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Science / Forces / Grade 7 / sf7f1

Static Friction: A Limit, Not a Fixed Force

Use separate paper-model trials of a box initially at rest on a stationary level floor. Only a horizontal push and friction act horizontally.

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Watch the lesson | Next: compare a box that is already sliding

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Static friction can balance the push up to its stated maximum. Below or exactly at that maximum, rest is possible in this ideal model. Above it, static friction cannot keep this box at rest. A maximum is not the actual force in every trial.

Worked example

With a 10 N static limit and a 6 N push right, friction is 6 N left. Net horizontal force is 0 N, not 4 N left. With no horizontal push, no horizontal friction is needed here.

A resting box has a six-newton push right and six newtons of friction left. The static limit is ten newtons.
Horizontal forces only: the actual friction balances 6 N, even though its maximum is 10 N.
Question 1 A separate resting box has static limit 14 N and push 4 N right. What is its friction?
Question 2 The same kind of resting box has static limit 14 N and no horizontal push or other horizontal force. What is its horizontal friction?
Question 3 A new trial starts at rest with a push of exactly 14 N right and static limit 14 N. Which statement fits the ideal model?
Question 4 A trial starts at rest with push 17 N right and static limit 14 N. Can static friction keep this box at rest?
Question 5 A box stays at rest with an 11 N rightward push and static limit 14 N. What is its net horizontal force?

Further inquiry

Make three separate resting trials with a new static limit of 16 N. Choose pushes below, exactly at, and above the limit. Explain what can and cannot be concluded without a sliding-friction value. Do not try this with furniture.

Review criteria

  • State that each trial starts at rest.
  • Compare each push with 16 N; distinguish the actual balancing force from its limit.
  • Above the limit, do not invent a sliding-friction value or add both types.