Question

why we need to be more careful while cycling down a hill than while cycling up the hill?

Answer 1

The force of gravity attracts everything to the center of the earth. So when a cycler moves down to the hill, the force of attraction gets more stronger and the speed increases due to this. This makes it hard for a cycler to handle the cycle as compare to moving up the hill. 

Answer 2

The slope of the hill, aided by gravity,would lead to rapid acceleration of the bicycle when cyclists go downhill. Hence, a cyclist has to be extra careful when going downhill. Going uphill requires extra effort as one goes up the slope and against gravitational pull.