1. Torque Physics 6A Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB

2. Torque
Torque is what causes angular acceleration (just like a force causes linear acceleration)
Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB

3. Torque
Torque is what causes angular acceleration (just like a force causes linear acceleration)
For a torque to be applied to an object, there needs to be a force that acts at some distance away from a pivot point.
For example, consider tightening a bolt with a wrench.
Which of the 3 forces shown will tighten the bolt?
Pivot Point FA FC FB
Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB

4. Torque
Torque is what causes angular acceleration (just like a force causes linear acceleration)
For a torque to be applied to an object, there needs to be a force that acts at some distance away from a pivot point.
For example, consider tightening a bolt with a wrench.
Which of the 3 forces shown will tighten the bolt?
Pivot Point FA
Force B will tend to rotate the bolt clockwise, which will tighten it. FC FB
Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB

5. Torque
Torque is what causes angular acceleration (just like a force causes linear acceleration)
For a torque to be applied to an object, there needs to be a force that acts at some distance away from a pivot point.
For example, consider tightening a bolt with a wrench.
Which of the 3 forces shown will tighten the bolt?
Pivot Point FA
Force B will tend to rotate the bolt clockwise, which will tighten it.
Notice that force A will tend to rotate the bolt counter-clockwise, loosening it.
What does force C do? FC FB
Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB

6. Torque
Torque is what causes angular acceleration (just like a force causes linear acceleration)
For a torque to be applied to an object, there needs to be a force that acts at some distance away from a pivot point.
For example, consider tightening a bolt with a wrench.
Which of the 3 forces shown will tighten the bolt?
Pivot Point FA
Force B will tend to rotate the bolt clockwise, which will tighten it.
Notice that force A will tend to rotate the bolt counter-clockwise, loosening it.
What does force C do?
Force C doesn’t cause any rotation at all – there is no torque generated by force C. Why not? FC FB
Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB

7. Torque
Torque is what causes angular acceleration (just like a force causes linear acceleration)
For a torque to be applied to an object, there needs to be a force that acts at some distance away from a pivot point.
For example, consider tightening a bolt with a wrench.
Which of the 3 forces shown will tighten the bolt?
Pivot Point FA
Force B will tend to rotate the bolt clockwise, which will tighten it.
Notice that force A will tend to rotate the bolt counter-clockwise, loosening it.
What does force C do?
Force C doesn’t cause any rotation at all – there is no torque generated by force C. Why not?
Force C points directly at the pivot point – no torque is created in this case. FC FB
Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB

8. Torque Formula for torque:
This is really just the magnitude of the torque. The angle in the formula is between the force and the radius (from the pivot point to where the force is applied).
Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB

9. Torque Formula for torque: FA θ
Pivot Point FA θ
This is really just the magnitude of the torque. The angle in the formula is between the force and the radius (from the pivot point to where the force is applied). r
Take a look at the diagram – r and θ are shown for force A.
Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB

10. Torque Formula for torque: FA θ
Pivot Point FA θ
This is really just the magnitude of the torque. The angle in the formula is between the force and the radius (from the pivot point to where the force is applied). r
Take a look at the diagram – r and θ are shown for force A.
There are 2 ways to interpret the formula.
If you group the Fsin(θ) together, that represents the component of the force that is perpendicular to the radius. To get the most torque, the force should be applied perpendicular (can you see why from the formula?)
Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB

11. Torque Formula for torque: FA θ
Lever Arm
Formula for torque:
Pivot Point FA θ
This is really just the magnitude of the torque. The angle in the formula is between the force and the radius (from the pivot point to where the force is applied). r
Take a look at the diagram – r and θ are shown for force A.
There are 2 ways to interpret the formula.
If you group the F sin(θ) together, that represents the component of the force that is perpendicular to the radius. To get the most torque, the force should be applied perpendicular (can you see why from the formula?)
The other option is to group the r sin(θ) together and call it the “lever arm” for the force. Think of this as the shortest distance from the pivot point to where the force is applied. This is the effective radius of the force. Again, to get maximum torque the angle should be 90°.
Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB

12. F1 is applied at the right end, and F2 is at the center. F2=30N
Example: Find the torque of each force shown with respect to the pivot point at the left end of the 2m long rod.
F1 is applied at the right end, and F2 is at the center.
F2=30N
120°
50°
F1=20N
Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB

13. F1 is applied at the right end, and F2 is at the center. F2=30N
Example: Find the torque of each force shown with respect to the pivot point at the left end of the 2m long rod.
F1 is applied at the right end, and F2 is at the center.
F2=30N
120°
50°
We can simply use our definition of torque here.
F1=20N
Notice the sign convention:
Counter-clockwise torque is positive.
Clockwise torque is negative.
Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB

14. Torque
We mentioned earlier that torques produce angular accelerations. We have a formula for this relationship:
This is really just Newton’s 2nd law applied to rotational motion. The moment of inertia, I, takes the place of the mass, and we use angular acceleration instead of linear.
Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB

15. F1 is applied at the right end, and F2 is at the center. F2=30N
Example: Find the angular acceleration of the 2m long, uniform rod (mass=5kg) when it is subject to the 2 forces shown.
F1 is applied at the right end, and F2 is at the center.
F2=30N
120°
50°
This is just like the last problem, so we can use the results here.
We need to add up all the torques on the rod, then solve for .
F1=20N
Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB

16. F1 is applied at the right end, and F2 is at the center. F2=30N
Example: Find the angular acceleration of the 2m long, uniform rod (mass=5kg) when it is subject to the 2 forces shown.
F1 is applied at the right end, and F2 is at the center.
F2=30N
120°
50°
This is just like the last problem, so we can use the results here.
We need to add up all the torques on the rod, then solve for .
F1=20N
Look up this formula for the moment of inertia of a rod, with the axis at the end.
Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB

17. Static Equilibrium
Sometimes an object is subject to several forces, but it does not accelerate. This is when the object is in equilibrium. We have done problems like this before, but we neglected the rotational motion. To incorporate this, we simply need to add a torque formula to our typical force formulas.
Here’s an example:
A uniform beam 4m long and weighing 2500N carries a 3500N weight 1.5m from the far end, as shown. It is supported by a hinge at the wall, and a metal wire running from the wall to the far end.
Find the tension in the wire, and find the horizontal and vertical components of the force that the hinge exerts on the beam.
1.5m
60°
Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB

18. A uniform beam 4m long and weighing 2500N carries a 3500N weight 1
A uniform beam 4m long and weighing 2500N carries a 3500N weight 1.5m from the far end, as shown. It is supported by a hinge at the wall, and a metal wire running from the wall to the far end.
Find the tension in the wire, and find the horizontal and vertical components of the force that the hinge exerts on the beam.
1.5m
60°
We need to draw a diagram of all the forces, then write down force and torque equations: T Hy Hx
2500N
3500N
T=Tension in wire
Hx and Hy are the components of the force that the hinge exerts on the beam.
Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB

19. We will save this equation and come back to it later.
A uniform beam 4m long and weighing 2500N carries a 3500N weight 1.5m from the far end, as shown. It is supported by a hinge at the wall, and a metal wire running from the wall to the far end.
Find the tension in the wire, and find the horizontal and vertical components of the force that the hinge exerts on the beam.
1.5m
60°
We need to draw a diagram of all the forces, then write down force and torque equations: T Hy
30° Hx
This could also be sin(60)
We will save this equation and come back to it later.
2500N
3500N
T=Tension in wire
Hx and Hy are the components of the force the hinge exerts on the beam.
Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB

20. We will save this equation and come back to it later.
A uniform beam 4m long and weighing 2500N carries a 3500N weight 1.5m from the far end, as shown. It is supported by a hinge at the wall, and a metal wire running from the wall to the far end.
Find the tension in the wire, and find the horizontal and vertical components of the force that the hinge exerts on the beam.
1.5m
60°
We need to draw a diagram of all the forces, then write down force and torque equations: T Hy
30° Hx
This could also be cos(60)
We will save this equation and come back to it later.
2500N
3500N
T=Tension in wire
Hx and Hy are the components of the force the hinge exerts on the beam.
Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB

21. A uniform beam 4m long and weighing 2500N carries a 3500N weight 1
A uniform beam 4m long and weighing 2500N carries a 3500N weight 1.5m from the far end, as shown. It is supported by a hinge at the wall, and a metal wire running from the wall to the far end.
Find the tension in the wire, and find the horizontal and vertical components of the force that the hinge exerts on the beam.
1.5m
60°
We need to draw a diagram of all the forces, then write down force and torque equations: T Hy
Before we can fill in the torque equation we need to choose a pivot point. A convenient choice is where the hinge attaches to the beam. This simplifies the torque equation because the 2 unknown hinge forces will not create any torque about that point.
Also, remember the sign convention – clockwise torques are negative and counterclockwise positive.
30° Hx
Pivot point here
2500N
3500N
T=Tension in wire
Hx and Hy are the components of the force the hinge exerts on the beam.
Now we can go back and substitute this value into the other equations to find the hinge forces.
Prepared by Vince Zaccone
For Campus Learning Assistance Services at UCSB