Physics Moment Question

symx SG

How to do question 2?

^tamago^ SG

The weight of the ruler produces a counter-clockwise moment around the pivot at the point where the mark is 50cm.
Let W be the weight of the ruler.
Counter-clockwise moment = Clockwise moment
15cm × 240g = (50-20)cm × W
15cm × 240g = 30cm × W
W = 120g

deteq SG

can explain more mah. i dont really understand. thanks.

iamapebble SG

Taking moments about the pivot,
Anti-clockwise moment = Clockwise moment.

The weight of the ruler (let it be X) at the 50cm mark acts like the 240g mass. If you do not want to be confused, convert all the values to their SI units. So we have:

240g x (20-5)cm = XN x (50-20)cm
NOTE: The perpendicular distance from the pivot is not read off directly from the metre rule.

=> 0.24kg x 0.15m = XN x 0.3m
      2.4 N x 0.15m = XN x 0.3m
      0.36 Nm = 0.3X Nm
      X = 1.2N
          = 0.12kg
          = 120g

Basically everything that ^tamago^ has did :)

deteq SG

actually , i dont understand the (50-20)cm the part.

^tamago^ SG

actually , i dont understand the (50-20)cm the part.

Moment = Weight × Distance to Pivot.

deteq SG

then why it isnt 100-20 but 50-20.

oh dear. i need my a1.

symx SG

Many thanks, now I understand.

deteq SG

but i dont.

deteq SG

oh yes i understand now. is it because the weight is acting on the cg of the ruler.

iamapebble SG

but i dont.

The mass of the ruler is in the middle, which is at the 50 cm mark, not at the 100cm mark. Perpendicular distance from the middle to the pivot is therefore 50 - 20, not 100 - 20. Anyway, you can try to make a sense of your working by looking at the values because 80cm is too long a length from centre to the pivot.

iamapebble SG

oh yes i understand now. is it because the weight is acting on the cg of the ruler.

Weight is a force that always acts downwards at the cg of the ruler, therefore for this uniform rule, it exactly at the centre. :)

^tamago^ SG

u understand the concept of centre of gravty of an object?

deteq SG

ah yes that clears everything up. oh yea there it is, my a1