The mass of a football player who weighs 1250n

Ah hah !  That depends on where he weighs 1250N, because
weight depends not only on mass but also on the local gravity.
So the same mass can have different weights in different places,
and the same weight can indicate different mass in different places.

       Weight = (mass) x (acceleration of gravity) .

Note:  1250N  is almost exactly  281 pounds.

-- If he weighs 1250N on Mars, then his mass is  336.8 kg.

-- If he weighs 1250N on Earth, then his mass is  127.5 kg.

-- If he weighs 1250N on the Moon, then his mass is  770.2 kg.