Stuck on MCQ HELP

1) https://imgur.com/M0NgXXN

2) https://imgur.com/jDmhIQF

1st one is relating to percentage uncertainty and I don't know which measurement gives the highest uncertainty?

I was thinking it has something to do with the measuring apparatus used to take the required reading values to find the wire's resistivity.

And since they said they used a ruler to work out the wire's length. I thought the ruler is the least sensitive instrument as to using something like a micrometer or a digital multimeter to take reading. Does that mean the length measurement gives the largest uncertainty?

For 2 do I have to make use of a triangle of vector forces and check which cannot give a resultant force of zero for all options given in the Q. I can't see how I would do this as there is no angles given in the Q?

Any help would be really appreciated. Thanks!

Reply 1

username3442196

For 1)
The ruler would be quite good for % uncertainty, as, even if it is not very accurate, the length is large, so % uncertainty would be small.
In this question, you are told both length and diameter are 1%.
What they are testing is that you need area which depends on diameter squared, so you double the % uncertainty.
I'll try 2) next :smile:

Reply 2

username3442196

2)(I wish it said 'forces of magnitude' as there's no indication of direction.)
I guess it is asking 'can you add these three forces to make a closed triangle'.
A looks like a 3 - 4 - 5 right angle triangle.
B looks like an equilatrial triangle.
C I can imagine making a triangle with two 10s and a 2.
D could you make a closed triangle with these forces?

Reply 3

Yatayyat

Thanks it makes sense to me now why the first Q has to be answer B as % uncertainty doubles for the diameter given cross sectional area is (pi*d^2) / 4

For 2), I can see now from your reply that you could draw out all the possible triangles that form from all the 4 given cases

So just to see again

A) right angle triangle
B) equilateral triangle
C) a isosceles triangle could be made since 2 of the 3 forces are the same

But for D I am thinking a force triangle can’t be made as one the forces or “side lengths” is either too big or smaller to be joined with the other forces to make a triangle.

It sort of make sense to me but does that then imply that any set of three forces that can be joined in as a triangle are always in equilibrium or does direction also play into this?

(edited 5 years ago)

Reply 4

username3442196

Yes, you can't make D into a triangle, the 3 and 6 can't match the 10N.
If you add three forces you add the vectors tip to tail, if they form a closed triangle, then the resultant is zero.
That's what the question's testing. Obviously, the directions of the forces are relevant, but this question just said 'is it possible'.
It must be fun dreaming up A level questions!