Final answer:
A forklift's lifting capacity decreases when the boom is fully extended due to increased torque, which could lead to tipping over or mechanical overloading.
Step-by-step explanation:
When using a boom attachment, a forklift's lifting capacity decreases when the boom is fully extended. This happens due to the principles of physics related to levers and moments: extending the boom increases the distance of the load from the fulcrum (which, in this case, is the forklift), thus increasing the moment (torque) exerted by the load. To prevent tipping over or mechanical overloading, the forklift's rated capacity is reduced as the boom is extended.
On the other hand, when a load is distributed evenly or the attachment is securely fastened, and the forklift is stationary, these conditions do not inherently reduce the lifting capacity. On the contrary, secure fastening and even load distribution are necessary to achieve optimal performance and safety.