Soldiers sometimes have heavy solid steel plates for armor. There could be potential in using water or another hydraulic fluid to reduce weight.
Water does not compress (much), so the only challenge would be to keep it from deforming. This might be solved by adding compartments.
Calculations
Water is significantly lighter than steel
One cubic foot of water contains 7.48 gallons of water, weighing in at 62.31 pounds.
The density of steel is 490 pounds per cubic foot.
Of course, if you don't believe it's lighter, find something steel and drop it in some water. It'll sink 'like a brick'. Water also doesn't compress very much. This is the principal behind hydraulics.
Preliminary Calculations:
Dimensions: 4 "rooms" 7*5*1in each. < to add rigidity. Whole thing: 14*10*1in. Approximated surface area: 350in^2 Projectile: Velocity: 2000ft/s = 609.6 m/s <about as fast as they get Weight: .5kg <significantly heavier than most .50 cal Momentum: 61m*kg/s Required stop time to stop it in 1cm: .00008197 seconds of impact using: ( .01m/TIME*.5kg=61m*kg/s ) Required force: 742980N using: ( F * .00008197 seconds of impact = 61m*kg/s ) Convert Newtons to KG: 75814 Convert KG to lbs: 167141 lbs find psi: 477psi 167141 lbs / 350in
Many hydraulic systems take 2000 psi. Aside from the obvious weight improvement, water also requires a substantial amount of energy to increase its temperature. Metals require very little. If the body armor was stored indoors, heat exhaustion would take far less time to set in.