Final answer:
The value of 18 g reported for Princess Diana's car crash likely represents the average acceleration, as acceleration during a car crash is not constant and can vary significantly throughout the event.
Step-by-step explanation:
The acceleration experienced by Princess Diana's car during the crash reported to be about 18 g is most likely an average acceleration rather than instantaneous or constant. During a car crash, acceleration can vary widely throughout the event as forces are applied and then dissipate.
The value of 18 g is a measure of the peak force experienced relative to gravity, but it would not be constant, as car crashes involve complex interactions of forces, leading to changes in acceleration over time.
Instantaneous acceleration would be the acceleration at any specific point in time during the crash, which can fluctuate rapidly. Average acceleration is the total change in velocity divided by the total time taken for that change, which seems most applicable to the reported value.
Constant acceleration is when an object's velocity changes by the same amount each second, which is very unlikely in the chaotic environment of a car crash.
To understand how acceleration affects the human body in accidents, tests often involve cars with dummies and simulations based on various speeds and scenarios, where the effects of different types of acceleration can be studied.