2b and 6c
http://www.vcaa.vic.edu.au/Documents/exams/physics/2013/physics_examrep13.pdf
Don't get 6c AT ALL. I get how they have assumed the spring potential to be 0 N as incorrect, but how do I prove that it is not 0?
2b:Look at the two components of the system separately.
So for m1, we've got the gravitational force working in one direction and tension in the string working in the other:
For m2, the only force acting on m2 is the tension in the string which is pulling it to the left:
The tension in a string is always the same, so we can sub in m2a=T into the first equation to get:
Now we can sub a back into the second equation:
So this is the simultaneous equation method, where you look at the forces acting on each block - alternatively you can look at the whole entire system as the whole, imagine both blocks are attached together and there's only a force of gravity acting on it, find the acceleration then look at individual blocks (the method in the examiner report).
6c:So they've assumed that when the mass is at position Q, that this will be a point of 0 spring potential which is incorrect. Because the spring has already been stretched by 0.5m, it is incorrect to say there's no energy there. So to prove that it's not 0, do a calculation and show that
- so there is in fact 1.25J stored in the spring, not 0. Furthermore you could show that to properly calculate the spring potential, you'd need to do
.