Copyright © Michael Richmond.
This work is licensed under a Creative Commons License.
Let's compare the quantities used in linear and rotational dynamics problems. Start with the linear ones, which we already know and understand.
| Linear | Rotational | ||||
| acceleration | a | (m/s2) | angular acceleration | α | (rad/s2) |
| mass | m | (kg) | moment of inertia | I | (kg * m2) |
| force | F | (N) = (kg * m / s2) | torque | τ | (N*m) = (kg * m2 / s2) |
We can put all the linear quantities together in Newton's Second Law, with which you should be very familiar:
Things you should note about this old friend:
In the rotational world, the equation looks very similar; just replace all the linear quantities with their rotational equivalents:
Once again,
We can use this equation to figure out just what a "torque" is. The units are given by its position in the equation. Notice that those tricksy radians disappear in the middle of our unit conversions, as usual.
So torque is some combination of "a force" and "a distance". We'll see that the torque applied to an object depends on several factors:
In fact, it will depend upon one more factor; but let's leave that for later, and just investigate these two quantities.
You stand in front of an ordinary door, which has hinges on the left-hand side and a doorknob near the right-hand side. Your job is to open the door, which swings out away from you.
Do you turn the knob, and then push the left-hand side of the door ... or push the right-hand side of the door?
Q: Which side of the door should you push? Q: Why?
Copyright © Michael Richmond.
This work is licensed under a Creative Commons License.