500
Imagine a very large Newton's cradle made of five bowling balls. Now imagine that the bowling ball on the end has a mass of 4 kg and is swung back to a height of 2 m above the other four balls. If the system doesn’t lose any energy to heat, light, sound, etc., how much kinetic energy will the bowling ball have as it strikes the other four?
The bowling ball will have ~80 J of kinetic energy.
If energy is conserved (none lost because of friction), the ball should have the same amount of kinetic energy at the bottom of its motion as the amount of potential energy it had to begin with. That potential energy is PE=mgh=4kg*~10m/s/s*2m=80J.