# Homework Help: Advantage of doorknobs with long handles

1. Sep 13, 2007

### deamonvivid

Torque question.
What is the advantage of of doorknobs with long handles instead of standard round ones in terms of decreased force requirements?

2. Sep 13, 2007

### learningphysics

What is the definition of torque?

3. Sep 13, 2007

### deamonvivid

the rotational motion of an object

4. Sep 13, 2007

### l46kok

True.. more specifically, how is that rotational motion caused?

What is the equation for torque?

5. Sep 13, 2007

### learningphysics

There is a definition of torque that is related to force and the distance from the rotation-axis that the force was applied...

6. Sep 13, 2007

### deamonvivid

rFsin()

7. Sep 13, 2007

### deamonvivid

the lever arm is the perpendicular distance from the axis of rotation to the line of action of the force

8. Sep 13, 2007

### deamonvivid

thats what you talking about right

9. Sep 13, 2007

### learningphysics

yes, exactly... so suppose I apply a certain torque with a certain force F1 at a radius r1... but then I double my r from r1 to 2r1... how much force do I need for the same torque as before (keeping the same angle)...

10. Sep 13, 2007

### learningphysics

yes exactly.

11. Sep 13, 2007

### deamonvivid

twice the force as well

12. Sep 13, 2007

### deamonvivid

oops, a smaller force

13. Sep 13, 2007

### learningphysics

exactly... only half the force is needed for the same torque...

14. Sep 13, 2007

### l46kok

Right. Apply this knowledge to your practical usage of doorknobs, the lever one VS. the turning one.

15. Sep 13, 2007

### deamonvivid

but i was thinking wasn't the axis of rotation always been considered was at the hinges

16. Sep 13, 2007

### deamonvivid

Ok! got it now.

17. Sep 13, 2007

### learningphysics

cool.

18. Sep 13, 2007

### deamonvivid

Thanks.

19. Sep 13, 2007

### l46kok

lol.. real time replying is awesome

20. Sep 13, 2007

### learningphysics

no prob.

21. Sep 13, 2007

lol. yup.