How Does a Spring Scale Measure the Total Force in a Balanced Meter Ruler Setup?

[xiaomeizi]

Homework Statement

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)

Homework Statement

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?

Homework Equations

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]

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]

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]

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...

Thanks for your reply.

 

[berkeman]

(two threads merged)