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PRACTICE / Static friction

Does friction always equal μN?

A 3 kg equipment case rests on a horizontal floor. A horizontal cable pulls it to the right with a steady 7 N force. The coefficient of static friction is 0.4. Take g = 10 m/s².

What friction force acts on the case?

Organize the givens & assumptions
  • Mass: 3 kg
  • Applied pull: 7 N right
  • Static coefficient: 0.4

The case starts at rest; the floor and case do not move relative to each other unless the static limit is exceeded. The cable has no vertical component.

  1. A0 N0\ \mathrm{N}
  2. B7 N left7\ \mathrm{N}\ \text{left}
  3. C12 N left12\ \mathrm{N}\ \text{left}
  4. D7 N right7\ \mathrm{N}\ \text{right}

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