Experiments at Cornell University found that an experienced mechanic could break a ______ bolt due to nut tightening:

The selection of the 5/8 inch bolt as the correct answer stems from empirical observations made during the experiments conducted at Cornell University. The research demonstrated that experienced mechanics, when applying their skills in nut tightening, can exert significant force that can lead to the failure of bolts.

A 5/8 inch bolt is relatively large and has substantial tensile strength compared to smaller sizes. Mechanics often use their judgment to apply the right amount of torque, but over-tightening can occur, especially with larger bolts, leading to failure. The specific findings reflect the point at which mechanics have typically been found to break bolts, indicating a point of failure characterized by material properties and the ideal handling techniques of larger bolt sizes.

In contrast, smaller bolts such as a 3/8 inch and a 7/16 inch would generally fail at lower torque levels due to their smaller cross-sectional area. They lack the strength attributes found in the larger 5/8 inch bolts, thereby making them less likely to be broken by tightening under typical mechanical practices. Similarly, a 1/2 inch bolt, while not as robust as a 5/8 inch, would also demonstrate a breaking point at less force than a 5/8 inch, reinforcing why that size is