A new piece of exercise equipment has been added to your gym, and you try it out. To use this machine, you lie horizontal on a mat with your feet against a platform perpendicular to the mat. The platform is held by a stiff spring that is compressed when the platform moves. Your workout invovles compressing the spring by pushing on the platform with your feet. After testing compressions of the equipment, you determine that you must do 85.0 J of work to compress the spring a distance 0.170 m from its uncompressed length. What magnitude of force must you apply to hold the platform stationary at the final distance given above