# 10m girder question (AQA physics summary questions)Watch

#1
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.

0
4 weeks ago
#2
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.
0
#3
(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.

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.
0
4 weeks ago
#4
Don't use tan! 10 is the hypotenuse, so try again with sin.
1
#5
(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!
0
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