forcemeter

[shellbell]

why does a forcemeter containa spring?

[Skids]

Do you mean a spring-balance?

[Gnu]

A spring's extension (x) is related to the tension (force, T) by the equation T = kx, so there is a linear relationship between force and extension.  This means the extension can be shown against a linear scale that indicates the force.

I seem to remember this is Hooke's law, and it holds as long as the spring does not exceed its 'elastic limit'.  A forcemeter would be designed so that this couldn't happen.

[brachiopod]

A force meter and spring balance wre effectively the same thing.

A spring requires a force to stretch it. The amount of stretch (the length of extension) of the spring is proportional to the force applied. (Hooke's Law)

So, if a force of 10N stretches a spring by 1cm, a force of 20N will stretch it by 2cm. By sticking an arrow marker  on the spring, you can calibrate your device by marking '10N', '20N', '30N' etc at intervals of 1, 2, and 3 cm intervals.

A spring balance is the same but is calibrated in Kg, pounds, stone etc, (though strictly speaking, these are units of mass.)

What a spring balance is really measuring is the force exerted by a mass as a result of the Earth's gravity - or what we commonly call 'weight'.