Andiio's Noob Questions :D

[Andiio]

A 4.0 kg magpie flies towards a very tight plastic wire on a clothes line. The wire is perfectly horizontal and is stretched between poles 4.0m apart. The magpie lands on the centre of the wire, depressing it by a vertical distance of 4.0 cm. What is the magnitude of the tension in the wire?


An old light globe hands by a wire from the roof of a train. What angle does the globe make with the vertical when the train is accelerating at 1.5 ms^-2?

Thanks!

[adelaide.emily10]

i got the answer!!!!

the total tension caused by the magpie is balanced by both sides of the rope:

t-tensions
2t x sin θ = mg
sin θ = depression / half the length of the string
= 4/ 200
= 0.02
2t x 0.02
=4*10
40/(0.02 x 2)

t = 1000N

[Andiio]

i got the answer!!!!

the total tension caused by the magpie is balanced by both sides of the rope:

t-tensions
2t x sin θ = mg
sin θ = depression / half the length of the string
= 4/ 200
= 0.02
2t x 0.02
=4*10
40/(0.02 x 2)

t = 1000N

Mm, but did you come to the conclusion of 2tsin(theta) being equal to mg through logic?

[adelaide.emily10]

yes because tension is a force in newtons, thus if you look at the inclined plane formulas you'll see that the vertical downward force can be determined as w=f=mg, i think it's in the textbook (heinemann)

[schnappy]

For 1, you just need to realise that the angle is the same in both the forces diagram (Use weight and tension forces in a triangle) and the distances diagram (length of wire, and the 4cm drop)

Check my diagram.

I remember doing the globe... triangle again. Let the vertical = 1... something to do with it being a ratio a maths wiz told me, I never really understood the question. Put 1.5 as your bottom length, hypotenuse unknown and the vertical as 1 and use trig to find the angle.