Science 7–10 · Year 7
Balanced and unbalanced forces: two newton meters pulling a trolley
Physical sciences — Forces (NSW Stage 4 focus area)
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The idea
When two opposing pulls on an object are equal it stays at rest or keeps its motion, and when they differ the object accelerates in the direction of the larger pull.
What you need
- dynamics trolley (about 1 kg), 1
- 1 kg mass to load the trolley, 1
- 0 to 10 N newton meters, 2
- string loops, 2
- bench 1.5 m long, clear
- masking tape and marker to mark start position
How to do it
- Hook one newton meter to each end of the trolley with string loops and place the trolley on the tape mark.
- Two learners pull gently in opposite directions until both meters read 2.0 N. Observe the trolley: it does not move. Draw the force diagram with two equal arrows.
- Increase one pull to 3.0 N while the other stays at 2.0 N. Observe the direction the trolley moves and how quickly. Record the net force (1.0 N) and its direction.
- Repeat with 4.0 N against 2.0 N and describe the difference in how quickly the trolley speeds up.
- Load the trolley with 1 kg and repeat the 3.0 N against 2.0 N trial; compare the motion with the unloaded trial.
- For each trial draw a labelled force diagram, state whether the forces are balanced, and give the net force in newtons with its direction.
What you should see
Equal pulls leave the trolley at rest. A 1.0 N net force on a 1.0 kg trolley produces a gentle acceleration of about 1.0 m/s^2 (ignoring the small rolling friction); 2.0 N net roughly doubles it, and adding 1 kg to the trolley roughly halves it. The learner knows it worked when motion appears only when the readings differ and is always toward the larger reading.
What changes
- What you change
- difference between the two pulls (N)
- What you measure
- direction and rate of change of the trolley's motion
- What you keep the same
- same trolley and surface
- meters held horizontal and in line
- pulls applied steadily
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- A stationary object has no forces acting on it.
- The larger of two forces cancels the smaller one completely.
- A constant net force makes an object move at constant speed.
Safety card
Hazards
- trolley rolling off the bench
- sudden release of a stretched meter
Controls
- a third learner stops the trolley at the end of the bench
- release both pulls together
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.
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)SC4-FOR-01SC4-WS-01SC4-WS-05
- Australian Curriculum v9AC9S7U04AC9S7I04
Sources
The pages the author read to write this activity.