Forces, Motion, Gravity and Simple Machines

Forces are pushes or pulls that can change an object's velocity. Students compare force magnitude and direction, investigate the effect of mass, measure forces in newtons, interpret force-arrow diagrams, explain Earth's gravity and examine how simple machines change the force needed for a task.

When the same net force is applied to different objects, lower-mass objects show a larger acceleration than higher-mass objects. When mass is kept the same, increasing net force produces a larger change in motion.

A fair investigation changes one major variable at a time and controls the surface, cart design and measurement method.

Balanced forces have a vector sum of zero. They cause no acceleration. Unbalanced forces have a non-zero net force and can start, stop, speed up, slow down or turn an object.

A force meter or spring scale measures force in newtons (N). Before measurement, check that the meter reads zero, use an appropriate range and read the scale consistently.

A good force-arrow diagram:

• shows the object being analysed;
• uses arrows pointing in the direction each force acts;
• uses relative arrow length to show magnitude;
• labels forces such as weight, support/normal force, friction, tension or applied force.

Important: a force arrow is not a motion arrow. An object can move right while its net force points left.

Gravity is an attractive interaction between masses. Near Earth's surface, Earth's gravitational force on an object points toward Earth's centre. This gravitational force is the object's weight.

Objects with greater mass have greater weight in the same gravitational field. Their mass does not change when they move to the Moon or another planet, but their weight can change.

Ignoring air resistance, objects in the same gravitational field have the same free-fall acceleration even though heavier objects have greater weight.

Simple machines can change force magnitude or direction. They do not create energy. In an ideal machine, reducing effort force requires the effort to act through a greater distance.

Gravity acts across space and shapes large-scale motion:

• moons orbit planets;
• planets orbit stars;
• stars move under the gravity of galaxies;
• galaxies interact gravitationally;
• black holes can produce extremely strong gravitational fields nearby because large mass is concentrated into a very small region.

An orbit is not a place where gravity has disappeared. Forward motion combines with inward gravitational acceleration to produce a curved path.

Model limitation: a black hole is not a universal 'vacuum cleaner'. At the same external distance from the centre, an object with the same mass produces the same simple gravitational pull; black holes are extreme because matter is concentrated and very small distances can be reached.

The curriculum asks students to analyse forces acting on boomerangs and connect airfoil design with flight. A useful physical model includes gravity, aerodynamic lift, drag, rotation and torque.

Returning boomerangs use carefully shaped rotating arms. Unequal aerodynamic effects around the rotating body can produce torque and a curved flight path. Not all boomerangs are designed to return; designs and purposes vary.

When teaching cultural context, use reliable, place-specific First Peoples sources for the actual design, purpose and community knowledge rather than assuming one description applies across Australia.

The curriculum includes investigating force through technologies such as spearthrowers used by First Peoples of Australia and bow-and-arrow systems used by Torres Strait Islander Peoples.

A spearthrower can be modelled as an extended lever that changes the movement of the projectile end and can increase launch speed. A drawn bow stores elastic potential energy that can be transferred to an arrow.

Science-and-source rule: keep the physical model separate from cultural claims. Use authoritative, community-linked or place-specific sources for who used a technology, how it was made and its cultural purposes.

1. Net force: 40 N right and 18 N left → 22 N right.
2. Balanced forces: 120 N left and 120 N right → 0 N; no acceleration.
3. Mass and acceleration extension: 20 N on 2 kg → 10 m/s²; 20 N on 10 kg → 2 m/s².
4. Weight extension: 50 kg on the Moon at 1.6 N/kg → 80 N.
5. Lever: 0.9 m effort arm and 0.3 m load arm → MA 3; 180 N load → 60 N ideal effort.
6. Pulley: 100 N load supported by 2 ideal rope segments → 50 N effort.
7. Terminal velocity: weight equals air resistance, so net force = 0 even though the object keeps moving.
8. Elevator: 6000 N up and 4900 N down → 1100 N net force upward.

AC9S7U04: investigate and represent balanced and unbalanced forces, including gravitational force, acting on objects, and relate changes in an object's motion to its mass and the magnitude and direction of forces acting on it.

• E1: different forces on familiar objects of same/different mass.
• E2: balanced and unbalanced forces and changes in motion.
• E3: force meters and force-arrow diagrams.
• E4: Earth's gravitational force and mass.
• E5: levers and pulleys changing required force.
• E6: gravity in space.
• E7: forces on boomerangs and airfoil design.
• E8: simple-machine/force applications in First Peoples and Torres Strait Islander technologies.

Extension note: numerical use of F=ma, W=mg, kinetic-energy equations and formal mechanical-advantage equations strengthens problem solving but is not presented here as the only required Year 7 evidence for the descriptor.

1. What is a net force?
2. Can balanced forces act on a moving object?
3. How does mass affect acceleration for the same net force?
4. How does force magnitude affect acceleration for the same mass?
5. What does a force-meter measure and in what unit?
6. What do arrow length and direction mean in a force diagram?
7. Why does gravity still act on a book resting on a table?
8. What is the difference between mass and weight?
9. Why can a planet orbit rather than fall straight into its star?
10. How can a lever reduce effort force?
11. How can pulleys reduce effort force?
12. What is the force-distance trade-off in an ideal simple machine?
13. Which forces are relevant to a boomerang flight model?
14. How can a spearthrower be represented as a lever system?
15. Why should cultural information about First Peoples technologies use place-specific sources?

• Balanced forces mean no motion. Correction: they mean no acceleration; constant velocity is possible.
• Zero net force means no forces exist. Correction: non-zero forces can cancel.
• Force arrows show travel direction. Correction: arrows show force direction.
• Friction always points opposite an object's velocity. Correction: friction opposes relative sliding or the tendency to slide at the contact; static friction can act forward on a walking foot.
• A force acting for longer creates larger acceleration. Correction: if force and mass stay constant, acceleration stays constant while the change in velocity accumulates.
• Simple machines reduce the ideal work to zero. Correction: they trade force for distance.
• All boomerangs return. Correction: boomerang designs and functions vary.

Curriculum equivalents for Investigate and represent balanced and unbalanced forces, including gravitational force...

Mapped skill: investigate and represent balanced and unbalanced forces, including gravitational force, acting on objects, and relate changes in an object’s motion to its mass and the magnitude and direction of forces acting on it

These references identify matching or closely related learning. Curriculum sequence, terminology and depth vary, so teachers should use the mapped skill and lesson difficulty to confirm suitability.

Australian Curriculum v9.0 is the canonical source for this SkillrHub lesson. Victoria and NSW entries name the closest published state codes or outcomes; international entries are planning references rather than claims of identical curricula.

• AC9S7U03 — Earth, Sun and Moon cycles
• AC9S7U05 — Particle theory and states of matter
• AC9S7U04 Homework
• Practice
• Test