← Senior Mathematics Studio

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If |u|=4, what is |−2u|?

Read the idea, work independently, then explain what changed.

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高一必修第二册(A版).pdf · 6.1 · PDF 9 / printed page 2

Teacher preparation and assessment

40-minute lesson

Minutes	Focus	Teaching and student action
5	Prediction and diagnosis	Compare two translated arrows and vote on equality.
8	Concept derivation	Derive head-to-tail addition and negative scaling using a closed route.
10	Guided application	Work the translated-arrow example; pairs explain the length counterexample.
12	Transfer and explanation	Solve the constrained river-crossing task and discuss when it fails.
5	Exit evidence	Write and check the exit statement about displacement.

Question sequence

If I slide this arrow to another desk, what changes?
Can travelling farther leave you closer to your start?

Board development

Draw east 3, north 4 and the single diagonal; label distance 7 and displacement 5.
Display boat + current = ground velocity, then impose the zero horizontal component.

Expected correct and incorrect responses

A student gives −6 as a scaled length. Ask them to locate a negative distance on a ruler.
Correct: “The length is 6; the negative multiplier reverses the arrow.”
Incorrect: “The length is −6 because the multiplier is negative.”

Follow-up questions

If the current is faster than the boat, can a due-north path still exist? Explain with a squared component.

Remediation

Use two opposite paper arrows, then separately measure the two lengths and the zero resultant.

Exit check

State one quantity preserved by translating an arrow and one route property it does not encode.

Assessment checklist and sample responses

Credit the chosen orientation, signed components and a nonnegative magnitude separately. Sample: (−3,4)+(3,0)=(0,4), so crossing time is 20/4.
Correct: “The length is 6; the negative multiplier reverses the arrow.”