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Physics Moments question

Can someone help me out with this question, I suck at moments -_- :
A uniform metre ruler has a pivot 0.3m from one end. If a 1N weight is positioned at 0.1m from the same end as the pivot, calculate the weight of the ruler.
The answer is 1N but I don't understand how :frown:
Original post by Azzy_wazzy
Can someone help me out with this question, I suck at moments -_- :
A uniform metre ruler has a pivot 0.3m from one end. If a 1N weight is positioned at 0.1m from the same end as the pivot, calculate the weight of the ruler.
The answer is 1N but I don't understand how :frown:


Let the co-ordinate x=0x = 0 be the left hand side of the metre ruler. Let WW be the weight of the metre ruler.

The pivot is at xp=0.3x_p = 0.3.

The 1 N weight is at xw=0.1x_w = 0.1.

The centre of mass of the metre ruler is at xm=0.5x_m = 0.5.

Take moments about the pivot:

1N×(xp−xw)=W×(xm−xp)1 \mathrm{N} \times (x_p - x_w) = W \times (x_m - x_p).

If you rearrange the above equation, you can obtain the weight of the meter ruler.
Reply 2
Avatar for Azzy_wazzy
Azzy_wazzy
OP
6
Original post by pleasedtobeatyou
Let the co-ordinate x=0x = 0 be the left hand side of the metre ruler. Let WW be the weight of the metre ruler.

The pivot is at xp=0.3x_p = 0.3.

The 1 N weight is at xw=0.1x_w = 0.1.

The centre of mass of the metre ruler is at xm=0.5x_m = 0.5.

Take moments about the pivot:

1N×(xp−xw)=W×(xm−xp)1 \mathrm{N} \times (x_p - x_w) = W \times (x_m - x_p).

If you rearrange the above equation, you can obtain the weight of the meter ruler.

You're amazing thank you! ♥ May I ask, where did you get the equation from?? Is it from the principle of moments i.e sum of clockwise moments equals to sum of anticlockwise moments??
(edited 7 years ago)
Original post by Azzy_wazzy
You're amazing thank you! ♥ May I ask, where did you get the equation from?? Is it from the principle of moments i.e sum of clockwise moments equals to sum of anticlockwise moments??


Yes.

However, in an identical fashion, you could similarly define one direction (clockwise or anticlockwise) as positive and the other as negative (anticlockwise and clockwise correspondingly). Next, you could say that the sum of all moments are zero i.e. moment equilibrium.
Reply 4
Avatar for Azzy_wazzy
Azzy_wazzy
OP
6
Original post by pleasedtobeatyou
Yes.

However, in an identical fashion, you could similarly define one direction (clockwise or anticlockwise) as positive and the other as negative (anticlockwise and clockwise correspondingly). Next, you could say that the sum of all moments are zero i.e. moment equilibrium.

I see, that makes sense. Thank you 🙇 🙇

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