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Isn't the horizontal force zero?

http://mei.org.uk/files/papers/m109ju_livn.pdf
Question 5 ii please help!
Original post by Kalabamboo


No. Why do you think it might be? Read the question again carefully.
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Kalabamboo
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Original post by ghostwalker
No. Why do you think it might be? Read the question again carefully.

because a =0i - 2j so a=-2j
F=ma = 1.5 x -2j =-3j N
This is vertical cause of j
also there isn't any horizontal acceleration so no horizontal force that's why i thought the horizontal force is zero
I am basically in denial with the question like i think the question is wrong but it isn't really
(edited 5 years ago)
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Kalabamboo
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Original post by Kalabamboo
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Did you do as @ghostwalker suggested and re-read the question? i and j are defined as perpendicular horizontal unit vectors.
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Kalabamboo
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Original post by old_engineer
Did you do as @ghostwalker suggested and re-read the question? i and j are defined as perpendicular horizontal unit vectors.

Oh! So sorry for asking this but how can you have perpendicular horizontal unit vectors-how does that work? It's just weird when I think about it cause I am so used to the i being horizontal and j being vertical and k being off the plane
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Original post by Kalabamboo
Oh! So sorry for asking this but how can you have perpendicular horizontal unit vectors-how does that work? It's just weird when I think about it cause I am so used to the i being horizontal and j being vertical and k being off the plane


The examiner's report says "many ignored the definition of the direction of the j direction, assumed it was horizontal and so said there was zero horizontal force", which would imply that the j direction is not supposed to be horizontal. More basically, if two vectors are horizontal then their directions are both the same (the horizontal direction), so they cannot be perpendicular. Thus the entire question and discussion surrounding it seems obscure and incoherent. @ghostwalker @Notnek can you clarify for both me and the OP please?
Original post by Kalabamboo
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Original post by Prasiortle
The examiner's report says "many ignored the definition of the direction of the j direction, assumed it was horizontal and so said there was zero horizontal force", which would imply that the j direction is not supposed to be horizontal. More basically, if two vectors are horizontal then their directions are both the same (the horizontal direction), so they cannot be perpendicular. Thus the entire question and discussion surrounding it seems obscure and incoherent. @ghostwalker @Notnek can you clarify for both me and the OP please?


For example, one vector could point Noth, and the other East, Both are horizontal. And they are perpendicular to each other.

Edit:
Examiners' report is incorrectly worded and should say "assumed it was vertical". It clearly states in the question that i,j are horizontal, and the mark scheme specifes the horizontal forces as being in the j direction.
(edited 5 years ago)
Original post by ghostwalker
For example, one vector could point Noth, and the other East, Both are horizontal. And they are perpendicular to each other.


How are they both horizontal? "Horizontal" in ordinary parlance means pointing this way <--------------->, i.e. east or west.
Original post by Prasiortle
How are they both horizontal? "Horizontal" in ordinary parlance means pointing this way <--------------->, i.e. east or west.


Not to me, it doesn't.

I edited my previous post to include more detail.
Original post by ghostwalker
Not to me, it doesn't.

I edited my previous post to include more detail.


The OED defines horizontal as "parallel to the plane of the horizon; at right angles to the vertical". So I still cannot fathom how any definition of horizontal can include both north and east.
Original post by Prasiortle
The OED defines horizontal as "parallel to the plane of the horizon; at right angles to the vertical". So I still cannot fathom how any definition of horizontal can include both north and east.


Any vector parallel to the plane of the horizon is automatically perpendicular to the vertical.

A Northerly vector fits this description, as does an Easterly.
Original post by ghostwalker
Any vector parallel to the plane of the horizon is automatically perpendicular to the vertical.

A Northerly vector fits this description, as does an Easterly.


Yeah I'm useless at visualising things in 3D space so all this lines/planes stuff doesn't really help. I appreciate that you tried your best though.
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Original post by Prasiortle
Yeah I'm useless at visualising things in 3D space so all this lines/planes stuff doesn't really help. I appreciate that you tried your best though.

Horizontal unit vectors are vectors that lie in a horizontal plane. Then vertical means straight up, perpendicular to the plane.

"Horizontal unit vectors" is a common thing to see in A Level mechanics questions e.g. in Edexcel you often see this:

[i and j are horizontal unit vectors due east and due north respectively]

I much prefer this to saying, "horizontal, perpendicular unit vectors" like this question has done.
Original post by Prasiortle
Yeah I'm useless at visualising things in 3D space so all this lines/planes stuff doesn't really help. I appreciate that you tried your best though.


It's impossible to know what technique would work for you, but one thing you could try is, rather than see it "out there", put yourself in the picture.

I.e. Imagine yourself standing facing North. One vector is straight ahead, going North. Another is going off to your right, going East. Both are horizontal, and are prependicular to each other. And perpendicular to the vertical, going straight up.
(edited 5 years ago)
Thank you guys!:smile:

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