# Spring balance: what will be the reading? [duplicate]

There are two cases:

1. When the two forces say L (for left) and R (for right) are equal. F=L=R What will be the reading? I know the reading would be F but why?

2. When the two forces are unequal that is L (not equal to) R What will be the reading? Well I have no clue;

## marked as duplicate by John Rennie, Kyle Kanos, Floris, ja72, Qmechanic♦Jul 24 '15 at 21:38

• More on forces and factors of two: physics.stackexchange.com/q/41291/2451 and links therein. – Qmechanic Jul 24 '15 at 16:45
• Note that the answer to your second question is not addressed by the duplicate, and in fact "it depends". Because the whole system would be accelerating, and without details of the construction one cannot know for sure how that translates into a reading on the scale (which is supposed to be stationary when used). – Floris Jul 24 '15 at 19:49

1. In terms of representing the answer: F would be for the reading because that is the basic notation for total force, and as there is no need to differentiate between the two forces Left or Right due to them being equal, then only F should be used. The actual answer as you probably will know is F = 0.

2. When L is not equal to R, the reading should be along the lines of F = L - R, where in this case -ve F, means in the direction to the right and +ve F, means a direction to the left.

If a force f (Net force) acts on the mass(Spring Balance), it will accelerate with a = f/ m. Spring balance will only show the reading if there is net force causing deflection of spring (F= K*x).

Here f = Net Force on Spring causing acceleration

F= Force that causes deflection of spring(Which always acts in pair - equal in magnitude and opposite in direction)

1. In this case force in either direction (which is equal in magnitude) will account for the spring deformation and thus the spring balance reading.

2. When L and R are unequal let L>R, Net force F=(L-R) causes acceleration of the mass. and spring balance will show the reading equal to R. e.g. L=30KN R=20KN