SubjectNest · Year 3 maths · Make it fit, then check
Day 23 · Mass without the mystery container
Which reading belongs to the thing inside?
Usual communication and wanted help are welcome. Choose one route; rest or finish when useful.
Full session and all three routes · PDF · Full readable session · HTML
Grown-up: full preparation, teaching and three choices
Day 23 · Mass without the mystery container
Which reading belongs to the thing inside?
Useful purpose: Read a marked mass scale and separate an object's mass from a container plus object reading.
1 · Find the starting point
Read 100, 150 and 200; revisit g for mass. Find an unknown part of 90 + something = 310 using earlier additive work.
2 · Prepare a suitable invitation
Adult checks a stable familiar scale, its g setting, readable increments, zero and capacity. Offer safe familiar sealed or dry non-food objects; avoid bodily weighing, dietary comments, powders, medicines, allergens and heavy lifts. No scale? Use the labelled model.
3 · Brief worked model
The MODEL scale has marks every 50 g. An arrow between 100 and 200 at the middle mark reads 150 g. An empty tray is 90 g and tray plus counters is 310 g, so the counters are 310 − 90 = 220 g; 90 + 220 checks 310. If I zero a real scale with the tray already on it, I must record that different procedure.
4 · Guided new try
Together read two labelled increments on a supplied or checked scale. Explain what the zero/tare control changes and what it does not change. Adult operates controls if needed; the child chooses what to compare.
5 · Own attempt
Choose one route. Say whether the reading is total or net, include g and check one result with an addition. Do not claim that a model mass was physically weighed.
6 · Observe and respond
Record interpretation of increments, zero/tare state and part-whole relationship. If an adult positions an object, observe the child's reading and reasoning separately from handling.
Specific next teaching
If the tray is counted as contents, draw a new whole with two parts, 60 g and an unknown making 240 g. If subtraction is secure, compare two safe real readings taken with the same zero procedure.
Pick a route
1 · Zero detective
What changed when we zeroed it?
With adult-reviewed kit, observe an empty safe tray and then a chosen object. State whether zero was set with or without the tray; compare readings using the same procedure. Directing the adult is fine.
Observe: Clear zero state and unit; no body mass requested.
Next teaching: Repeat with a new familiar safe object and ask why a different tray matters.
Evidence boundary: Actual reading/directing if observed; help recorded.
2 · Packing tray puzzle
Find the contents, not the tray.
MODEL: empty tray 60 g; tray plus paper pieces 240 g. Find contents 180 g and check 60 + 180 = 240. Explain which number includes the tray.
Observe: Part-whole reason, 180 g and correct inverse check.
Next teaching: Use a new drawing with different values before another check.
Evidence boundary: Supplied mass calculation, not weighing.
3 · Scale repairer
Which arrow reading makes sense?
A MODEL scale labels 0, 100, 200 and 300 g with a middle 50 g mark in each gap. Arrow sits at 200 g. A label says 20 g. Repair it and explain the marked interval.
Observe: 200 g, with 50 g gaps understood.
Next teaching: Read a new pointer between labels and let the child request a clearer description.
Evidence boundary: Marked model reading only.