10m girder question (AQA physics summary questions) Watch

Navboi
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#1
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Hi guys,

while going through some summary questions, this one ended up giving me a bit of a headache.

https://imgur.com/qnw1IM3

For a, I got 6.0 kN (which is correct). but I can't seem to figure out how to prove b.

I've tried taking moments about where the cable connects to the girder, but that didn't work out so I'm completely stumped.

Thanks for your time!
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old_teach
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For part b I'd just look at horizontal and vertical components of the forces (so you need to calculate the angle and the components of the tension force). Don't think part b is a moments question.
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Navboi
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(Original post by old_teach)
For part b I'd just look at horizontal and vertical components of the forces (so you need to calculate the angle and the components of the tension force). Don't think part b is a moments question.
Thanks for the reply man

I've tried taking the components of the tension force like you said. This is what I did:

Angle between the ground and girder is 31 degrees because Tan(angle) = 6/10

therefore the angle between the crane cable and the horizontal plane is 59 degrees (since the cable is perpendicular to girder)

Tension in cable is 6000 N (from part a) , so Vertical componant: 6000sin(59) = 5140 N Horizontal componant: 6000cos(59) = 3090 N

I tried subtracting the vertical component from the weight ( 15 000 N - 5140 N = 9860 N) but it still didn't give me 10.2 kN and I didn't get 3.6 N for the horizontal component either.

I probably misunderstood what you meant, just need a bit of clarification.
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old_teach
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Don't use tan! 10 is the hypotenuse, so try again with sin.
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Navboi
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(Original post by old_teach)
Don't use tan! 10 is the hypotenuse, so try again with sin.
oh my god, that was such a dumb mistake.
thanks so much, everything works out now!
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