A crate skids quickly across a floor. Later, the same crate skids across it slowly.
Most people expect the fast skid to meet more friction. Measured, the two friction forces come out about the same.
Kinetic friction has exactly two inputs: μ_k and the normal force. Speed and contact area are not on the list.
Here is the first measurement. A box skids across a floor at a walking pace, and the kinetic friction on it is measured. The same box skids again at a running pace, and the friction is measured again.
The two readings come out about the same. The kinetic friction force is roughly independent of the sliding speed.
Tilda says: 'A sled gliding fast and the same sled gliding slowly feel about the same kinetic friction from the snow.' Is she correct?
Here is the second measurement. A hardcover book slides on its wide front cover, and then on its narrow spine, pressed by the same normal force.
Again the friction comes out about the same. The kinetic friction force is roughly independent of the contact area too.
Why does the area drop out? A wide face spreads the same normal force over more area, so the surfaces press together less strongly at each spot, and the two changes cancel.
Imre says: 'Sliding a crate on its larger face roughly doubles the kinetic friction on it.' Is he correct?
So what does set the kinetic friction? Only μ_k for the pair and the normal force on the object, through f_k = μ_k N.
Which two quantities set the kinetic friction on a slider?
'Roughly' is honest. At extreme speeds, or between soft surfaces, the friction can drift. In this topic, f_k is set by μ_k and N alone.
A hockey puck crosses the ice at 8 m/s, and later at 2 m/s. Compare the kinetic friction on it in the two crossings.
Kinetic friction's inputs are μ_k and the normal force.
Both crossings have the same pair of surfaces and the same normal force.
Speed is not an input.
The two friction forces are about the same.
A wooden plank is slid across a patio on its wide face, then on its narrow edge, with the same normal force. Compare the two kinetic friction forces.
Same pair of materials: same μ_k. Same normal force: same N.
Contact area is not an input to f_k = μ_k N.
About the same on the face and on the edge.
Which change raises the kinetic friction on a sliding crate?
Which statement about kinetic friction is correct?
A moving box slows as it skids across a floor. As its speed falls from 3 m/s to 1 m/s, what happens to the kinetic friction on it?