AC9S7U04 • Homework
Forces, Motion, Gravity and Simple Machines
Use force vocabulary precisely. When a question involves motion, identify the forces, determine whether the net force is zero or non-zero, and connect the net force to the change in velocity.
Part A — 2-mark questions
- Explain why the same net force produces different accelerations when applied to objects of different mass. (2)
- Give one example of balanced forces and one example of unbalanced forces acting on an object. State what happens to the object's motion in each case. (2)
- Describe a force-arrow diagram for a box sliding to the right on a floor. Include weight, support force, applied force and friction. (2)
- What is Earth's gravitational force and in which direction does it act on an object near Earth's surface? (2)
- Explain how increasing the effort-arm length of a lever can reduce the effort force needed to lift a load. (2)
Part B — 5-mark questions
- Design an investigation to compare how different applied forces affect carts of the same mass and how the same force affects carts of different mass. Identify the independent variable, dependent variable, controls, measurements and expected pattern. (5)
- Analyse how balanced and unbalanced forces can make an object start, stop, speed up, slow down or change direction. Use at least three examples and refer to net force. (5)
- Explain how to use a force meter accurately, then describe a complete force-arrow diagram for a stationary hanging object. Explain what the arrow lengths and directions mean. (5)
- Explain how gravity affects motion in space. Include a moon orbiting a planet, a planet orbiting a star and one example involving a galaxy or black hole. (5)
- Using reliable, place-specific information, explain how a spearthrower used by First Peoples of Australia or a bow-and-arrow system used by Torres Strait Islander Peoples can be analysed using force, leverage and/or energy ideas. Keep cultural claims separate from the physics model. (5)
Part C — Enrichment
11. Integrated forces analysis. Analyse how balanced and unbalanced forces, gravity and simple machines influence motion in everyday life and technology. Your response should include: the effect of mass and force magnitude on acceleration; how force diagrams represent interactions; how gravity shapes motion on Earth and in space; one accurately sourced First Peoples or Torres Strait Islander engineering example; and why an ideal simple machine trades force for distance rather than creating energy.
Answers and marking guidance
1 — 2 marks
1 mark: states that lower mass changes motion more for the same net force. 1 mark: links this to acceleration depending on both net force and mass (extension formula F=ma acceptable).
2 — 2 marks
1 mark for a valid balanced example such as equal push and friction with no acceleration; 1 mark for a valid unbalanced example such as a larger forward force producing a change in velocity.
3 — 2 marks
Weight downward and support force upward; applied force right and friction left. Arrow direction shows force direction and relative length shows magnitude.
4 — 2 marks
Gravity is an attractive force between masses; near Earth's surface Earth's gravitational force acts toward Earth's centre.
5 — 2 marks
A longer effort arm increases the turning effect/mechanical advantage, so a smaller effort force can balance the same load. In an ideal machine this is traded for greater effort distance.
6 — 5 marks
A strong response: varies applied force while controlling cart mass and surface; then varies mass while controlling force; measures acceleration or another consistent change-in-motion measure; repeats trials; predicts greater acceleration for larger net force and smaller mass.
7 — 5 marks
Balanced forces give zero net force and no acceleration. Unbalanced forces give non-zero net force and change velocity. Examples should correctly cover starting/stopping, speed changes and/or direction changes with the force direction stated.
8 — 5 marks
Check/zero the force meter, use correct units and scale, stay within range and read consistently. For a stationary hanging object, upward tension equals downward weight. Opposite equal arrows show zero net force.
9 — 5 marks
Gravity provides inward acceleration that curves orbital motion. Moons orbit planets and planets orbit stars through gravity plus forward motion. Stars and galaxies also interact gravitationally; black holes can create very strong nearby gravitational fields because mass is highly concentrated.
10 — 5 marks
Credit an accurately sourced technology and community/context; a correct force/energy model; explanation of leverage, elastic energy or launch motion as appropriate; a clear separation between source-supported cultural information and the student's physical model; and scientifically accurate terminology.
11 — enrichment
High-level responses integrate net force, mass, acceleration, force diagrams, gravity, simple-machine trade-offs and one reliably sourced cultural engineering example. They should avoid claims that balanced forces mean no motion, that machines create energy, or that all boomerangs are designed to return.