Our Komatsu trucks (4,000Lb & 5,000Lb cap) have a decal on the overhead guard stating that the FOPS has been tested to withstand an "impact of 16,000 foot pounds".
In interpreting that statement, the simple explanation is something like "a 16,000 pound object dropped from a height of one foot".
The extrapolations from that point are infinite and my own examples are certainly not going to be mathematically accurate, but will serve to show how a "16,000 foot pound impact" can be generated with less than 16,000 pounds of mass.
16,000 Lb dropped from height of 1 foot = 16,000 foot pound
8,000 Lb dropped from height of 2 feet = 16,000 foot pound
4,000 LB dropped from height of 4 feet = 16,000 foot pound
2,000 Lb dropped from height of 8 feet = 16,000 foot pound
1,000 Lb dropped from height of 16 feet = 16,000 foot pound