physics (urgent~)

1)
F = ma
Only at acceleration and deceleration, a (the value of acceleration) is not equal to zero, so there is a force acting on people when MTR starts to move or starts to stop, as a result, people hold the hanger to countact with the force.
On constant speed (no change in speed), a (the value of acceleration) is equal to zero, so there is NO force acting on people when MTR move in constant speed, as a result, people can stand without holding the hanger.
2)
There is alway friction between the girl and the slide, the difference is only whether it is large or little.
(a)
The girl comes down the slide faster and faster means that energy give out from change of potential is larger than the frictional energy lost, so kinetic energy gain and speed increase.
i.e. Friction force is small.
(b)
The girl comes down slowly at a constant speed means that energy give out from change of potential is equal to the frictional energy lost, so no kinetic energy gain and no speed change.
i.e. Friction force is medium.
(c)
The girl sits on the slide and does not move means that the force from gravity acting on the girl in the direction of movement (parallel to the direction of the slide) is smaller than the limiting frictional force. As a result, the girl does not move.
i.e. Friction force is large.
I hope this can help with your understanding. =)