Check these out:http://www.imagesco.com/sensors/force-sensors.html
I was a little curious about force sensors myself today. These guys have different sizes up to 100lbs (44 newtons?) and they are apparently read as a resistor with a range between 5M and 5k. Soooo, divide the max rating by the difference in impedance, and you get a resolution of around 0.0000088 newtons per ohm. But since the Arduino has a 10-12 bit adc, the resolution does get brought down. If the change is 4995000 ohms, then it would take a change of 4877 ohms to register on an Uno. So you'd be looking at 0.0429 newtons per byte, which is kinda close... but the smaller ones may get better resolution, and a chip with a 12 bit adc can get you up to 0.0107. Just a couple ideas! Good Luck!