Unit 6B - Help with 2Qs please

1)

http://pastpapers.edexcel.com/conten...e_20120307.pdf

In this paper, Q4bii) says:
"State why this is a suitable instrument for this measurement."

For which the mark scheme says:
"0.01 s « human reaction time (accept 0.1 s) "

But people were saying that if the precision of a stopclock is less than reaction time, we have to use the human reaction time as the absolute uncertainty of the measurements, so regardless of the values we get, 0.1 would be the uncertainty... So the mark scheme doesn't make sense to me. Can someone help please?

2)

http://pastpapers.edexcel.com/conten...e_20120307.pdf

For Q1c, I really don't understand the mark scheme.

It says:

Measure the coiled length of spring with callipers and divide by the number of
loops (do not credit use of micrometer screw gauge)
Sensible precaution for their choice of instrument
eg repeat and average
measure in different places/orientations

But measuring the coiled length of spring with calliper and then dividing by the number of loops would actually give me the width of each spring, right? And what we're looking for is the diameter ..

Reply 1

Joinedup

Original post by jtbteddy

first the uncertainty is not limited by the stopclock, it's limited by human reaction time. as long as you rely on a human to trigger the stopclock you wouldn't get less uncertainty by using a stopclock with more resolution... so that stopclock is as good as any for this task.

second it appears from the photo that the turns of the spring are touching, so the pitch (distance between centre of one turn and the next) is equal to the diameter of the spring wire. you could ensure the coils were touching during measurement by lightly squeezing them.

Reply 2

jtbteddy

Original post by Joinedup

I agree with the stopwatch explanation, buut then why does the MS say that its suitable because the precision of the stopwatch is 0.01 which is less than human reaction time? How does being less than the reaction time make it suitable?

I don't understand the explanation for the 2nd one :frown:

( the diameter is the width of the spring, why does the mark schemeimply we have to measure the length? Please explain it clearly, I'm unable to picture it :frown:

Reply 3

Joinedup

Original post by jtbteddy

( the diameter is the width of the spring, why does the mark schemeimply we have to measure the length? Please explain it clearly, I'm unable to picture it :frown:

markschemes aren't model answers as such, they're just there to remind the markers what to award points for.

---

This section asks...

"(c) She now wants to measure the diameter of the wire that makes the spring."

it is a question about the wire the spring is made out of - not a question about the diameter of the coils.

I've indicated the measurement I think is required as x

(edited 9 years ago)

Reply 4

jtbteddy

Original post by Joinedup

If I'd measure that distance and divide by the number of loops, I'd get the thickness of each loop, and the question asks for the diameter .. I'm confused :frown:

Reply 5

Joinedup

Original post by jtbteddy

If I'd measure that distance and divide by the number of loops, I'd get the thickness of each loop, and the question asks for the diameter .. I'm confused :frown:

The wire in the spring looks like it's circular in cross section. the thickness of a circular wire is the same thing as its diameter.

Reply 6

jtbteddy

Original post by Joinedup

The wire in the spring looks like it's circular in cross section. the thickness of a circular wire is the same thing as its diameter.

http://www.youtube.com/watch?v=XvvealZwfhk

I even watched this video but I didn't get it.

Even in the diagram you posted, the diameter of the coil of wire looks a lot bigger than the thickness of each coil.. so I can't understand

I can see that it's circular in cross-section, but I don't then understand how the thickness can give me the diameter

OHHH!!! I GOT IT :biggrin:

D they want the diameter of the wire!!! I got it, thanks so muuuch!! And sorryy!!!

(edited 9 years ago)

Reply 7

Joinedup

Original post by jtbteddy

The earlier part of the question concerns the diameter of the coil spring, d in the diagram.

the later part of the question concerns the diameter of the wire the spring is made out of...

it's about two separate diameters,

1. the diameter of the coil, d in the first picture
2. the diameter of the wire the coil is made from - shown in the second picture.

Reply 8

jtbteddy

Original post by Joinedup

The earlier part of the question concerns the diameter of the coil spring, d in the diagram.

the later part of the question concerns the diameter of the wire the spring is made out of...

it's about two separate diameters,

1. the diameter of the coil, d in the first picture
2. the diameter of the wire the coil is made from - shown in the second picture.