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Science / Forces / Grade 7 / sf7f3

Static Friction: Balance a Leftward Push

Consider separate invented trials of a crate initially at rest on a stationary level floor. Only the push and friction act horizontally. Use paper models, not furniture.

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5 questions0m 0s
Learn the skill

First check that the crate starts at rest. Static friction can balance a push up to the stated maximum, including equality in this ideal model. It points opposite the attempted slide. A leftward push needs rightward balancing friction, not an automatic leftward force.

Worked example

A different crate has a static limit of 16 N. A 10 N push left is balanced by 10 N of friction right, so the net horizontal force is 0 N. The unused 6 N of possible friction is not another force.

A resting crate has a ten-newton push left and ten newtons of friction right. The static limit is sixteen newtons.
Separate worked example: equal horizontal forces balance. The questions use an 18 N static limit.
Question 1 A resting crate has static limit 18 N and push 7 N left. What is its friction?
Question 2 A new resting trial has static limit 18 N but no horizontal push or other horizontal force. What is the horizontal friction?
Question 3 A separate trial begins at rest with exactly 18 N pushing left and static limit 18 N. What does the ideal model allow?
Question 4 A trial starts at rest with push 21 N left and static limit 18 N. Which conclusion is supported without a sliding-friction value?
Question 5 A crate stays at rest with push 12 N left and static limit 18 N. What is its net horizontal force?

Further inquiry

Use a new static limit of 20 N. Draw separate leftward-push trials below, at and above that limit. Identify the balancing force where rest is possible and explain what is unknown once sliding begins.

Review criteria

  • State that each trial starts at rest and only two horizontal forces act.
  • Use equal rightward friction below or at the limit, not always 20 N.
  • Above the limit, do not invent a sliding-friction value.