1. changing rotational motion
Torque
changing rotational motion
§ 10.1–10.2

2. Torque An influence causing angular acceleration
angular analogue of Newton’s second law:
tnet = Ia
t = torque
I = moment of inertia
a = angular acceleration
units?

3. Torque Definition t = Fl units? F = tangential component of force
l = lever arm or moment arm
units?

4. Lever Arm Shortest distance from line of action to point of interest
Source: Young and Freedman, Figure 10.2

5. Poll Question
Force P is applied to one end of a lever of length L. What is the magnitude of the torque about point A?
PL sin q.
PL cos q.
PL tan q.
PL sec q.
PL cot q.
PL csc q.
Source: Young and Freedman, Test Your Understanding §10.1

6. Torque Vector Turning influence = torque = radius  force = r  F
Units: Nm (not J)
Source: Hewitt, Conceptual Physics

7. Poll Question
What is the direction of the torque about point O from force F1?    
Source: Young and Freedman, Figure 10.2

8. Poll Question
What is the direction of the torque about point O from force F2?    
Source: Young and Freedman, Figure 10.2

9. Whiteboard Work
A 10,000-N truck is stalled 1/4 of the way across a 100-m bridge.
What torque does its weight apply about the far support? r

10. Whiteboard Work
What upward force must the near support provide to cancel the truck’s torque about the far support? r F

11. Whiteboard Work
What upward force must the far support provide to support the weight of the truck?
Hint: Several ways will work:
forces on the bridge
torques about the near support r F