.,i'm not quite sure as i didnt attend school today and this is part of my homework from my physics workbook)

4. Mar 17, 2008

### Staff: Mentor

And you already solved for that force X. (Even if you didn't recognize it as such.)

5. Mar 17, 2008

### tyana21

oh,i see.but then the question is phrased in two parts.does this mean that i can just bring forward the answer?

6. Mar 17, 2008

### Staff: Mentor

What are the two parts? State the complete problem exactly as given.

7. Mar 17, 2008

### tyana21

a) Calculate the moment of the force X about A.

b) what is the minimum firce X that must be applied in order to stop the door from turning?

8. Mar 17, 2008

### Staff: Mentor

Good. In the light of my previous posts, can you answer each part?

9. Mar 17, 2008

### tyana21

so does that mean that both questions the answers are the same?
because you previously said that i actually answered the question without me knowing

10. Mar 17, 2008

### Staff: Mentor

No, the answers are not the same. The first part asks for a moment; the second part for a force.
Right. You actually solved for the force (X) when you thought you had solved for the moment.

I interpret the first question as a general question that helps you answer the second question. For a force X that is a certain distance from the axis, what's the moment?

11. Mar 17, 2008

### tyana21

so the answer is 28Nm,right?

12. Mar 17, 2008

### Staff: Mentor

That's the moment needed to produce equilibrium, but I don't think that's what they are looking for in question a. (They don't even bring up the idea of preventing the door from turning until question b.)

I think they just wanted a general answer. The moment of a force (F) at a perpendicular distance (d) from an axis is Fd. So just plug in what you know about the force X. (At this point, X is just some unknown force. But you do know the distance from the axis.)

13. Mar 17, 2008

### tyana21

ah i see
thanks fr the help!
i got to go now :)
once again thanks!