# Classical mass rope mass problem

1. Sep 20, 2011

### kasperrepsak

hey,

This question is about a system where one mass is on an incline and another hanging of it. They are connected with a rope.. Its a classical mass rope mass problem. My reasoning was that since the mass hanging of the cliff is pulling on the rope with a force of mg, the rope will be pulling the mass on the cliff with that same force.. so T (Tension force) on the mass on the cliff = weight of mass hanging of the cliff.
Using this reasoning i ended up with bad results for acceleration and tension force for the system. In books other approaches to these problems are used.. but i would like to know where my reasoning is wrong? Does it have to do with part of the force being neutralised by the edge of the cliff? (or in some problems a massless/frictionless pulley). Ofc we totally disregard friction and mass of rope in these problems.

thanks

2. Sep 20, 2011

### Hootenanny

Staff Emeritus
Welcome to Physics Forums.
Let's assume that the mass-rope system is initially at rest. If the rope exerts a force mg on the mass and gravity exerts a force mg in the opposite direction, will the mass ever move?

3. Sep 20, 2011

### kasperrepsak

which mass r ye talking about?

4. Sep 20, 2011

### Hootenanny

Staff Emeritus
The mass hanging off the cliff.

5. Sep 20, 2011

### kasperrepsak

ofc it wont.. obviously there is something wrong here in my reasoning.. but could you tell me where my reasoning is wrong as opposed to showing me what weird consequences that would have?

6. Sep 20, 2011

### Hootenanny

Staff Emeritus
That is precisely where your reasoning is wrong. You have assumed that the tension in the rope is equal to mg, which it isn't.

7. Sep 20, 2011

### kasperrepsak

aha ok, thank you :)