Apile of rocks stays together because of which type of friction

Gordon is going for a run through the park, but it is cold outside. the low outside temperature could affect his personal safety.

To practice problem-solving strategy 10.1 for energy conservation problems. a sled is being held at rest on a slope that makes an angle θ with the horizontal. after the sled is released, it slides a distance d1 down the slope and then covers the distance d2 along the horizontal terrain before stopping. find the coefficient of kinetic friction μk between the sled and the ground, assuming that it is constant throughout the trip.

What is the quantity which is measured by the area occupied below the velocity- time graph?