Giant Mallet Conundrum: Calculate Length for 800J Impact

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I'm currently building a roughly 5m tall mallet for testing the strength of various pipes against impact force. I've done all my math and with a 4m long pipe and a weighted head of about 20.3Kg I can achieve 800J on impact (test requirement), I haven't taken into account the weight of the pipe used as the shaft.

I'm wondering if anybody here can help me figure out how long I need to make my pipe for the head to be 20Kg and hit the test pipe with 800J. The specs for the pipe are as follows:

Outer diameter: 60mm
Internal diameter: 34mm
Length: 4000mm
Mass: 60.273Kg
Material: Stainless Steel

A quickly put together inventor drawing of the assembly in attachments.
 

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Weight

Actually keep the 4m but how heavy does the head need to be to achieve 800J.

cheers
 
It will depend on the path taken by the mallet head.
Does it start at the top and swing through 90 or through 180 degrees?

Ignoring the handle, the kinetic energy available will be the potential energy due to the change in height of the head. But a 60kg rigid tube appears to be a dominant component.

I would be concerned about the health of the tubular handle as it will be subjected to some bending at the time of impact. There will be a wave travel from the head to the fulcrum and reflect back when it hits. Ideally it would be arranged for the handle to be insignificant since energy stored in the handle will take time to reach the head and you need only one clear pulse.