A car traveling at a velocity of 35 m/s, encounters an emergency and comes to a complete stop in 2.5 seconds. What is the car’s acceleration?
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Answer ⤵️
It may not seem like much, but driving even a few kilometres per hour above the speed limit greatly increases the risk of an accident.
Many of us cheat a little when driving. We figure that while the speed limit is 60 km/h the police won't pull us over if we sit on 65. So we happily let the speedo hover just above the speed limit, unaware that by so doing we are greatly magnifying our chances of crashing.
Using data from actual road crashes, scientists at the University of Adelaide estimated the relative risk of a car becoming involved in a casualty crash—a car crash in which people are killed or hospitalised—for cars travelling at or above 60 km/h. They found that the risk approximately doubled for every 5 km/h above 60 km/h. Thus, a car travelling at 65 km/h was twice as likely to be involved in a casualty crash as one travelling at 60 km/h. For a car travelling at 70 km/h the risk increased fourfold. For speeds below 60 km/h the likelihood of a fatal crash can be expected to be correspondingly reduced.
STOPPING DISTANCE CALCULATOR
Small conditions can make a big difference to the time it takes you to stop your car, such as going a few km/hr slower or being alert on the road.
INTERACTIVEInitial speed
60 km/h
Reaction time
2.7 sec
Rate of deceleration
10 m/s2
metres travelled before car stops
metres travelled before brakes are fully applied
The physics that drive you
REACTION TIME
One reason for this increased risk is reaction time—the time it takes between a person perceiving a danger and reacting to it. Consider this example. Two cars of equal weight and braking ability are travelling along the same road. Car 1, travelling at 65 km/h, is overtaking Car 2, which is travelling at 60 km/h. A child on a bicycle—let's call him Sam—emerges from a driveway just as the two cars are side-by-side. The drivers both see the child at the same time and both take 1.5 seconds before they fully apply the brakes. In those few moments, Car 1 travels 27.1 metres and Car 2 travels 25.0 metres.
The difference of 2.1 metres might seem relatively small, but combined with other factors it could mean the difference between life and death for Sam.
The figure of 1.5 seconds is the reaction time of average drivers. A driver who is distracted, for example listening to loud music, using a mobile phone or has drunk alcohol may take as long as 3 seconds to react.