A car is travelling along a straight road when the driver suddenly slams on the brakes. A passenger without a seatbelt keeps moving forward and jolts into the dashboard, even though the car itself has already stopped. Which law of motion explains why the passenger's body keeps moving after the car stops?
- Newton's Second Law -- the acceleration produced depends on the size of the force and the mass being pushed
- Newton's First Law -- an object continues at rest or at a constant velocity unless an unbalanced force acts on it
- Newton's Third Law -- every force one object exerts on a second object is matched by an equal and opposite force back
- The law of conservation of momentum -- the total momentum of a system stays the same unless an external force changes it
Why B? And why not the others?
Correct answer: B. Newton's First Law -- an object continues at rest or at a constant velocity unless an unbalanced force acts on it
A passenger without a seatbelt has no force holding them back, so their body keeps travelling forward at the car's original speed even as the car itself decelerates beneath them - this is Newton's First Law of motion (the law of inertia): an object continues at constant velocity unless an unbalanced force acts on it, and the dashboard only supplies that stopping force at the moment of collision. The option about force, mass and acceleration is wrong because it explains how much the car itself slows down for a given braking force, not why a loose object inside the car carries on moving independently once that braking force stops acting on it specifically. The option about paired action-reaction forces is wrong because that law describes two objects each pushing on the other with equal and opposite forces, which isn't what is happening to an unrestrained passenger who has nothing pushing back on them yet. The option about conserved momentum is wrong because although momentum is genuinely conserved in the full system, it isn't the concept a school-level explanation reaches for here - the direct answer to 'why does the passenger keep moving' is inertia, stated by the first law specifically.
Source: BBC Bitesize GCSE Physics: Forces and motion -- Newton's Laws (First Law / inertia)