# Cycling on the Moon, no air resistance to cause terminal velocity.

1. Jul 28, 2013

### mrcotton

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.

2. Jul 28, 2013

### voko

The wheels of the bicycle will experience rolling resistance, just like they do on the Earth.

Note the terminology: rolling resistance, not friction.

3. Jul 28, 2013

### mrcotton

So is this rolling resistance providing a force in the opposite direction of the motion?

4. Jul 28, 2013

### voko

Yes, it always does, as the term "resistance" would suggest.

5. Jul 28, 2013

### haruspex

Not exactly. It provides a torque opposing the rotation of the wheel.
In reality, the contact of wheel with road is not a single point. The vertical force is spread across a short distance fore and aft of the nearest point of road to axle. Because of losses in the compression/decompression of the surfaces, the force is greater ahead of the midpoint than behind it. That creates a torque.