How Do You Calculate the Tension in a Wire Supporting a Sign?

[Lana Elcic]

Homework Statement

[/B]
A sign with a mass of 27 kg is hanging in two wires both making an angle of 25 degrees with the vertical. Find the tension in one of the wires. Give your answer in Newtons (N) and with 3 significant figures.

Homework Equations

Ftens=gravity/sin(x) ?

The Attempt at a Solution

Force of gravity is 49N, half (finding one wire) is 24.5. The 25 degrees is with the vertical cos(25).

24.5N/cos(25)

This equals 27.0N

The answer should be in the hundreds.
I'm confused on whether I should be using the mass at all, or if I should be converting to Newtons, which would be 264.78, but the division of gravity is already converted to Newtons so I wouldn't see why I'd have to convert it again.

 

[Simon Bridge]

How did you determine the force of gravity?

 

[Lana Elcic]

How did you determine the force of gravity?

I read it on a guided problem. That it was the standard.

 

[haruspex]

I read it on a guided problem. That it was the standard.

That 49N is a standard weight? Standard for what?
The sign has a mass of 27kg. What is its weight?

 

[Lana Elcic]

That 49N is a standard weight? Standard for what?
The sign has a mass of 27kg. What is its weight?

Standard for Gravity is what I read.
Weight. That should be 9.8(27) right? 264.6

 

[haruspex]

Standard for Gravity is what I read.
Weight. That should be 9.8(27) right? 264.6

Right. So how did you get 49N before?

 

[Lana Elcic]

Right. So how did you get 49N before?

I don't know it said on physics classroom that it was standard gravity in Newtons. So if I plug that in instead of 49 I should be correct?

 

[haruspex]

I don't know it said on physics classroom that it was standard gravity in Newtons. So if I plug that in instead of 49 I should be correct?

Yes. Please post the physics classroom link.

 

[Simon Bridge]

It is important to make sure the theory you are using makes sense for the problem you are trying to solve.

You have probably noticed that different things may have different weights in the same gravity... does it make sense to use, as you did above, a single standard weight for all objects? If the wire had to support a car, or a twig, wouldn't you expect the tension to be different? Asking this sort of question is a core skill in science.