Motion and forces — Advanced

Score: 0 / 19

Level 3: Advanced

Evaluate, justify, synthesise, propose — combine ideas, judge claims and methods (senior secondary style).

1. Evaluate the claim: wearing a seat belt is pointless if the car is going slowly. The strongest response uses:

   A)Low-speed crashes mainly bruise soft tissue, so belts worsen outcomes by concentrating stopping force on the torso.B)Even at low speeds, a rapid stop still means large acceleration; belts limit motion and cut injury risk.C)Seat friction already locks people to the car at low speeds, so belts add no real protection in town driving.D)Restraint systems are validated mainly for highway speeds, so they provide little benefit under about 40 km/h.

2. A sprinter pushes backward on the track. The track pushes the sprinter forward (third law). The sprinter accelerates because:

   A)The forward push from the track can exceed opposing forces, giving a non-zero forward net force on the sprinter.B)The backward push on the track and forward push on the sprinter are a third-law pair, so on the sprinter they cancel and prevent acceleration.C)Forward acceleration requires a net force aimed backward along the track, matching the direction the foot pushes the ground.D)The sprinter loses mass while driving, so $F=ma$ stops applying until the stride ends and mass returns.

3. Justify why crash tests use instrumented dummies and controlled impact speeds:

   A)Dummies standardise appearance so analysts can apply one subjective injury score without using sensors.B)Human volunteers would yield fairer biomechanics, but rules keep dummies only for historical comparability.C)They allow repeatable tests and measured estimates of forces and accelerations on bodies in fixed scenarios.D)Fixing impact speed forces every vehicle model to produce the same damage pattern with no extra interpretation.

4. Lower speed limits near schools can reduce pedestrian injury risk. Which reason is best supported by the physics ideas in this topic?

   A)Injury severity in pedestrian impacts often rises sharply with vehicle speed, so small speed cuts can matter a lot.B)Driver reaction distance shrinks in exact proportion to speed, and that alone explains most injury reduction.C)Pedestrian injury depends only on vehicle mass, so changing speed within a school zone barely changes outcomes.D)Once a car is moving, any impact speed above walking pace produces the same harm regardless of how fast the car was going.

5. Critique the statement: heavier cars are always safer because $F = ma$ means larger mass gives smaller acceleration for the same force.

   A)Mostly correct — a larger car mass by itself guarantees lower injury risk for occupants in every crash scenario.B)Correct for the car’s acceleration, but wrong for people because seat belts remove impact forces inside the cabin.C)Partly true for the car’s acceleration, but safety also depends on structure, restraints, the other object, and stopping time.D)Not valid — $F = ma$ only applies to steady cruising, not to the forces during a collision.

6. Evaluate how air bags and crumple zones reduce injury risk in a frontal crash:

   A)Strip away passenger motion in essentially zero time so the body stops without any gradual slowing phase.B)Shorten stopping time so peak forces on the body rise, wrongly treating that as how cushioning lowers injury risk.C)Remove the need for seat belts in frontal crashes because the bag alone governs how the occupant slows.D)Lengthen the time over which the occupant’s motion changes, so peak contact forces on the body tend to be smaller.

7. Two ice skaters with different masses push off each other from rest on frictionless ice. Which statement is most accurate?

   A)During the push, each feels a force on them from the other of equal magnitude; the lighter skater usually gains higher speed.B)The heavier skater must move faster after the push because larger mass always demands higher speed from the same push.C)Each skater feels a different-sized push from the partner because their masses differ, contradicting Newton’s third law.D)Internal forces cannot alter motion within the pair, so after the push both skaters must remain at rest together.

8. Synthesise Newton’s first and second laws: an object speeding up in a straight line must have:

   A)Zero net force on it, consistent with the first law even while its speed is increasing along the line.B)Balanced forces in every direction so that all pushes and pulls on the object cancel pair by pair.C)Non-zero net force pointing in the same direction as the acceleration along that straight-line path.D)Net force directed opposite to velocity so the object can still speed up along the same straight line.

9. A car speeds up along a straight road. A student says friction on the tyres must be zero because the net force on the car is forward. The best reply is:

   A)Forward net force is impossible if any friction acts on the tyres, so the student must be correct.B)Forward net force means forward forces exceed backward ones along that line; friction may still be present.C)Friction and engine thrust are a third-law pair on the car, so they always cancel and net force stays zero.D)Kinetic friction always drops to zero the moment speed is not zero, so only air resistance opposes motion.

10. Evaluate the idea that heavier objects always fall faster near Earth’s surface in everyday drops.

    A)Well supported in vacuum, where removing air lets heavier objects accelerate downward more than lighter ones.B)Often misleading — drag can slow light or high–surface-area objects; without air, $g$ is nearly the same for different masses.C)False near Earth because gravity does not pull on heavy objects, only on light ones in everyday drops.D)True in general because larger weight always means larger gravitational acceleration for every object.

11. Critique: Newton’s third law means forces always cancel, so nothing can accelerate.

    A)Fully correct, because equal and opposite partners always remove net force from every object in the interaction.B)True only for objects at rest, since motion would break the symmetry that keeps third-law pairs balanced.C)It confuses interaction partners on different bodies with the net force on a single chosen body.D)False overall, because Newton’s third law forbids contact forces from ever forming between touching objects.

12. Justify why crumple zones and longer stopping distances can reduce injury in a crash.

    A)Spreading the same overall slowing over more time usually means a smaller peak average force.B)They remove the need for seat belts, because controlled crushing alone keeps occupants from moving relative to the car.C)They make the car’s speed vanish without work being done on the vehicle or the people inside it.D)They guarantee the occupant experiences zero acceleration, even when the car’s speed changes rapidly.

13. A rocket accelerates in empty space with no air. The strongest explanation is:

    A)It relies on pushing against outside air molecules, which should still be present even in deep space.B)The third-law force pairs both act on the rocket at once, so they add internally and drive acceleration.C)The rocket’s mass drops to zero during thrust, so $F = ma$ stops being a usable relation for the motion.D)The rocket pushes exhaust backward, and the exhaust pushes the rocket forward as the interaction partner.

14. Synthesise walking forward on rough ground: friction from the ground on your shoe is often forward because:

    A)Static friction always balances your weight, so its forward component appears whenever you stand upright.B)You push the ground backward; the ground’s friction on you is the horizontal partner that can accelerate you forward.C)The normal force from the ground has a forward component that propels you whenever you take a step.D)Air resistance against your body pushes you forward on calm days, adding to the ground’s horizontal support.

15. Evaluate using how strongly injury risk rises with impact speed: why can a small drop in driving speed matter a lot for pedestrians?

    A)Injury risk is effectively flat once the car is already moving at urban speeds.B)Halving speed halves the harmful effect in a simple one-to-one way every time.C)Harm often increases markedly as impact speed rises, so modestly lower speeds can substantially cut severity.D)Speed mainly changes how far a car travels before the driver reacts, not how violent a pedestrian impact is.

16. Critique: a rigid helmet with no padding is best because it does not deform.

    A)Very stiff shells can shorten impact time and raise peak forces on the head; liners spread load and lengthen contact time.B)Always true — a harder outer shell always lowers injury risk regardless of how abruptly the head stops.C)Soft inner materials cannot store or dissipate energy, so they only make helmets heavier without safety gain.D)How long the head is in contact with the liner does not affect the forces for a given change in motion.

17. Justify why a head restraint can reduce whiplash risk in a rear-end collision.

    A)It cancels inertia for the head, so the head no longer lags when the car suddenly gains forward speed.B)It supports the head early so the neck undergoes less sharp flexion as the torso is driven forward.C)It adds effective mass behind the head, which lowers acceleration during the rear-end impulse on the car.D)It removes normal and friction forces from the seat back, letting the torso move freely without torque on the neck.

18. Evaluate: passengers lean forward when a bus brakes because braking removed their forward force.

    A)Correct — once engine thrust drops away, passengers should stop instantly with the bus and not slide on the seat.B)Misleading — passengers tend to keep moving forward while the bus slows; braking forces act on the bus, and seats or belts exert forces on people.C)False as stated — inertia only influences heavy passengers, so lighter people should not lurch forward when the bus brakes.D)True — no horizontal forces from the seat or belt act on passengers during braking, so torsos stay fixed.

19. Synthesise how force and contact time interact: a hard catch versus a soft hands catch of the same ball at the same speed differs mainly because:

    A)The ball’s change of motion is larger when hands are soft, because giving with the ball adds extra backward effect.B)Local gravity on the hands changes between a stiff catch and a cushioned catch, altering how weight acts.C)Soft hands lengthen contact time, which tends to lower peak force for the same overall change in motion.D)A stiff catch removes motion energy without exerting force, because rigidity lets the ball stop without a push.

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