Forces in sport

Weight, ground reaction, friction, drag and buoyancy: the forces acting on an athlete.

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

Weight: what is it?
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The Earth's pull, equal to mass times g (9.81 m/s²).
It is in newtons, not kilos.
On landing from a jump, the ground pushes the athlete much harder than his weight. Why?
- His falling speed must be stopped in little time
- His weight increases at impact
- Gravity is stronger at ground contact
- The ground sends his fall's energy back to him
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His falling speed must be stopped in little time
Cancelling his falling speed in little time takes a net upward force, so the ground reaction exceeds his weight (F × Δt = m × Δv).
Ground reaction force: what is it?
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The force the ground exerts back on the foot or body.
In running it can reach 2 to 3 times body weight.
Why does drag matter little when walking but a lot in cycling?
- It rises with the athlete's mass
- It rises with the square of speed
- It depends only on body shape
- It rises with the length of the effort
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It rises with the square of speed
Eight times faster (40 km/h against 5 km/h) means about sixty-four times more drag: at 40 km/h, air makes up most of the resistance.