[求助]FORCES(physics)

COULD ANYONE PLEASE ANSWER ME FOR I DON'T UNDERSTAND WHY SHOULDN'T WE CONSIDER mgsin(angle) and friction AS WELL?

At the top of a slope, a trolley(mass:20kg) is released from rest and slides down freely along the slope (the runway is 25m long and makes an angle of 15 degrees with the horizontal, the frictional force is 20N).
1. Find the acceleration of the trolley.(1.59)
2. Find the speed of the trolley when it is half way down the slope.(6.3)

Then, a person catches the trolley when it is half way down the slope and makes it stop at the foot of the slope. What is the AVERAGE PULLING FORCE?
v=0,u=6.3,s=12.5
Since v^2-u^2=2as,
a= -1.59
Average pulling force=20(1.59)=31.8
I DON''T UNDERSTAND WHY WE NEEDN'T HAVE
Net force = ma -- Pulling force-mgsin15+f=20(-1.59)

THANK YOU VERY MUCH!!!