# How Pulleys Affect Force

1. Oct 6, 2008

### keemosabi

1. The problem statement, all variables and given/known data
A 100 gram mass is hanging vertically from a spring scale and exerts a certain force. Another 100 gram mass is set up in the following situation (there is a pulley, spring scale, mass, and ring stand holding up the spring scale):
Do the weights on each setup exert equal forces on the spring scale, neglecting the weight and friction of the pulleys?

2. Relevant equations

3. The attempt at a solution
I thought that I would have to separate the second one in to vectors to figure out if there is a difference.

2. Oct 6, 2008

### LowlyPion

What is the tension in the string that holds the weight in each situation?

In the one the tension goes straight up to the scale.
In the other if there is a difference in the tension at the weight and the tension at the scale, what will the weight be doing? Will it be in motion?

3. Oct 6, 2008

### keemosabi

When we did the experiment the only measurement we were told to make was reading the spring scale. In the first situation the spring scale read .98 N, while in the second situation the scale read .8N. However, my teacher said that the readings should have been the same the same in both situations, but that doesn't make sense.

4. Oct 6, 2008

### LowlyPion

That's where friction and the real world intrudes on things.

Oil the pulley next time?

5. Oct 6, 2008

### keemosabi

So the measurement in Newtons should have been the same both times? Why is that? Don't you need to break down the force that the 100 g mass exerts into vectors to get the x any y directions of force?

6. Oct 6, 2008

### LowlyPion

The only direction of real interest with tension is the direction of the cable.

7. Oct 6, 2008

### keemosabi

Wouldnt the mass being hung at an angle from the spring scale decrease the amount of force it exerts on the scale in horizontal direction?

8. Oct 6, 2008

### LowlyPion

The weight is hanging straight down.

What angle?

The force of the weight is along the tension translated back to the scale.

If the scale is supporting with less force than the weight ... is the cable growing, is the weight in motion? Otherwise the sum of the forces are 0.

9. Oct 6, 2008

### keemosabi

The angle between the weight and the spring scale. The hypotenuse of the string in the vertical and horizontal directions.

So the force of the weight is translated up through the cable, and then horizontally to the scale? Making the force that the weight exerts the sum of the tension of the strings in the vertical and horizontal directions?

The system is at equilibrium.

Thank you for your help thus far.

10. Oct 6, 2008

### LowlyPion

Sorry it doesn't work that way.

11. Oct 6, 2008

### keemosabi

So then why do you not need to break down the weight's pull into vectors?