Communities

Writing
Writing
Codidact Meta
Codidact Meta
The Great Outdoors
The Great Outdoors
Photography & Video
Photography & Video
Scientific Speculation
Scientific Speculation
Cooking
Cooking
Electrical Engineering
Electrical Engineering
Judaism
Judaism
Languages & Linguistics
Languages & Linguistics
Software Development
Software Development
Mathematics
Mathematics
Christianity
Christianity
Code Golf
Code Golf
Music
Music
Physics
Physics
Linux Systems
Linux Systems
Power Users
Power Users
Tabletop RPGs
Tabletop RPGs
Community Proposals
Community Proposals
tag:snake search within a tag
answers:0 unanswered questions
user:xxxx search by author id
score:0.5 posts with 0.5+ score
"snake oil" exact phrase
votes:4 posts with 4+ votes
created:<1w created < 1 week ago
post_type:xxxx type of post
Search help
Notifications
Mark all as read See all your notifications »
Q&A

Why does tension change from 15 N to 17 N when forces are replaced by weights?

+3
−0

In the image, I have shown how to visualise the system when two masses of weight 15 N and 20 N are hung with the same massless string, and found out that the tension in the string is not 15 N but 17 N.I'm working on a problem involving a pulley system, and I’m confused about how the tension changes when forces are replaced by masses.

Initially, if I apply forces of 20 N and 15 N directly at the ends of a massless string (without any pulley or masses), the tension in the string is exactly 15 N, as expected.

However, when I introduce masses (which exert 20 N and 15 N due to gravity) and place them over a pulley, the tension comes out to be 17 N instead of 15 N. I understand how to solve this mathematically, but why does the tension increase to 17 N when masses are used, and why does the pulley play such a crucial role?

I tried working through the mechanics, but I can't quite grasp the intuition behind the change in tension. I've attached an image of my attempt to solve the problem, but I am struggling to understand why placing the system over the pulley makes this difference.

Please see the attached image.

History
Why does this post require attention from curators or moderators?
You might want to add some details to your flag.
Why should this post be closed?

0 comment threads

2 answers

+1
−0
Initially, if I apply forces of 20 N and 15 N directly at the ends of a massless string (without any pulley or masses), the tension in the string is exactly 15 N, as expected.

No, it's not. You've got a problem without a solution, like dividing by 0. You can't apply 20 N to pull a string and then say the tension is 15 N. It is 20 N by definition of applying 20 N. Of course on the other end you have the same thing, resulting in 15 N. That's an impossible situation for a massless object. It simply can't be.

In the real world, there would be a net force of 5 N in one direction. That divided by the mass of the object would tell you its acceleration. In your case you have an impossible situation since the mass is 0.

why does the tension increase to 17 N when masses are used

Because of motion. The 1.5 kg mass is being accelerated, so more than just its weight of 15 N must be applied to it. This is of course assuming you are on earth where 1 kg weighs about 10 N.

History
Why does this post require attention from curators or moderators?
You might want to add some details to your flag.

1 comment thread

Sir, I understand that the tension would change—that is clear to me—but is there a method to understa... (1 comment)
+0
−0

The increase in tension from 15 N to 17 N happens because when you introduce masses and place them over a pulley, the dynamics of the system change. Without the pulley, forces directly oppose each other, and the tension matches the smaller force. However, adding masses introduces gravitational force, and the pulley allows for the redistribution of forces.

With the masses, the tension must balance both the weight of the masses and the gravitational pull. The pulley plays a crucial role because it changes the direction of the forces and allows the system to reach equilibrium at a new tension value. Essentially, the tension now has to counteract not just the direct forces, but also the effect of gravity on the masses, resulting in a higher tension value of 17 N.

History
Why does this post require attention from curators or moderators?
You might want to add some details to your flag.

0 comment threads

Sign up to answer this question »