Australian Curriculum v9 / ACiQ Year 7 Science - Unit 9 - Multiple forces and changes in motion

Multiple forces and changes in motion

Represent multiple contact and non-contact forces and relate balanced or unbalanced net force to changes in motion.

Updated 2026-07-24 - 8 min read

Represent multiple contact and non-contact forces and relate balanced or unbalanced net force to changes in motion. This note connects the core scientific model to a worked example, an inquiry design and the limits of the evidence.

Core model

Identify every interaction, draw labelled arrows, compare opposing magnitudes, determine net force, then explain the change in velocity. Balanced describes net force, not whether an object is moving.

Accessible diagram description: A trolley is described with an upward support arrow and equal downward gravity arrow. Horizontally, a long rightward pull arrow and shorter leftward friction arrow produce a rightward net force. Arrow length indicates relative magnitude.

A model is useful because it highlights relationships that help explain or predict evidence. It is not a perfect copy of reality. Always state what the representation includes, what its arrows or symbols mean, and one relevant limitation.

Authored representation workshop

Forces on a pulled trolley

Accessibility description: The trolley has equal upward support and downward gravitational forces. A 6-newton pull points right and a 2-newton friction force points left, producing a 4-newton net force to the right. Arrow direction shows force direction and relative length shows magnitude.

Represented parts

  • Trolley
  • Support force 5 N upward
  • Gravitational force 5 N downward
  • Pull 6 N right
  • Friction 2 N left

Represented relationships

  • Trolley -> Support force 5 N upward (5 N up)
  • Trolley -> Gravitational force 5 N downward (5 N down)
  • Trolley -> Pull 6 N right (6 N right)
  • Trolley -> Friction 2 N left (2 N left)

Learner action: Construct the force diagram from the four labelled forces, combine opposing forces to find the net force, and explain why balanced vertical forces do not cancel the unbalanced horizontal force.

Trolley mass and speed change

Accessibility description: With the same pulling force and time, speed change generally decreases as mass increases. Trial 2 at 1.5 kilograms is unusually large and should be checked.

| Trolley mass (kg) | Trial 1 speed change (m/s) | Trial 2 speed change (m/s) | Trial 3 speed change (m/s) | | --- | --- | --- | --- | | 0.5 | 1.20 | 1.16 | 1.18 | | 1.0 | 0.62 | 0.59 | 0.61 | | 1.5 | 0.39 | 0.91 | 0.41 | | 2.0 | 0.31 | 0.29 | 0.30 |

*Same trolley, track, pulling-force method, pull interval and starting speed.*

Learner action: Plan a teacher-approved trolley investigation that keeps pulling force and time consistent while changing mass. Use a stop block and keep feet clear; analyse the supplied trials, identify the anomalous 1.5-kilogram result and compare motion change without treating mass and weight as the same quantity.

The representation and table are supplied evidence. Do not replace a represented value, relationship or anomaly with an expected result. If a visual version is created, preserve this text-equivalent information and define every symbol, arrow, heading and unit.

Worked example

The reasoning routine is:

  1. identify the observation, measurement or represented relationship
  2. select the relevant science idea
  3. connect the idea to the evidence in a complete sentence
  4. qualify the answer when the evidence or model has a boundary

Inquiry connection

Question: How does trolley mass affect its change in speed when the same pulling force acts for the same time?

Reasoned hypothesis: If trolley mass increases, its change in speed will decrease because the same net force produces less acceleration for a larger mass.

  • Independent variable: trolley mass
  • Dependent variable: change in speed over the pull interval
  • Relevant controls: pulling force, time, trolley, track and starting speed
  • Hazard: moving trolleys and hanging masses can strike feet or fall
  • Risk control: use a stop block, keep feet clear and handle masses only when the trolley is stationary
  • Reproducibility detail: state masses, force method, timing interval, track setup, sensor precision and repeats

Possible data finding: speed change decreased as mass increased, except one heavy-trolley trial with an unusually large change.

Evidence-based interpretation: The overall decrease supports the hypothesis; the anomalous trial should be checked for a stronger pull or sensor error.

Limitation: Friction may not have been identical as mass changed, and only a small mass range was tested.

This investigation frame recurs across Year 7 Science. A sound response names a testable relationship, matches the hypothesis to the same variables, manages a realistic risk, specifies quantities and measurement rules, analyses the full data pattern, supports a claim with evidence and states what the evidence cannot establish.

Common errors and corrections

  • Error: Drawing a forward 'motion force'. Correction: Return to the core model and identify the exact evidence or relationship before answering.
  • Error: Saying balanced forces require the object to be stationary. Correction: Return to the core model and identify the exact evidence or relationship before answering.
  • Error: Treating mass in kilograms as the same quantity as weight force. Correction: Return to the core model and identify the exact evidence or relationship before answering.
  • Error: Choosing only the largest arrow and ignoring the directional sum of all forces. Correction: Return to the core model and identify the exact evidence or relationship before answering.

Practice

  1. Classify four forces as contact or non-contact.
  2. Interpret a text force diagram.
  3. Find net direction for opposing forces.
  4. Compare motion change for two masses under the same force.

Transfer task

Apply the topic to the investigation below without relying on a teacher or AI to mark the reasoning.

Evidence status: activity only. Use the task-specific rubric below for self-review or teacher feedback; completion does not automatically award mastery.

Context: How does trolley mass affect its change in speed when the same pulling force acts for the same time?

  1. Rewrite the question if it does not clearly name the relationship and measurable outcome.
  2. Write the matching reasoned hypothesis: include the expected direction and the science idea that justifies it.
  3. Identify the independent variable, dependent variable and at least three relevant controlled variables.
  4. Write a six-step reproducible method. Include equipment, quantities, units, an ordered measurement rule and at least three repeated trials for each condition.
  5. State the hazard, possible harm and a practical control. The control must address the stated hazard rather than being generic advice.
  6. Design a results table with headings and units. State which graph or other representation would best show the relationship and why.
  7. Use this possible finding: speed change decreased as mass increased, except one heavy-trolley trial with an unusually large change. Describe the overall pattern, identify any anomaly and state a reasonable check.
  8. Write a conclusion using claim, specific evidence and scientific reasoning. Finish with this limitation: Friction may not have been identical as mass changed, and only a small mass range was tested.

Correction guide: Compare the question and hypothesis to confirm they use the same two variables. In the method, circle every quantity and unit and underline every controlled condition. In the data response, separate the broad pattern from any anomalous result. In the conclusion, highlight the evidence sentence and box the limitation. If one of these parts is absent, revise that part before checking the scientific vocabulary.

An excellent transfer response is precise without pretending the evidence is perfect. It makes the chain from question to method to data to claim visible, and it explains how the core model applies in the unfamiliar context.

Task-specific inquiry rubric

  • Question and hypothesis for trolley mass and change in speed over the pull interval: Names the same measurable relationship in both and gives a scientific reason. Revise if needed: Align the changed factor, measured outcome and predicted direction.
  • Fair, safe and reproducible method: Controls pulling force, time, trolley, track and starting speed, manages "moving trolleys and hanging masses can strike feet or fall", and specifies quantities, units, measurement rules and repeats. Revise if needed: Replace vague directions and generic safety advice with operational detail.
  • Data and analysis plan: Provides labelled headings and units, an appropriate representation, a way to describe the whole pattern and a rule for checking anomalies. Revise if needed: Show how evidence will answer the question before collecting it.
  • Scientific interpretation and boundary: Links the expected evidence to multiple forces and changes in motion and anticipates this boundary: Friction may not have been identical as mass changed, and only a small mass range was tested. Revise if needed: Explain the science link and state what the design cannot establish.

Self-check

For each response, check:

  • Did I use the correct scientific vocabulary?
  • Did I refer to the specific evidence, feature or data?
  • Did I explain the link rather than only naming it?
  • Did I avoid claiming more than the model or data support?
  • If an investigation is involved, did I address question, hypothesis, variables, safety, reproducibility, data, evidence and limitations?

Sources