Sunday, November 25, 2012

The force exerted on the rugby ball by Francois Steyn (The longest rugby conversion kick)


Tinashe Nyanhete

Steyn’s record 66m kick.  http://www.youtube.com/watch?v=muI0ULBSWmw
Horizontal distance (x) = 68m
Time of flight of ball (t) = 4s
Vxo = x/t = 68m/4s =17m/s
Vy0 = gt = 9.8ms-2 * 4s = 39.2m/s
V = 43m/s
The approximate change in velocity of the ball after Francois struck the ball was 43m/s, which is higher than the normal reported value of 30m/s, since the ball travelled the highest ever recorded horizontal distance for a conversion kick.
Now to determine the force on the ball due to the kick (Use Newton’s 2nd and 3rd laws):
F = ma, and action = -reaction (Force exerted on ball = force exerted on the foot by the ball)
F = mv/t
Ft = mv, where t – time of contact between the foot and the ball; v – change in velocity of ball; m – mass of ball
Time of contact with the ball = 0.013s
Mass of ball = 0.44kg
Change in velocity of ball = 43m/s
Therefore, F = mv/t = 1455N
This is larger than the normal estimated force of 1015N, which explains why Francois Steyn’s ball travelled a record distance.
Reference: http://www.coachesinfo.com/index.php?option=com_content&id=228&Itemid=147

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