As the programs were executed directly by the computer, there was no physical mechanism in place to prevent cheating.

The focus of research during the last century has been to understand the physical mechanisms behind friction.

The physical mechanisms involved in forming these canyons were not well understood until the 1960s.

This paper goes the furthest in terms of physical mechanism, and that's an important step.

The physical mechanisms behind these echoes are still (as of August 2008) a mystery.

Increasingly, scientists are trying to pin down the physical mechanisms that might be at work.

There's no other physical mechanism out there that can give the same effect.

The physical mechanism in this interaction need not be specified.

Generally this position requires new physical mechanisms to explain the observed warming.

Medical scientists are already looking for possible physical mechanisms that could link people's emotional states to their state of health.