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A runner decelerates as he runs into a windy exposed area, from a speed of 4 m/s to 2 m/s in 8 seconds. His mass is 60 kg. Assuming he also experiences a frictional force of 3 N, find the size of the force exerted by the wind on the runner.

I have found the resultant force to be 15N but to calculate the wind I'm sure it cannot just be 12N because the force pushing the man forwards has to be taken into account as well. I'm all rather confused about it!

There is also this question which I am a bit stuck with:

A stone of mass 50g is dropped into some liquid and falls vertically through it with an acceleration of 5.8 m/s^2. Find the force of resistance acting on the stone.

Many thanks!