Suppose the skateboarder and all his equipment have a combined mass of 85 kg.
The questions:
**Kinetic energy:** How much kinetic energy does he have at that speed, expressed in joules and the equivalent energy in TNT?
**Stopping distance:** If he brakes with a constant deceleration of 0.8g, how many metres does he need to stop? How long does it take?
**Impact:** If he crashes into a rigid barrier and comes to a complete stop in 0.1 seconds, what average force acts on his body, expressed in newtons and multiples of his body weight?
**The helmet problem:** Suppose his helmet can safely absorb 150 J of energy before its protective capacity is exceeded. What fraction of his total kinetic energy would that represent?
**The tricky & strange part:** If his speed increased by just 10%, by what percentage would his kinetic energy increase?