9.3 Torque Torque Puzzle |
| Instructions: Click on the "Start Simulation" button to open the applet window. To start the applet, select the direction of rotation of the "spool." "Left" refers to rotation in the counterclockwise direction and "right" refers to rotation in the clockwise direction. The spool might not rotate in the direction you have expected! The aim is to explain the result. You may change the slope of the string by positioning your mouse cursor where the string touches the green region (pulley attachment), clicking on the left mouse button, and dragging up or down. You can see the line of action of the force along the string and the axis of rotation by enabling the "show" checkbox. Navigation: You can come back to this window by pressing the "Close Simulation" button on the bottom frame of the utility. |
| Explanation: As discussed in section 9.3, the magnitude of the torque is equal the product of the magnitudes of the forces on the lever arm. The direction of the torque depends on the direction it tends to rotate the object. You can identify the direction of rotation by examining the relationship of the line of action of the force to the axis of rotation. A judicious choice of the axis of rotation makes the task much easier. For instance, this problem can be easily understood if we consider torques relative to the point of contact with the ground (instead of center of mass). Changing the slope of the string results in lines of action at different orientations relative to the axis of rotation; this in turn results in different directions of rotation. There is even a string inclination that will result in no rotation. |
| Source: Fu-Kwun Hwang Go to the web site and link to the Online Library for further information. |