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AQA AS Physics A Unit 2 25/05/12 - PHYA2

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Reply 140
Avatar for kingm
kingm
10
Original post by uxa595
What do you lot find harder, mechanics or materials/waves?
I've just finished the spec and am going through papers now.


It's just waves and optics for me. A little bit of a ball ache with all the material to learn. I love mechanics and materials :smile:. Then again I love maths so that probably helps a lot.
Can I have some help with this question (p.107 of the textbook, summary question 3):

A crane is used to raise one end of a 15Kn girder of length 10m off the ground. When the end of the girder is at rest 6m off the ground, the crane cable is perpendicular to the girder as shown in the diagram below:

question picture.gif

EDIT: sorry I've just realised how to find tension. I have another question relating to this question and the above diagram though, which direction would the reaction/support force from the ground be pointed at? - perpendicular with the girder or just vertically upwards?

You see, I've always thought the support force from the ground would act vertically upwards, I don't know if any of you do M2 for maths but when you get a ladder, the support force from the ground on that ladder (when its leaning against a wall) is always vertical. But from the second question (which asks me to find the horizontal and vertical component of the support force), this doesn't appear to be the case :s-smilie:, I'm really confused now.
(edited 11 years ago)
Reply 142
Avatar for uxa595
uxa595
21
Original post by kingm
It's just waves and optics for me. A little bit of a ball ache with all the material to learn. I love mechanics and materials :smile:. Then again I love maths so that probably helps a lot.


Same here. I've already done M1 so much of the mechanics was really easy to learn but with waves in particular, there is just so much wordy stuff to learn.

I need about 86ish (given i got at least 30 ums on my ISA) to get my B and i think that's doable if i ace the mechanics section even though i'm a little off on the rest of it all.
Reply 143
Avatar for amish123
amish123
Original post by internet tough guy
Can I have some help with this question (p.107 of the textbook, summary question 3):

A crane is used to raise one end of a 15Kn girder of length 10m off the ground. When the end of the girder is at rest 6m off the ground, the crane cable is perpendicular to the girder as shown in the diagram below:

question picture.gif


EDIT: sorry I've just realised how to find tension. I have another question relating to this question and the above diagram though, which direction would the reaction/support force from the ground be pointed at? - perpendicular with the girder or just vertically upwards?

You see, I've always thought the support force from the ground would act vertically upwards, I don't know if any of you do M2 for maths but when you get a ladder, the support force from the ground on that ladder (when its leaning against a wall) is always vertical. But from the second question (which asks me to find the horizontal and vertical component of the support force), this doesn't appear to be the case :s-smilie:, I'm really confused now.


How did you find the tension? Using moments? I don't understand how to work out the tension
Reply 144
Avatar for uxa595
uxa595
21
Original post by internet tough guy
Can I have some help with this question (p.107 of the textbook, summary question 3):

A crane is used to raise one end of a 15Kn girder of length 10m off the ground. When the end of the girder is at rest 6m off the ground, the crane cable is perpendicular to the girder as shown in the diagram below:

question picture.gif

EDIT: sorry I've just realised how to find tension. I have another question relating to this question and the above diagram though, which direction would the reaction/support force from the ground be pointed at? - perpendicular with the girder or just vertically upwards?

You see, I've always thought the support force from the ground would act vertically upwards, I don't know if any of you do M2 for maths but when you get a ladder, the support force from the ground on that ladder (when its leaning against a wall) is always vertical. But from the second question (which asks me to find the horizontal and vertical component of the support force), this doesn't appear to be the case :s-smilie:, I'm really confused now.


Use moments about the ground.
So
5(15000sin53.1)=10x
x= 6000N
Original post by amish123
How did you find the tension? Using moments? I don't understand how to work out the tension


Yeah, I only just figured out that tension could be found by taking moments from the girder's point of contact with the ground :tongue:.
Reply 146
Avatar for amish123
amish123
Sorry I just got it, didn't realise you had to take moments from the centre of mass which is 4m
Original post by uxa595
Use moments about the ground.
So
5(15000sin53.1)=10x
x= 6000N


Thank you, I got another question though, why is it that the reaction/support force from the ground is perpendicular to the girder? I mean, shouldn't it just be vertical?
Reply 148
Avatar for kingm
kingm
10
Original post by internet tough guy
Can I have some help with this question (p.107 of the textbook, summary question 3):

A crane is used to raise one end of a 15Kn girder of length 10m off the ground. When the end of the girder is at rest 6m off the ground, the crane cable is perpendicular to the girder as shown in the diagram below:

question picture.gif

EDIT: sorry I've just realised how to find tension. I have another question relating to this question and the above diagram though, which direction would the reaction/support force from the ground be pointed at? - perpendicular with the girder or just vertically upwards?

You see, I've always thought the support force from the ground would act vertically upwards, I don't know if any of you do M2 for maths but when you get a ladder, the support force from the ground on that ladder (when its leaning against a wall) is always vertical. But from the second question (which asks me to find the horizontal and vertical component of the support force), this doesn't appear to be the case :s-smilie:, I'm really confused now.


Original post by amish123
How did you find the tension? Using moments? I don't understand how to work out the tension


I got the tension as 60kN instead of 6kN?? I can't understand what's happened.

Here's what I did:

angle between girder and the floor is Sin(6/10) = 36.87

T = 5 x 15000 cos (36.87) = 60000?????? and not 6000 like in the answer.
Reply 149
Avatar for uxa595
uxa595
21
Original post by kingm
I got the tension as 60kN instead of 6kN?? I can't understand what's happened.

Here's what I did:

angle between girder and the floor is Sin(6/10) = 36.87

T = 5 x 15000 cos (36.87) = 60000?????? and not 6000 like in the answer.


Look at my answer...
Reply 150
Avatar for kingm
kingm
10
OHHHH, i forgot to do 10T = 5 x 15kN cos (36.87).

Silly.
Reply 151
Avatar for kingm
kingm
10
Ha, yeh cheers. We are all typing at the same time!
Reply 152
Avatar for kingm
kingm
10
Original post by internet tough guy
Thank you, I got another question though, why is it that the reaction/support force from the ground is perpendicular to the girder? I mean, shouldn't it just be vertical?


I must admit, i don't understand it either.
Original post by kingm
I must admit, i don't understand it either.


yeah really doesn't make sense to me, the ground is level, how the hell could the support/reaction force be anything other than vertical :confused: :confused::confused::confused::confused::confused:
Reply 154
Avatar for uxa595
uxa595
21
Original post by internet tough guy
yeah really doesn't make sense to me, the ground is level, how the hell could the support/reaction force be anything other than vertical :confused: :confused::confused::confused::confused::confused:


The support force is from the ground dot again facing directly upwards. See if that helps.
Reply 155
Avatar for kingm
kingm
10
Original post by internet tough guy
yeah really doesn't make sense to me, the ground is level, how the hell could the support/reaction force be anything other than vertical :confused: :confused::confused::confused::confused::confused:


I've tried lot's of calculations to get the 3.6N and 10.2kN that they're after as well, but it's just not happening for me. I'm gonna go back to optics now, but will have another stab at it later. Good luck.
Original post by uxa595
The support force is from the ground dot again facing directly upwards. See if that helps.


Sorry, I don't quite get what you mean by that, are you saying that the support force is acting vertically?
Reply 157
Avatar for uxa595
uxa595
21
Original post by internet tough guy
Sorry, I don't quite get what you mean by that, are you saying that the support force is acting vertically?


The support force is from the ground at the point where the girder meets the ground, vertically upwards.
Reply 158
Avatar for uxa595
uxa595
21
The forces you should get are 10200N vertically, and 36000 horizontally (not 3.6 like in the book, that's ridiculous)
Original post by uxa595
The support force is from the ground at the point where the girder meets the ground, vertically upwards.


but the question suggests that the support force is not acting vertically because it then asks me to show the horizontal and vertical components of this support force

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