# Physics question on Moment of force (1 Viewer)

### Users Who Are Viewing This Thread (Users: 0, Guests: 1)

#### xiaomeizi

1. The problem statement, all variables and given/known data
A uniform metre ruler of weight of 1.2N suspended at its mid-point from a spring scale which is calibrated in newton(N).
An apple with weight of 0.75N, suspended from the metre rule at the 10cm mark, is balance by a weight of 1N, which is suspended from the 80cm mark. Assume g=10N/kg.
Q. what is the reading on the spring balance?

The reading on the spring balance= weight of both apple and the 100N. (0.75N+1N=1.75N)
I don't understand the answer. what about the weight of the metre ruler, balanced out by the upwards force ??? I thought the spring scale is used to measure the tension, if that is the case, the tension should include the weight of the metre rule, isn't it ??? could anyone dismiss my doubts??? Thanks you. :D

#### xiaomeizi

Turning effect of force (Moments)

1. The problem statement, all variables and given/known data
A uniform metre ruler of weight of 1.2N suspended at its mid-point from a spring scale which is calibrated in newton(N).
An apple with weight of 0.75N, suspended from the metre rule at the 10cm mark, is balance by a weight of 1N, which is suspended from the 80cm mark. Assume g=10N/kg.
Q. what is the reading on the spring balance?

2. Relevant equations

3. The attempt at a solution

The reading on the spring balance= weight downwards due to gravity
the answer suggested that (0.75N+1N=1.75N)
BUT, I don't understand the answer. what about the weight of the metre ruler, balanced out by the upwards force ??? I thought the spring scale is used to measure the tension, if that is the case, the tension should include the weight of the metre rule, isn't it ??? could anyone dismiss my doubts??? Thanks you. :D

#### PhanthomJay

Homework Helper
Gold Member
Yes, you are correct, the scale should read 2.95 N. Perhaps the scale was precalibrated to 0 with the meter stick alone on it? Like when you weigh a bunch of potatoes on a spring scale in a food store, the scale is adjusted to 0 with the pan weight alone on it, so you don't get overpriced. But the problem is not clear in that regard.

#### issacnewton

Re: Turning effect of force (Moments)

hi xiao

yes, the answer seems wrong. since the ruler is neither moving nor rotating, we have to use
the stability condition for the force. when we do that , the answer given seems wrong as it neglects the weight of the ruler...

#### xiaomeizi

Re: Turning effect of force (Moments)

hi xiao

yes, the answer seems wrong. since the ruler is neither moving nor rotating, we have to use
the stability condition for the force. when we do that , the answer given seems wrong as it neglects the weight of the ruler...

Mentor

### The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving