HSC Exam Practice

Sample response. Inertia is the tendency of an object to resist changes to its state of motion — it resists both being set in motion from rest and being brought to rest from motion. Inertia is directly proportional to mass: a greater mass means greater inertia, making the object harder to accelerate or decelerate. This property is described by Newton’s First Law, which states that an object remains at rest or continues at constant velocity unless acted upon by a net external force.

Marking notes. 1 mark for a correct definition of inertia (resistance to change in state of motion); 1 mark for correctly relating inertia to mass (proportional, or more mass = more inertia); 1 mark for naming Newton’s First Law (accept “law of inertia”) with a correct statement.

Sample response. Static equilibrium: the object is at rest and the net force acting on it is zero — example: a 5 kg book resting on a desk (normal force from desk exactly balances weight). Dynamic equilibrium: the object moves at constant velocity (constant speed in a straight line) and the net force acting on it is also zero — example: a car cruising at a steady 100 km/h on a straight, flat road where engine force equals air resistance and friction. In both cases F_net = 0, but the objects differ in whether they are stationary or in uniform motion.

Marking notes. 1 mark for correct definition of static equilibrium (at rest, F_net = 0); 1 mark for a valid example of static equilibrium; 1 mark for correct definition of dynamic equilibrium (constant velocity, F_net = 0); 1 mark for a valid example of dynamic equilibrium.

Sample response. Three criteria: (1) The forces must be equal in magnitude. (2) The forces must be opposite in direction. (3) The forces must be the same type of force (both contact, or both gravitational, or both magnetic, etc.) and — critically — must act on different objects (one force on object A, the other on object B).

Marking notes. 1 mark per criterion. Award 1 for equal magnitude; 1 for opposite direction; 1 for same force type AND acting on different objects (must mention both for the third mark). Do not award the third mark if “different objects” is omitted.

Sample response. Newton’s Third Law pairs act on different objects, so they cannot be combined when finding the net force on one object. To calculate net force we can only add forces acting on the same object. For example, when a skateboarder pushes a wall, the wall pushes back on the skateboarder with an equal and opposite force. These two forces act on different objects (wall and skateboarder). The net force on the skateboarder includes only the reaction force from the wall — there is no equal and opposite force on the skateboarder to cancel it — so the skateboarder accelerates away from the wall.

Marking notes. 1 mark for stating that Third Law pairs act on different objects; 1 mark for explaining that net force is only the sum of forces on the same object; 1 mark for applying this to a specific named example with a correct conclusion about motion. Accept any valid example (rocket and exhaust gases, swimmer and water, horse and cart, etc.).

Sample response. A contact force requires physical contact between the two interacting objects; it acts only when surfaces touch. Examples: normal force (perpendicular push between a book and a table) and friction (opposing relative motion between a sled and snow). A field-mediated force acts across a distance without contact, transmitted via a field. Examples: gravity (gravitational field — Earth pulling a ball downward) and magnetic force (magnetic field — a magnet attracting a steel nail).

Marking notes. 1 mark for correct description of contact force (requires physical contact / acts only when touching); 1 mark for two valid examples of contact forces; 1 mark for correct description of field-mediated force (acts across a distance through a field); 1 mark for two valid examples of field-mediated forces with the field named for each.

Sample response. The student is incorrect for two reasons: (1) the weight of the book is a gravitational (field-mediated) force, while the normal force from the table is a contact force; a genuine Third Law pair must be the same type of force. (2) Both forces act on the same object (the book), whereas Third Law pairs always act on different objects. The correct Newton’s Third Law pair for the weight of the book is: Earth pulls the book downward with a gravitational force of mg, and the book pulls Earth upward with an equal gravitational force of mg (acting on Earth, not on the book).

Marking notes. 1 mark for identifying that the weight and normal force are different force types; 1 mark for explaining that a genuine Third Law pair must act on different objects (both weight and normal force act on the book); 1 mark for stating the correct Third Law pair (Earth pulls book down / book pulls Earth up — both gravitational, equal magnitude, different objects). Accept the table’s Third Law pair: normal force on book from table (contact) / book’s contact force pushing table down (contact), as the Third Law pair for the normal force (not for the weight — must be specific).

Sample response (a). During the drive phase (0–0.9 s), the force rises steeply from 0 N at t = 0 to a peak of approximately 600 N at t = 0.4 s, then decreases gradually back to 0 N at t = 0.9 s [describe trend and peak].

Sample response (b). By Newton’s Third Law, the water exerts a force of 600 N on the oar blade [1], directed forward (in the direction of the shell’s motion) [1]. This forward reaction force is transmitted through the oar to the rowing shell, producing a net forward force on the shell that causes it to accelerate forward during the drive phase [1].

Sample response (c). During the recovery phase (0.9–1.4 s), with no oar force acting, there is no net horizontal force on the shell (ignoring water resistance) [1]. By Newton’s First Law, the shell will continue moving forward at constant velocity — the velocity it had at the end of the drive phase [1].

Marking notes. Part (a): 1 mark for describing the rise-then-fall trend; 1 mark for correctly identifying the peak (~600 N at ~0.4 s). Part (b): 1 mark for stating the reaction force magnitude (600 N) using Newton’s Third Law; 1 mark for correct direction (forward); 1 mark for explaining how this causes the shell to accelerate. Part (c): 1 mark for predicting constant velocity (not stopping); 1 mark for citing Newton’s First Law as the justification.

Sample response. The statement is incorrect. It confuses the concept of a Third Law pair (forces on different objects) with two forces on the same object that might cancel. Newton’s Third Law states that for every action force there is an equal and opposite reaction force — but crucially, these forces act on different objects. Because they act on different objects, they cannot be combined in a net force calculation for any single object, and therefore they do not prevent motion. Consider an AFL player kicking a football: the player’s boot exerts a contact force forward on the ball, and Newton’s Third Law requires the ball to push back on the boot with an equal force. However, the force on the ball (forward) and the force on the boot (backward) act on different objects. The net force on the ball is the forward kick force alone — no cancellation occurs — so the ball accelerates forward. Similarly, when a rocket expels exhaust gases downward at high speed (contact force), the gases push the rocket upward (reaction force). The force on the gases and the force on the rocket are an equal-and-opposite pair, but because they act on different objects, the rocket’s net upward force is non-zero and the rocket accelerates. This is why rockets work in the vacuum of space, where there is nothing to “push off” in the contact-force sense. The relationship with Newton’s First Law is key: Newton’s First Law tells us that a net force changes an object’s velocity. If the Third Law pair really cancelled (acted on the same object), there would be no net force on any object, and nothing could ever accelerate — this contradicts observable reality. Only when two forces acting on the same object are equal and opposite do they cancel — this is the condition for equilibrium. A book on a table is in static equilibrium because its weight (gravity on book) and the normal force (table on book) are equal and opposite forces on the same object; these are not a Third Law pair. The Third Law pair for the book’s weight is the book pulling Earth upward — acting on Earth, not on the book, and therefore not cancelling the book’s weight. In summary, the statement conflates two distinct concepts: Third Law pairs (different objects, cannot cancel) and equilibrium forces (same object, can cancel). Net motion is entirely possible under Newton’s Third Law, as demonstrated by every rocket, swimmer, and rowing shell in the world.

Marking criteria (7 marks). 1 = identifies the fundamental flaw in the statement: Third Law pairs act on different objects, not the same object. 1 = explains that net force is calculated only from forces acting on the same object, so Third Law pairs cannot cancel each other. 1 = provides first valid Australian or real-world example with correct Third Law analysis (e.g. AFL kick, rowing, rocket). 1 = provides second valid example, different from the first. 1 = correctly distinguishes between Third Law pairs and equilibrium forces (same object), with an example (e.g. book on table). 1 = links Newton’s First and Third Laws: explains that net force (not a Third Law pair) determines whether an object accelerates or remains in equilibrium. 1 = reaches an explicit evaluative judgement that the statement is incorrect and that net motion is possible and consistent with Newton’s Third Law.