Calculating Moments in AS Physics Homework Question

[seiei]

Homework Statement

Was doing some past papers in preparation for mock Cambridge exams when I ran into this question. Image should be on here I hope...

Homework Equations

Moment=Force x Perpendicular distance from pivot point?

The Attempt at a Solution

Dunno how to start this one, anyone that can point me in the right direction will be much appreciated!

 

[rock.freak667]

Homework Equations

Moment=Force x Perpendicular distance from pivot point?

Yes, this is all you need

The Attempt at a Solution

Dunno how to start this one, anyone that can point me in the right direction will be much appreciated!

Well firstly start by taking clockwise moments as positive (or negative, it doesn't really matter)

So which forces would cause a clockwise moments and what are these moments equal to? (write them separately and then add them; also remember your convention so these are positive; negative if you chose otherwise )

Similarly, which forces cause anticlockwise moments, write them down with the appropriate sign?


If you are having trouble with finding the distances, draw a vertical line and horizontal line through P, then for each for each force's direction draw a perpendicular line to it until it intersects with your line through P, this will the perpendicular distance. (Say if the force is vertical, you would draw a horizontal line)

 

[seiei]

Using positive moments as clockwise rotation, The 20N force x 3m distance makes 60Nm. Then with the 5N and 10N forces become -10Nm and -20Nm respectively. 60-20-10= 30Nm which was the correct answer. Thanks heaps!