# Min force to lever to lift weight

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1. Aug 31, 2015

### toforfiltum

1. The problem statement, all variables and given/known data

2. Relevant equations
Concept of couple?

3. The attempt at a solution
First, I calculated the couple about disc, though I'm not sure if I can use this approach. Since 900 N is pulling the string of pulley downwards, I take the couple about disc as 900 x 0.4, which gives me 360 Nm. To balance this couple, the couple by F must be equal to 360. So 1.2 x F = 360 which gives me 300 N as the answer. But I'm wrong, since the answer is B. Where did I go wrong?

2. Aug 31, 2015

### Bystander

Any particular reason for using diameter?

3. Aug 31, 2015

### toforfiltum

I thought the 900 N exerts a couple on the disc, though I'm not sure. I'm just guessing.

4. Aug 31, 2015

### Bystander

What are/would be the two elements of the couple?

5. Aug 31, 2015

### toforfiltum

Two forces acting in opposite directions separated by a perpendicular distance.

6. Aug 31, 2015

### Bystander

What specific forces? And what distance?

7. Aug 31, 2015

### toforfiltum

In this case there's only one force which is 900 N rotating the disc in anticlockwise direction.

8. Aug 31, 2015

### Bystander

What's holding the disc?

9. Aug 31, 2015

### toforfiltum

The lever turning in opposite direction?

10. Aug 31, 2015

### Bystander

What's the "spindle" doing?

11. Aug 31, 2015

### toforfiltum

Transferring the energy?

12. Aug 31, 2015

### Bystander

No sideways forces on it?

13. Aug 31, 2015

### toforfiltum

I totally don't know. From the picture, as the spindle rotates, it turns discs in opposite direction to the wight of 900 N on disc.

14. Aug 31, 2015

### Bystander

What happens when you pull the starter rope on a lawnmower without putting your foot on the mower to hold it in place?

15. Aug 31, 2015

### toforfiltum

The lawnmower will jump?

16. Aug 31, 2015

### Bystander

Any particular direction?

17. Aug 31, 2015

### toforfiltum

I've never used a lawnmower before so I'm not too sure.It will jump away from the person pulling it?