# Physics

## Physics regards the physical aspects of the natural world. It includes topics that deal with forces on different bodies within the universe and phenomena that explain how the universe works.

### If a child ran into the road 65 to 70 feet ahead of your vehicle what is the highest speed from which you could stop with good brakes before hitting him?

You could stop at 20 miles per hour without hitting him.
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Assuming The Highway Code formula for stopping distances:
stopping_distance = thinking distance + stopping time
= speed + speed²/20 ft (where speed is in mph)
→ 65 = speed + speed²/20
→ speed² + 20speed + 1300 = 0
→ speed = -10 + 10√14 mph ≈ 27.4 mph
(The negative square root can be ignored as it gives a negative
speed which is nonsense for this problem.)
However, this assumes that you observe, think and can start
braking within 15/22 (≈ 0.68) seconds of the child running into the
road (1ft for every 1 mph of speed), and your braking matches the
Highway Code assumptions for good tyres, brakes and dry road
conditions.