2008 Module 2 Layout Calculation Attempt

[reuben]

I am inviting guidance and correction on the attached attempt on the calculations part of the 2008 paper; especially with a view to make it achievable within 18 minutes for a novice practising to finish the paper in 90 minutes.

Feedback

General:

remember to declare your variables
well laid out

Part a

A good approach: work out everything at the start, for all given speeds, because you can't yet predict which outcomes you might need later in the process, also it's much quicker to do them all when you've got the formulae in your head / the conversion factors in the calculator.

Laying out your answers in a tabular form as you have done is also helpful. You could probably save a bit more time by just writing down the generic formula you used for each step, instead of writing it out in full with all the actual figures - the examiner can see that you're just applying the same formula to different input figues.

Part b(i)

I always recommend a very quick sketch diagram here, showing the 2 successive trains and the relevant quantities. this shows that you understand what you're doing, instead of just plucking a formula out of the air; it also helps to declare your variables.

b(ii)

Again, good to draw a diagram here.

The graph must show both trains - if you just show one train, it doesn't demonstrate the time relationship between two trains. also must make clear if it's showing the back or front of each train (you're interested in the back of Train 1 and the front of Train 2.)

t3, i think is intended to relate to the dwell time - if this is the case, this phase of the journey should be truly vertical on the graph, not the vaguely angled line shown

error 1:

The question requires Ht for fast following stopping - if this is the case, you need to allow for Train 1 to accelerate back to full speed rather than just clearing the overlap - it continues to delay the train behind it until it reaches full speed

error 2:

Your calculation for t4 is wrong, it should allow for acceleration rather than travelling at full speed.

error 3

you need to allow for the train length as well as the overlap length; you've only allowed for one of these, I can't tell which because they're both 180m

error 4:

if you're using the "extra time" method, which i recommend because it's very quick, you must account for the time Train1 would have taken anyway, if travelling at full speed instead of stopping. Your calculated values for t2 and t4 are the absolute times, not the extra times compared to non-stopping. Your method partly deals in absolute times, and partly deals in relative times, the two are incompatible.

The actual Textra value should be (2 * (38.89/0.5) )/2 + 30 = 107.8 seconds.

Note:

the first factor of 2 accounts for the extra time during both deceleration and acceleration, you can do this because the acceleration values (0.5) are the same magnitude, even though opposite sign.

The second factor of 2 accounts for the time the train would have taken anyway, if travelling at full speed.

these two factors of 2 cancel each other out, so your answer is co--incidentally about right!

because of error 1, you're not actually interested in the train accelerating through the overlap only, so errors 2 & 3 no longer apply. You've already accounted for train and overlap length at full speed in part b(i), so under the "extra time" method, you don't need to count them again.

Overall, a good answer, most candidates would not attempt b(ii) at all.

hope this helps