questions relating to mechanics

[NE2000]

This is from VCAA Exam 2 2005: http://www.vcaa.vic.edu.au/vce/studies/mathematics/specialist/pastexams/2005specmaths2.pdf
Specifically I'm looking at the second last question. It's got a conveyer belt carrying packages up from A to B.

The question says:
If the conveyor belt accelerates at 0.8 m/s2, the package moves up the incline with the same acceleration.
If the acceleration is greater than 0.8 m/s2, the package will slip on the conveyor belt.
Find, correct to two decimal places, the coefficient of friction µ between the package and the conveyor belt.

I originally got the answer right by doing


But then I thought that didn't look right because the friction is causing acceleration, so I changed it. Turned out my original answer was correct.

Having mulled over it for a while I've come up with an explanation. Is this right? I would like someone to confirm:
The friction between the conveyer belt and package is what keeps the package in sync with the conveyer belt. That is as the conveyer belt moves upwards the friction ensures that the package also moves upwards. That is why in this case the friction can actually cause the motion to keep the acceleration as 0 relative to the belt's acceleration.

The max. friction possible for the surface is reached when a = 0.8 m/s2. Beyond this acceleration, the friction cannot increase more so the package's own acceleration remains at 0.8 m/s2 even if the belt moves faster.

So is that right?

My second question relates to the next part of that question which was pretty standard. However knowing that I was unsure about my value of from the previous question. I worked out the value for m in terms of and then at the final step explicitly said I was subbing in my value for from the previous question and that is how I got my final answer. My question is: will I get full marks for this next part or will I lose one for getting the answer wrong? Essentially, do you get marks for consequential errors?

[Mao]

1. correct. The conveyor belt is under a driving force (motor of some sort) that accelerates the belt itself. Objects on this belt are hence subjected to friction. The object would naturally want to slide against the motion of the belt, hence friction is in the direction of acceleration of the belt, hence the object is under the same acceleration (in a way, the driving force is 'transferred' through friction to the object). This is subjected to the limit on maximum static friction.

2. you do get method marks, but some marks are specifically for correct answers, so you may not be awarded full marks.

[NE2000]

Thanks Mao