# Kinetic to Static Friction

#### steel_cow

Are there any fans of truck or tractor pulls out there? I am trying to understand what is happening to the coefficient of friction as the sled transitions from kinetic friction back to static friction.

It is a fairly complicated model...

The weight imposed on the front plate of the drag is increasing at a constant rate as the sled and tractor move down the track. As the load on the sled increases, and is transfered to the tractor, there is a resultant drop in speed. Eventually, the entire system comes to a hault.

What does the transition from Kinetic friction back to Static friction look like?

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#### 256bits

Gold Member
The weight imposed on the front plate of the drag is increasing at a constant rate as the sled and tractor move down the track.
The track that moves the weight forward from the rear of the sled to the front is on a track geared directly to the rear wheels of the sled. The weight will move forward in direct proportion to the forward speed of the sled. If the sled forward movement did not slow down as the pull prgresses, then yes that would be a constant rate forward or constant velocity forward of the weight. If the weight had a mass M, then if the sled was being pulled at constant velocity, then theoretically you could plot on a graph the normal force of the sled on the ground as a function of time or distance travelled as a straight line plot from 0 to Mg or even greater than Mg if there is some kind of overhang.

The sled does speed up and does slow down though so there is nothing about a constant rate.

You would have to know the velocity profile of the sled as the tractor is pulling it. Perhaps studying some of the videos you could get a grasp how it does paly out. I would expect there is an initial acceleration, a plateau of something similar to a constant velocity and a deceleration to a complete stop, with a lot of black exhaust smoke and dust.

Assuming the coefficient of kinetic fricion is constant, the sled would keep moving forward until the kinetic friction matches the drawbar of the tractor and the tractor wheels start spinning on the earth and the the thing quickly comes to a halt. At that point the sled being stopped would require a larger force to move irt to overcome static friction.

Hopefully that gets you started.

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