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# M3 edexcel Further dynamics watch

1. Please tell me the book is in error.

This is the question 6c and their answer. I am so rubbish at mechanics that I doubt everything I do but shouldn't the answer be zero?
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3. (Original post by maggiehodgson)
Please tell me the book is in error.

This is the question 6c and their answer. I am so rubbish at mechanics that I doubt everything I do but shouldn't the answer be zero?
Sketch the graph - you'll want the absolute area.

Look at ghostwalkers post.
4. (Original post by maggiehodgson)
Please tell me the book is in error.

This is the question 6c and their answer. I am so rubbish at mechanics that I doubt everything I do but shouldn't the answer be zero?
it will move you backwards and forwards for equal length of time
(sketch the graph of cos4t)

MAGNITUDE of impulse is the total area which is 1 but the workings there are not correct
Need to integrate from 0 to pi/8 then pi/8 to pi/4

the analogy I can give you is the displacement is zero but the distance is not
5. As Zacken said.

Edit: Just seen TeeEm's response now.
6. (Original post by maggiehodgson)
Please tell me the book is in error.

This is the question 6c and their answer. I am so rubbish at mechanics that I doubt everything I do but shouldn't the answer be zero?
Won't let me attach the picture in my other post:

7. (Original post by ghostwalker)
As Zacken said.

Edit: Just seen TeeEm's response now.
No reply for 35 minutes, then three replies all at once. The workings of this forum.
8. (Original post by Zacken)
No reply for 35 minutes, then three replies all at once. The workings of this forum.
Well short of the record.

Having looked at the question again, I think that they've used the term "force of magnitude F" erroneously, and it should just be "force of F Newtons"

My thinking is that magnitude of impulse is then just the straight integration from start of end; then take the modulus.

So, I still go with 0 as the answer.

If we're going to mean the area between the curve and the axis as always being positive regardless of whether it's above or below the axis, then we also need to revisit part a, where F is initially going negative.
9. (Original post by ghostwalker)
x
I'll admit to not being well versed in this area, but I think I can see where you're coming from and I do agree with you. Afterall, we say the magnitude of the displacement is 0 even though it has a non-zero distance.

I'll defer to your expertise here and delete my post.
10. (Original post by Zacken)
I'll admit to not being well versed in this area, but I think I can see where you're coming from and I do agree with you. Afterall, we say the magnitude of the displacement is 0 even though it has a non-zero distance.
Displacement is a vector quantity, as is impulse.

The only issue with the question that I see is the use of the word magnitude when refering to the force. Which would imply the use of , rather than , and I think that's just an error in the wording of the question.

I'll defer to your expertise here and delete my post.
I claim no great expertise, and your post is fine, IMO.

Edit: Posts #2 and #4 together now look rather odd - verging on an infinite loop
Edit2: It's changed again!
11. (Original post by TeeEm)
it will move you backwards and forwards for equal length of time
(sketch the graph of cos4t)

MAGNITUDE of impulse is the total area which is 1 but the workings there are not correct
Need to integrate from 0 to pi/8 then pi/8 to pi/4

the analogy I can give you is the displacement is zero but the distance is not
THANKS.

Such a lot to learn.
12. (Original post by ghostwalker)
Well short of the record.

Having looked at the question again, I think that they've used the term "force of magnitude F" erroneously, and it should just be "force of F Newtons"

My thinking is that magnitude of impulse is then just the straight integration from start of end; then take the modulus.

So, I still go with 0 as the answer.

If we're going to mean the area between the curve and the axis as always being positive regardless of whether it's above or below the axis, then we also need to revisit part a, where F is initially going negative.
That's gone over my head for the moment but I will try to understand it in due course.
13. (Original post by Zacken)
Won't let me attach the picture in my other post:

This I get but I would never have thought of it.

Many thanks.
14. (Original post by ghostwalker)
Displacement is a vector quantity, as is impulse.

The only issue with the question that I see is the use of the word magnitude when refering to the force. Which would imply the use of , rather than , and I think that's just an error in the wording of the question.

I claim no great expertise, and your post is fine, IMO.

Edit: Posts #2 and #4 together now look rather odd - verging on an infinite loop
Okay - so, to clear this up (for myself and the OP):

It's normally magnitude of an impulse is given by where is the force.

However, if you're only given as being the magnitude of the force, then your impulse is

Is that it?
15. (Original post by Zacken)
Okay - so, to clear this up (for myself and the OP):

It's normally magnitude of an impulse is given where is the force.

However, if you're only given as being the magnitude of the force, then your impulse is

Is that it?
IMO.

Caveat:
The first form gives the impulse. If you want its magnitude, drop any sign after integrating.

The second form is something I've never seen nor would I expect to, but is my attempt to interpret the "magnitude of force" statement, which I think is erroneously in the question.
16. (Original post by ghostwalker)
IMO.

Caveat:
The first form gives the impulse. If you want its magnitude, drop the sign after integrating.

The second form is something I've never seen nor would I expect to, but is my attempt to interpret the "magnitude of force" statement, which I think is erroneously in the question.
Fair enough - we can chalk this down to improper textbook wording then.

(I'm going to edit in the modulus signs into my post)

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