# Homework Help: Mechanical Principles help please.

1. Jun 21, 2008

### crutts

1. The top pulleys of a Weston differential pulley block have diameters of 210mm and 190mm. Determine the effort required to raise a load of 150kg if the efficency of the system is 35%.

What is the work done in overcoming friction when the load is raised through a height of 2.5m?

I did some working out and got:

effort = load/number of pulleys = 150/2 = 75 (what units?)

and work done= (150x9.8) x 2.5 = 3677.6 N/M

I did 150x9.8 to get newtons.

Is this correct?

Cheers Joe

2. Jun 21, 2008

### series111

you must work out using these formulas

i only no this as i did the same question in class

you need to work out the velocity ratio of the machine first

vr = 2 d1/(d1-d2)

then the mechanical advantage

effiency of the system = mechanical advantage/ velocity ratio x 100 %

you need to transpose this

effort of the machine = load / mechanical advantage

this is for the first part

second part is

wd = force x distance moved

then effiency of the system = work output/work input

you also need to transpose this
i no how you feel this is a hard subject and i am also finding it hard !!!!

3. Jun 21, 2008

### crutts

I take it d stands for diameter? Should it be this or should it be circumfrence?

Joe

4. Jun 21, 2008

### series111

d is the diameter of each pulley

5. Jun 21, 2008

### crutts

Should I use diameter or circumfrence? As i seem to remember circumfrence being mentioned in class.

6. Jun 21, 2008

### series111

when i calculated these i used the diameters and the answers were correct compared to the answers i had.

7. Jun 21, 2008

### crutts

Ok cheers.

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