Im too young for physics, sorry!
<u>Answer:</u>
2.39 kg
<u>Explanation:</u>
There is conservation of momentum here in this problem so we will use the following problem:
where the mass of the student 
is 48.5 kg,
the mass of the skateboard 
is kg,
the initial speed of the student 
is 4.25 m/s; and
the speed of the student and skateboard 
is 4.05 m/s.
So substituting the given values in the above formula to get:
Therefore, the mass of the skateboard is 2.39 kg.
Potential energy = m · g · h
-- When you held the ball at 2.0 meters above the floor, it had
(0.5 kg) · (9.8 m/s²) · (2.0 m) = 9.8 Joules of potential energy.
-- After it bounced and went back up as high as it could, it was only 1.8 meters above the floor. Its potential energy was
(0.5 kg) · (9.8 m/s²) · (1.8 m) = 8.82 Joules
-- Between the drop and the top of the bounce, it lost
(9.8 - 8.82) = <em>0.98 Joule</em> .
-- The energy was lost when the ball hit the floor. During the hit, 0.98 joule of kinetic energy turned to <em>thermal energy</em>, which slightly heated the ball and the floor.
Rotational kinetic energy <span>is the kinetic energy of an object, proportional to the object's moment of inertia and the square of its angular velocity.</span>
