Friction — Science definition

A force that opposes motion between two surfaces in contact.

Where it is taught

In use

Friction resists relative sliding, or the tendency to slide, between contacting surfaces.

Gravity & Friction

Questions using friction

Problem 1

Which surface pairing would create the MOST friction?
  1. ice on ice
  2. rubber on dry pavement
  3. oiled metal on metal
  4. polished wood on polished wood
Answer and explanation

rubber on dry pavement

Rough, grippy surfaces like rubber on dry pavement create much more friction than smooth or lubricated ones.

Problem 2

A box slides right across a rough floor. State the direction of kinetic friction from the floor. Show the reasoning, not just the conclusion.
Hints

The book's weight is still present even though it is not falling.

Answer and explanation

Left, opposing the box's sliding relative to the floor.

Problem 3

An illustrative 2 kg object has weight 19.6 N using Earth g=9.8 N/kg and 3.2 N using a modeled Moon g=1.6 N/kg. Both trials use the same object. Explain the mass and weight comparison, then identify a friction test that would isolate surface type.
Answer and explanation

Mass remains 2 kg, while gravitational force depends on the local gravitational field. Weight equals mass × g, giving the supplied forces. To compare surface friction, match object load, contact procedure, travel direction and measurement, changing only the intended surface. Weight and friction are distinct forces, and air resistance can add another interaction.

Mass remains 2 kg, while gravitational force depends on the local gravitational field. Weight equals mass × g, giving the supplied forces. To compare surface friction, match object load, contact procedure, travel direction and measurement, changing only the intended surface. Weight and friction are distinct forces, and air resistance can add another interaction.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 4

Why must steady speed be stated when using a pull to estimate friction?
Answer and explanation

At steady straight-line speed the net force is zero, so the pull balances resistance in the stated model. During acceleration the pull need not equal friction.

At steady straight-line speed the net force is zero, so the pull balances resistance in the stated model. During acceleration the pull need not equal friction.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

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