1. The problem statement, all variables and given/known data Imagine a cyclist (in spacesuit) cycling on the level on the Moon (so no drag). The external forces acting on them are contact force upwards (from Moon) and weight downwards (from Moon), which balance. Plus an unbalanced friction force forwards (from Moon on the tyres). I can't think of any other external horizontal forces which could balance this frictional force. How can he ever travel at constant speed on a level surface on the Moon? 2. Relevant equations F=ma An unbalanced or resultant force will cause an acceleration. Once the bike is traveling no force is necessary to keep it moving. 3. The attempt at a solution While the bike is in motion there must be a frictional force exerted on the road by the tyre.