# Finding forces acting on spring/pulley system

## Homework Statement

There is a spring attached to a hanging block by a rope over a pulley, with a stop on the other end. Using basic physic principles, determine the relationship between spring extension and weight of the block.

F=-kd
w=mg

## The Attempt at a Solution

I know that if the system is in equilibrium, the sum of forces acting on the block/spring should be zero. Also, since the force of the rope on the block and the force of the rope on the spring are third law pairs, they should be equal in magnitude but in opposite directions. Not quite sure where to go from here though.

## Homework Statement

There is a spring attached to a hanging block by a rope over a pulley, with a stop on the other end. Using basic physic principles, determine the relationship between spring extension and weight of the block.

F=-kd
w=mg

## The Attempt at a Solution

I know that if the system is in equilibrium, the sum of forces acting on the block/spring should be zero. Also, since the force of the rope on the block and the force of the rope on the spring are third law pairs, they should be equal in magnitude but in opposite directions. Not quite sure where to go from here though.

Not like this.

Hint: First consider the axle of the pulley as the system and not the whole pulley. Consider equilibrium of spring and axle of pulley. What equation can you make for this ?

SammyS
Staff Emeritus
Science Advisor
Homework Helper
Gold Member

## Homework Statement

There is a spring attached to a hanging block by a rope over a pulley, with a stop on the other end. Using basic physic principles, determine the relationship between spring extension and weight of the block.

F=-kd
w=mg

## The Attempt at a Solution

I know that if the system is in equilibrium, the sum of forces acting on the block/spring should be zero. Also, since the force of the rope on the block and the force of the rope on the spring are third law pairs, they should be equal in magnitude but in opposite directions. Not quite sure where to go from here though.
Hello uglies1992 . Welcome to PF !

The force of the rope on the block and the force of the rope on the spring are not a third law pair .

The force of the rope on the block and the force of the block on the rope is a third law pair .

Similarly, the force of the spring on the block and the force of the spring on the rope is a third law pair .

There are also the force of the rope on the pulley and the force of the pulley on the rope to consider.

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