At the top:
Potential Energy = (mass) x (gravity) x (height)
= (30 kg) x (9.8 m/s²) x (3 meters)
= 882 joules
At the bottom:
Kinetic Energy = (1/2) x (mass) x (speed)²
= (1/2) x (30 kg) x (3 m/s)²
= (15 kg) x (9 m²/s²)
= 135 joules .
He had 882 joules of potential energy at the top,
but only 135 joules of kinetic energy at the bottom.
Friction stole (882 - 135) = 747 joules of his energy while he slid down.
The seat of his jeans must be pretty warm.