A Question About Pulleys and Spring Scales

[mcnealymt]

Homework Statement

The question shows two 5kg masses at rest hanging from two pulleys on opposite ends of a table. The string is massless and the pulley is frictionless.

Homework Equations

They want to know what the spring scale reads (in the middle of the masslessstring).

The Attempt at a Solution

This is what I thought:

Fspring= w=mg
5kg*9.8m/s^2= 49N
This answer was wrong.

Would the answer be 98N since both masses are the same?

 

[Doc Al]

They want to know what the spring scale reads (in the middle of the masslessstring).

Assuming I understand the setup, the spring scale will read the tension in the string.

Fspring= w=mg
5kg*9.8m/s^2= 49N
This answer was wrong.

I'd say that answer was correct. Why do you think it's wrong?

Would the answer be 98N since both masses are the same?

No.

 

[mcnealymt]

Would it help if I posted a picture? My description may not be as specific as the pictorial representation. http://session.masteringphysics.com/problemAsset/1070759/4/07.Q12.jpg

 

[Doc Al]

Would it help if I posted a picture? My description may not be as specific as the pictorial representation. http://session.masteringphysics.com/problemAsset/1070759/4/07.Q12.jpg

That's exactly the setup I had in mind.

 

[mcnealymt]

I must be going crazy . What about the mass of the system? Wouldn't it be 10?

 

[Doc Al]

What about the mass of the system? Wouldn't it be 10?

If by "system" you mean both 5 kg masses together, then sure.

What are the exact questions you are being asked?

 

[mcnealymt]

The string is massless and the pulley is frictionless. The spring sclae reads in kg. What is the reading of the scale?

 

[Doc Al]

The spring sclae reads in kg.

There's the problem: They want the answer in kg, not Newtons.

 

[mcnealymt]

So wouldn't the answer just be five because 49N is 5kg.

 

[Doc Al]

So wouldn't the answer just be five because 49N is 5kg.

Yes, the answer will be 5 kg.

 

[mcnealymt]

Thank you Doc Al. I highly appreciate it.