Motion and forces — Advanced

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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 cause bruising, so restraints add risk by concentrating forceB)Forces and accelerations on the body still depend on how quickly motion changes in a crash; lower speed helps but does not remove injury riskC)At low speed, inertia is overcome by friction with the seat, so occupants move with the vehicle without extra supportD)Seat belts are only rated for highway speeds; below 40 km/h they are not designed to engage

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

   A)The forward force from the track (if it exceeds other opposing forces) gives a net force on the sprinterB)The backward push on the track and forward push on the sprinter are a third-law pair, so they cancel on the sprinterC)Acceleration requires a net force backward relative to motion, matching the foot’s push directionD)The sprinter’s mass decreases while accelerating, so $F=ma$ no longer applies during the drive phase

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

   A)Dummies remove variability so every test can use the same subjective injury ratingB)Real volunteers give more ethical data than instruments, but regulations prefer dummies for traditionC)To make reproducible comparisons and estimate forces/accelerations on bodies in controlled scenariosD)Controlled speeds guarantee identical crash outcomes across vehicle models without further analysis

4. Two students pull a rope in opposite directions with equal magnitudes. The rope does not accelerate. The net force on the rope is:

   A)Equal to one student’s pull — the other student’s pull acts on the environment, not the ropeB)Equal to the vector sum of both pulls along the rope, which is twice either pull in the forward directionC)Non-zero if the rope is horizontal, because gravity adds a separate net force along the ropeD)Zero — equal and opposite pulls along the same line cancel on the rope

5. Justify with energy ideas why lower speeds in school zones can reduce injury severity in collisions.

   A)Kinetic energy scales strongly with speed ($E_k \propto v^2$); less energy to dissipate in a crash often means less violent changes in motion for occupantsB)Kinetic energy is proportional to speed only ($E_k \propto v$), so halving speed halves severity in a simple wayC)Most crash energy goes into sound and heat, not body deformation, so speed has little effect on injuryD)Lower speed mainly reduces trip time, which gives drivers more reaction time rather than changing impact energy

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

   A)Fully correct — mass alone decides injury risk in every crashB)Correct for the car, wrong for passengers because seat belts remove all forcesC)Incomplete — crash safety also depends on crumple zones, restraints, what you collide with, and how forces are distributed over timeD)False — F = ma does not apply to vehicles in collisions

7. A cart is pulled by two students at angles so their force vectors are not collinear. To find net force on the cart you should:

   A)Add the magnitudes onlyB)Add forces as vectors (components or diagram)C)Use the larger force and ignore the smallerD)Average the forces and multiply by mass

8. Evaluate using impulse ideas: air bags and crumple zones help because they:

   A)Remove all momentum from the passenger instantlyB)Increase peak force on the body to shorten collision timeC)Eliminate the need for seat belts in frontal crashesD)Increase contact time so the same momentum change can occur with smaller peak force (for a given $\Delta p$)

9. Two ice skaters push off each other from rest on frictionless ice. After the push, which statement is most accurate?

   A)They have equal-magnitude momentum in opposite directions (if no external horizontal impulse)B)The heavier skater always has greater speedC)Both skaters feel different-magnitude forces because masses differD)Internal forces cannot change the skaters’ combined momentum, so both must remain at rest

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

    A)Zero net force, by the first lawB)Balanced forces in every directionC)Non-zero net force in the direction of accelerationD)Net force opposite to velocity

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