# A rod is moving in space and an insect is on it. How many degrees of freedom does the insect have?

Is the answer 7? The number of degrees of freedom of a system can be viewed as the minimum number of coordinates required to specify a configuration. Applying this definition, we have:

• For a single particle in a plane two coordinates define its location so it has two degrees of freedom.
• A single particle in space requires three coordinates so it has three degrees of freedom.
• Two particles in space have a combined six degrees of freedom.
• If two particles in space are constrained to maintain a constant distance from each other, such as in the case of a diatomic molecule, then the six coordinates must satisfy a single constraint equation defined by the distance formula. This reduces the degrees of freedom of the system to five, because the distance formula can be used to solve for the remaining coordinate once the other five are specified.
• Where did you get 7? – Mike Apr 6 '17 at 14:15
• Can the insect move along the rod? Is there only one parameter specifying the location of the bug relative to the rod? – ja72 Apr 6 '17 at 14:34
• Obviously, 3 coordinates locate a point in space, but are additional specifications required to indicate rotation (or lack thereof)? – David White Aug 7 '17 at 17:01
• Can the insect walk around the rod? Does the rod have ends, or is it an infinite line? – ja72 Aug 7 '17 at 20:11