A car brakes hard. The coffee cup on the dashboard keeps going: it slides forward and hits the windshield. Nobody threw it. What happened?
The cup's story is every passenger's story. Inertia explains both, and seat belts and headrests are the answers engineers built for it.
The brakes act on the car. They exert no backward force on the cup, and none on an unbelted passenger.
So while the car slows, the cup's velocity persists: the cup keeps moving forward at the car's old speed.
The cup does not speed up. The car slows underneath it, so the cup slides forward relative to the car.
An unbelted passenger does the same. In a hard stop, the passenger keeps moving forward at the car's old speed until something exerts a backward force on the passenger.
A pickup truck brakes hard. Why does an unbelted passenger keep moving toward the windshield?
A seat belt is that something. The belt exerts the backward force on the passenger, so the passenger slows together with the car.
Arlo says: in a hard stop, the seat belt exerts the backward force that slows a passenger together with the car. Is he correct?
A sudden turn tells the same story sideways. The car turns, the passenger's velocity persists straight ahead, and the belt or the door exerts the sideways force that turns the passenger with the car.
A headrest answers the reversed case. When a car is hit from behind, the seat drives the body forward while the head tends to keep its old velocity.
Without a headrest, the body is driven forward while the head lags, and the neck takes the strain. The headrest exerts a forward force on the head, so the head speeds up together with the body.
In a rear-end collision, what does a headrest do?
A bus slows sharply at a stop. A standing rider with no handhold stumbles toward the front. Explain why, naming the forces.
The brakes exert their force on the bus, not on the rider.
With no backward force on the rider, the rider's velocity persists at the bus's old speed.
The bus slows underneath the rider, so the rider moves forward relative to the bus.
Holding a rail lets the rail exert the backward force that slows the rider together with the bus.
The rider stumbles forward because the rider's velocity persists while the bus slows; a handhold supplies the missing backward force.
A car makes a sudden left turn. An unbelted passenger slides toward the right-hand door. Why?
Which statement says why a seat belt protects a passenger in a hard stop?
A parked car is hit from behind and jolts forward. The driver's seat has no headrest. Why can the driver's neck be strained?