Edexcel Mechanics 2 question Watch

SN-RAVER
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A cyclist starts from rest and freewheels down a hill inclined at arcsin 1/20 to the horizontal. After travelling 80 meters the road becomes horizontal and the cyclist travels a further 80 meters before coming to rest without using the brakes. The cyclist and the bike have a combined mass of 80kg. Find the resistive force (assumed constant throughout the motion).

Anyone fancy a crack at that badboy? I don't see where I'm going wrong.
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ztibor
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(Original post by SN-RAVER)
A cyclist starts from rest and freewheels down a hill inclined at arcsin 1/20 to the horizontal. After travelling 80 meters the road becomes horizontal and the cyclist travels a further 80 meters before coming to rest without using the brakes. The cyclist and the bike have a combined mass of 80kg. Find the resistive force (assumed constant throughout the motion).

Anyone fancy a crack at that badboy? I don't see where I'm going wrong.
Let the acceleration down the hill be a1 and the decceleration on the horizontal road be a2. The travelling was 80m both down the hill and on the horizontal road.
Starting from the rest and coming to the rest means that a1=a2
and a1=(F1-Fr)/m a2=Fr/m, where Fr is the resistive force (assumed constant Fr,
but really it is not).
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