Science 7–10 · Year 10

Newton's third law: balloon rocket, paired newton meters and trolleys pushing apart

Physical sciences — Waves and motion: motion and Newton's laws (NSW Stage 5 focus area)

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Forces come in pairs: when one object pushes or pulls on another, the second pushes or pulls back on the first with an equal force in the opposite direction.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

falling or tipping from a chair on castors
fishing line at eye height
trolleys off the bench

Controls

the chair extension seated only, on a clear level floor, with a spotter behind each chair and hands pushing hands, never the body
line tied above head height or clearly flagged
catch trolleys at the bench ends

Note

No hazardous chemicals or naked flames are used. Complete the school's risk assessment for the activity before the lesson; the NSW Department of Education Science safety and compliance page points to CSIS 1.7 (Risk assessment – a pre-requisite for risk control) for how to carry it out. Skateboards are not used for the extension: the NSW Department of Education School Sport Unit guideline for skateboarding requires a hard-shell helmet of a Standards Australia approved design, knee, elbow and wrist protection for inexperienced participants, a teacher with expertise or training in teaching skateboarding, a teacher holding CPR accreditation HLTAID009 and a teacher to student ratio no greater than 1:20, which a classroom third-law demonstration is not set up for.

What you need

long balloon, drinking straw, 5 m of fishing line and sticky tape, 1 set
0 to 10 N newton meters, 2
dynamics trolleys of equal mass, one with a spring-loaded plunger, 2 (plus a 1 kg bar to load one)
metre rule and stopwatch, 1 each
office chairs on castors, on a level hard floor, 2 (for the extension)

How to do it

Before each part, predict what will happen and write down the reason.
Thread the line through the straw and tie it taut across the room. Inflate the balloon, hold the neck closed, tape it to the straw, and release. Observe the balloon's direction and the direction the air escapes.
Hook two newton meters together. Two learners pull gently in opposite directions; each reads their meter. Then one learner pulls harder while the other only holds on; read both again.
Place the two trolleys touching, compress the plunger of one, and release it against the other. Measure how far each travels before stopping. Repeat with the 1 kg bar on one trolley.
Extension: two learners sit on the office chairs facing each other with their feet off the floor and a spotter standing behind each chair; one pushes the other's hands while the other keeps their arms still. Observe both.
For each case name the two objects, state the pair of forces and their directions, and explain why the motions can differ while the forces are equal.

What you should see

The balloon moves along the line away from its open neck, in the opposite direction to the escaping air. Hooked newton meters show equal readings, within their reading precision, however the two learners pull. Equal trolleys pushed apart travel about equal distances. The push on each trolley is the same, so with the 1 kg bar added the loaded trolley moves off more slowly: if each trolley is 1.0 kg, the loaded one has twice the mass and moves off at half the speed, and if rolling friction slows both at the same rate it travels about a quarter as far. On the office chairs both learners roll apart, the lighter one faster. The learner knows it worked when the two meters agree even when only one person is pulling.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The larger or stronger object exerts the larger force.
Action and reaction forces cancel so nothing can move.
The balloon moves because the air pushes on the room's air.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC5-WAM-02 SC5-WS-02 SC5-WS-06 SC5-WS-08
Australian Curriculum v9AC9S10U05AC9S10I01AC9S10I07AC9S10I08

Sources

The pages the author read to write this activity.

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