Gravel Running: Friction & Velocity Questions

[danielandpenn]

Okay, so I was running the other day and thinking about friction as I hit a patch of gravel. I'm a new physics student so I know enough to think about some things but not enough to know the answers yet. Why is it that if friction works to slow down my velocity, then when there is less friction (ie...I hit the gravel as opposed to the dry pavement) why don't I go faster? I can't go faster on loose gravel...just trying to make a connection here. Thanks for your input!

 

[Stonebridge]

Well, if I was cycling on a hard pavement and then went on to gravel, I don't think it would make my progress easier. The opposite, I would imagine. More opposition to my motion: slower speed. This is because the wheels dig into the gravel and have to move gravel stones out of the way in order to roll forward.
On the other hand, when trying to run on a surface, you need plenty of grip between your shoes and that surface. The gravel is not so helpful there either and would make it more difficult to run. This is because the gravel stones tend to slip and slide over each other.

 

[DThielke]

Friction is required in transferring energy from your foot into your forward motion. When running on a surface with low enough friction, you begin to waste energy in this transfer because your foot spends some time sliding rather than staying planted. In this sense, a certain amount of friction is required to run, just as it is required for an object to roll down a hill. This is different from the role friction plays when sliding an object, which is the way friction is treated in most intro mechanics courses. In this case, friction gets in the way of sliding and slows down the object.

 

[Astronuc]

Friction is required to achieve 'traction', otherwise one's foot or bicycle's (motive) wheel, would slide or slip.

Similarly, in braking, one would slide, or otherwise have a reduced magntide of deceleration without friction.

Friction is a shear force, i.e. it acts parallel to a surface, rather than a normal force acting normally or perpendicularly to the surface. Pushing (normally) against an object does not require friction to work, but applying a shear force requires 'friction'.

The friction between a foot or wheel and the ground/surface is static, as opposed to sliding/dynamic friction.

In running or riding on gravel, the force from the foot or wheel would go into moving the gravel, either translationally or rotationally. There is friction between the foot/wheel and the gravel.

 

[jack action]

Why is it that if friction works to slow down my velocity, then when there is less friction (ie...I hit the gravel as opposed to the dry pavement) why don't I go faster? I can't go faster on loose gravel

You are mixing the traction force with the resistance force. They are both the same but opposite. If you want to push a big rock, it is a lot easier on ice than on asphalt, because the friction force is less on ice than asphalt and its putting less resistance. But when you push the rock with your feet on ice, you also have less friction than on asphalt, so you also produce less tractive force.

When you rely on traction (hence friction) to initiate a motion, the higher the friction force, the the faster you can go. But if you rely on another form of force to initiate a motion (say the wind pushing a sail) the friction becomes an resisting force and the lower the friction force, the faster you can go.

 

[rcgldr]

Rolling resistance is related to the amount of deformation (deformation with hysteresis so that energy is consumed by the deformation and restoring of the tread orientation on a tire) and/or displacement involved, a separate issue than friction. When driving through gravel, you displace a lot of rocks, so it takes more energy. I've done this on motorcycle, on a gravel bed next to train tracks, and with sufficient speed you start to plane, similar to water or sand, reducing the rolling resistance, and making the bike more stable (it's a bit wobbly till you get up to speed).