A packing box slides across the floor while Imre applies a steady force to it.
His friend sets a second identical box on top. The same slide now dies almost at once.
The floor has not changed, and the surfaces are the same. What changed?
Friction's size is set partly by the normal force pressing the surfaces together. Change that force, and the friction changes with it, in proportion.
Give the boxes numbers. A sliding box feels 40 N of kinetic friction from the floor while the floor's normal force on it is 100 N.
Stack a second identical box on top. The floor now supports twice the weight, so the normal force doubles to 200 N.
The kinetic friction doubles with it, to 80 N.
Here is the rule: increasing the normal force pressing the surfaces together increases the friction force in proportion.
Double the normal force, double the friction. Triple it, triple the friction.
A sliding crate feels 28 N of kinetic friction. A load is added so the normal force on it doubles. Calculate the new kinetic friction.
Write down the values in the question:
old friction = 28 N
the normal force doubles
new friction = ?
Write down the rule:
friction changes in proportion to the normal force
Substitute in the values:
new friction = 28 × 2
Simplify:
new friction = 56 N
The new kinetic friction is 56 N.
A sliding storage bin feels 36 N of kinetic friction from the floor. An identical bin is stacked on top of it, doubling the normal force. Enter the new kinetic friction in N.
The link runs both ways. Take weight off the slider and the normal force drops, and the friction drops in the same proportion.
The same crate is instead unloaded until the normal force on it is half its first value. Calculate the kinetic friction then.
Write down the values in the question:
old friction = 28 N
the normal force halves
new friction = ?
Write down the rule:
friction changes in proportion to the normal force
Substitute in the values:
new friction = 28 ÷ 2
Simplify:
new friction = 14 N
The kinetic friction falls to 14 N.
A sliding serving cart feels 8.0 N of kinetic friction while loaded. Unloading it halves the normal force on it. Enter the new kinetic friction in N.
Nothing else about the pair changed: same box bottom, same floor. Only the press between the surfaces grew, and the friction grew with it.
Tilda says: 'If the normal force on a sliding box triples, the kinetic friction on it triples too.' Is she correct?
A sliding pallet feels 24 N of kinetic friction while the normal force on it is 80 N. Loading it raises the normal force to 200 N. Enter the new kinetic friction in N.
The normal force on a slider is raised to four times its old value. What happens to the kinetic friction on it?
A sliding crate feels 15 N of kinetic friction on its own. Two identical crates are stacked onto it, making the normal force three times as large. Enter the new kinetic friction in N.