Charlatan wrote:Richard wrote:
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However torque is needed if you want to know the force in something.
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I think this is where you are missing the point; Power is Force times Speed, no reason to involve Torque to figure out the Force.
If you are only considering the motive force, you are correct, but I think he made it clear that he is not necessarily only considering the motive force, because he is not necessarily only considering the performance. If you want to verify that the teeth on a gear is strong enough to handle the loads to which it is exposed, you basically need to study the tangential force which is equal to the torque multiplied by the radius of the gear. It's not enough to know the power in that case, you need the torque. Of course, you may calculate the torque from the same formula you keep quoting, but it's still the torque you need to consider. If you want to ensure that the tortional stiffness of the drive shaft is sufficiently hight, it's the torque, not the power, you need to take into account.
There may be other applications, such as traction control (anti spin) systems where the torque curve is of particular interest. Although the absolute values are not interesting, the shape of the curve is. Such systems are often quite complex, but one simple effect which is easy to understand is that a torque curve with a negative slope is stable, while a torque curve with positive slope is unstable. So with a negative slope, if the wheels start spinning, the torque will decrease as the rpm increases. But with a positive slope, wheel spinn will cause higher torque as the rpm increases. In formula 1 the technical regulations prohibit a slope less than -0.045Nm/rpm.
Another effect regarding stabilty, which you may feel in any car is that if you are in the regime of negative torque slope, you don't need to work as much with the throttle in order to maintain a certain speed for example when arriving a steep hill. When the resistance increases, the speed goes down and as a result the torque goes up, i.e. a stable system. If you are in the regime of positive slope, the increased resistance will still cause lower speed, but this will again cause lower torque which in turn will mean even lower speed, amplifying the effect, i.e an unstable system. The means you either need to shift down or use rather large corrections on the throttle.
At all times, the current force is equal to the current power output divided by the current speed, but that equation alone does not allow you to make considerations such as the above.
On a side note, it has already been pointed out that torque is the equivalent of force, in rotational form. But I would like to add that the torque can also be seen as energy per revolution, or more precisely; energy per radian. The torque in Nm is actually equal to the energy in joules per radian. This is why, when you multiply the torque with the angular velocity in radians per second, you get the power in watts. When it comes to performance, it all boils down to how much work you can do in a given period of time and therefore energy per angle is of no interest before you multiply it with angle per time.
