Physics 11–12 · Year 11

Newton's second law: constant net force on a cart with hanging masses

Module 2: Dynamics (Forces, Acceleration and Energy)

Practical, model not builtLow risk

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The idea

A fixed hanging mass applies a constant net force to a cart-and-mass system, and the measured acceleration is proportional to that force and inversely proportional to the total mass.

What you need

Dynamics cart (measure its mass, about 0.500 kg) on a level 1.2 m track
Pulley clamped at the end of the track, light string, a 50 g hanger and six slotted 50 g masses (0.350 kg in all)
Two light gates and timer, or a motion sensor, or ticker timer
Electronic balance, metre rule

How to do it

Level the track so the cart does not drift; tilt it very slightly until a gentle push gives constant speed (this compensates friction).
Place 0.300 kg on the cart and hang 0.050 kg over the pulley; release from rest and measure the acceleration from the light-gate speeds over a 0.500 m separation, a = (v2^2 - v1^2)/(2 s).
Keep the total mass constant by moving 0.050 kg at a time from the cart to the hanger; record a for hanging masses 0.050 to 0.250 kg.
Plot a against the net force m g; the gradient should be 1 over the total mass.
Second series: keep the hanging mass at 0.100 kg and add masses to the cart; plot a against 1 over total mass.

What you should see

With cart 0.500 kg and hanging 0.100 kg the acceleration is 0.100 x 9.806 65 / 0.600 = 1.634 m/s^2 and the string tension is 0.817 N (not the hanging weight of 0.981 N). In the first series the cart, its load and the hanger always total 0.850 kg, so the a against F line passes through the origin with gradient 1/0.850 = 1.18 per kg, and a 0.100 kg hanger gives 1.154 m/s^2; a measured gradient within about 10 per cent of this, with an intercept near zero, shows friction was compensated.

What changes

What you change: hanging mass (net force) or total mass
What you measure: acceleration
What you keep the same: total mass constant in series one
same track, pulley and string
release from rest

Common misconceptions

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

The force on the cart equals the weight of the hanging mass; the tension is less because the hanging mass is also accelerating.
Doubling the hanging mass doubles the acceleration; it also increases the total mass being accelerated.

Safety card

Low riskLearners carry it out

Hazards

falling masses on feet
cart striking the pulley

Controls

stop the cart with a padded stop before the pulley
keep feet clear beneath the hanger

Note

Record the activity in RiskAssess (https://www.riskassess.com.au/) and follow the Science ASSIST risk management information sheet (https://asta.edu.au/resource/ais-risk-management-and-risk-assessment/).

Curriculum references

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

Physics Stage 6 Syllabus (2017), current: Year 11 until the end of 2026, Year 12 until Term 3 2027PH11-9 PH11/12-2 PH11/12-4
Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-11-01 PY-11WS-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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