Chestermiller said:
I apologize. It's just that you seemed so cavalier about it.
I think if you reconsider answering my questions in post #14, you will find that it was not as useless as you originally thought, and that it will actually help you solve your problem. It is very close to what gneill was trying to help you do.
Again, sorry for not having more patience.
Chet
I apologise in turn. I was cavalier about it, because I thought that this was getting seriously overblown for a 1 mark. But I've just learned that this question has been taken from a practice guide, not an exam paper. It would probably pass for more in an exam.
Anyway, I thought about this some more. (Note to self: it helps to have a paper to scribble on rather than wrestling everything out in the mind.)
The distance traveled in the first 2 seconds = 0.5 * a * 2
2 = 2a
The distance traveled in the first 1 second = 0.5 * a * 1
2 = 0.5a
The first distance minus the second distance should give me 15m.
2a - 0.5a = 1.5a = 15m. a = 10 ms
-2.
Therefore the distance traveled in each of the 5 seconds should go something like:
S
1 = 5m
S
2 = 15m
S
3 = 25m
S
4 = 35m
S
5 = 45m.
The sum of all of these is 125m, which is consistent with s = 0.5 * 10 * 5
2 = 125.
So I'd say the answer is 45m. The mark scheme disagrees with me and says it's 75m. Yet I took this to both of my physics teachers, separately, and they agreed with my line of reasoning and said the mark scheme could well be wrong -- again, this was just a practice set taken from online.
Even if I'm correct, I have some doubts:
1) If the acceleration is in fact 10, why wouldn't the distance covered in the first second from rest be 10? Acceleration is the change in velocity per second and the car starts at rest.
2) Wouldn't the distance covered in the first two seconds = 3a? a would be covered in second 1, a + a would be covered in second 2. Maybe I'm making the same fallacy in this and the above question.
3) If we consider the first two seconds, v = 15m/s, u = 0 and t = 2s. (15 - 0) / 2 = 7.5ms
-2. I'd guess the error is in taking 15m/s for v, but I'm not sure why.
Thanks to both of you for getting me this far, and thanks to anyone who has the patience to respond to this.