newton mechanics?

Question

When the two ends of a spring balance are pulled by two equal forces F,what is the force indicating on the spring balance?

Answer 1

F

you need 1 time F to create the force, and the other F at the other side of the spring is the reaction force

Answer 2

Imagine one end of the spring is connected to a wall. The force--say a weight--stretching the spring is F, and the wall must exert F on the spring the other way--otherwise the spring must accelerate. Now imagine that the spring itself is the spring balance--or that you have attached a little pointer to the end of the spring.

The extension of the end of the spring shows how far the spring has been stretched, and gives the force, because of Hooke's Law, F=kx. The spring must be exerting exactly this force on the weight, because otherwise the weight would be accelerating.

The confusing thing about this problem is that you are tempted to describe the force on the spring--which is zero total--when what you really want is the force which the spring is exerting on the weight.