A tank doesn't go from standing still to a constant movement at once, does it?
Physics has a lot of properties, like gravity, elasticity, friction. You could play around with these.
You could also set your tank as an immovable object and move it with move forward or any other move command if physics shouldn't affect it, but it should affect physics.
Is this a top-down or side-view game and is using physics uour best choice? Platform behaviour and bullet behaviour also have gravity, so for some games choosing either of those would be better.