STRIMMER ANSWER

The answer to this question lies in the speed with which the nylon spins.  By looking at the rev counter in a car, you can see that petrol engines can rotate thousands of times a minute. 

If the nylon cord is about 20cm long and we assume a modest engine rotation of 1000 revs per minute, the speed of the end can be calculated using the formula Speed = Distance / Time

The distance the end of the cord travels for each revolution is the circumference of a circle of radius 20cm (0.2m):

Distance = 2pr = 2 x 3.142 x 0.2 = 1.26m

That is a surprising long way!  Added to which, it makes this distance in just one thousandth of a minute, which in seconds is 60/1000 = 0.06s, so:

Speed = Distance / Time = 1.26 / 0.06 = 21m/s

So, the end of the wire is travelling at 21 metres per second!  As there are 60 seconds in a minute, and 60 seconds in an hour, this is equal to:

21 x 60 x 60 = 75600 metres per hour = 75.6 kilometres per hour

Of course, a blade of grass has little chance of survival whatever hits it at that speed!