# Torque and Rotational Kinetic Energy Relationship

• I
• alichoudhry57
In summary, it is not possible to calculate the Kinetic Energy or Rotational Kinetic Energy of an object with just the given information of Power (kW), Torque (Nm), and Speed (RPM). Additionally, Power is a measure of energy transfer and does not directly indicate the amount of energy or angular momentum an object contains. However, it is possible to calculate power from torque and speed, and given any two of the three values, the third can be calculated.

#### alichoudhry57

I am wondering if it is possible to calculate either the Kinetic Energy or Rotational Kinetic Energy of an object if we have the Power (kW), Torque (Nm), and Speed (RPM) of the object.

alichoudhry57 said:
I am wondering if it is possible to calculate either the Kinetic Energy or Rotational Kinetic Energy of an object if we have the Power (kW), Torque (Nm), and Speed (RPM) of the object.
Depends, but probably not. Do we have a continuous function for those? Is there any more info you can provide on your scenario?

topsquark
alichoudhry57 said:
I am wondering if it is possible to calculate either the Kinetic Energy or Rotational Kinetic Energy of an object if we have the Power (kW), Torque (Nm), and Speed (RPM) of the object.
Welcome @alichoudhry57 !
Objects can’t have those by themselves, as power refers to a transfer of energy, you need at least another object or substance receiving that energy.

russ_watters and topsquark
alichoudhry57 said:
Power (kW), Torque (Nm), and Speed (RPM) of the object.
Power can be computed from torque and speed, so it is redundant.

Finding the kinetic energy requires knowing the speed, and the moment of inertia of the object.
https://en.wikipedia.org/wiki/Moment_of_inertia

alichoudhry57 said:
I am wondering if it is possible to calculate either the Kinetic Energy or Rotational Kinetic Energy of an object if we have the Power (kW), Torque (Nm), and Speed (RPM) of the object.
Power tells you the rate at which energy is flowing through the object. It will not tell you how much energy or angular momentum the object contains. A drive shaft with an attached flywheel and a second drive shaft without can have the same power and torque passing through and can be rotating at the same rate. But the amount of kinetic energy and angular momentum that the two contain can be dramatically different.

If you have torque in Nm and rotation rate in RPM then you can get power in kilowatts. Indeed, given any two of the three, you can calculate the one you do not know.

For power, first convert RPM to radians per second (multiply by 0.104719755). Multiply torque in Nm by rotation rate in rad/s to get power in watts. Then divide by 1000 to get power in kilowatts.

Edit: scooped by @Baluncore on the power from torque and speed calculation.