Definition of force, torque, work, power

　・In Japanese

■Force [N,kgf]

According to Newton's law, mass m [kg], force F [N:Newton], and acceleration a [m/s²] are as follows.


1N is defined as the force that can move an object with a mass of 1kg at an acceleration of 1m/s2. Therefore, N may be expressed as follows.



The gravitational acceleration that occurs when falling freely on the earth is 9.8 [m/s²]. The downward force exerted by gravity on an object of mass 1 kg is 9.8 N. The force exerted on a mass by gravity is called a load, and the force exerted on a 1kg object is defined as 1kgf, and the following relationship holds.



Note that the relationship no longer holds true on the moon because the gravity is different. Also, kg is a unit of mass, whereas kgf is a unit of force, which can be confusing, so N is generally used. The international system of units is also N.

■Torque [Nm]

As shown below, the force applied in the direction of rotation is called torque (or moment), and the torque when a force of 1N is applied from a point 1m away is defined as 1Nm (Newton meter). When torque is T [Nm], distance is r [m], and force is F [N], the following relationship holds true.

■Power [PS , J/s , kW]

Power is work done per unit time. 1W = 1J/s.

＜For engines and motors＞
When the number of rotations per minute is [rpm] * 2π [rad], the power is the work (torque * distance traveled) divided by the time, and can be expressed as follows.


The relationship between kW and PS is as follows.


＜For electricity＞
As below.



＜What is rated power?＞
Rated power is the maximum output value that can be output continuously for a specified period of time. In the case of an engine, it indicates the maximum value indicated by the output characteristics, and in the case of AC power, it indicates the effective value.

■Work [Nm , J , kWh]

Work is force (N) multiplied by distance (m), so the unit is Nm, and 1Nm = 1J(Joule) (=0.239cal). Also, 1J is defined as 1W output for 1 second. Although the torque explained above is also the same unit, they are different. The work of a rotating object is defined as the force multiplied by the rotational distance [rad].

In the field of electricity, kWh is work (amount of electricity), and 1kWh is defined as the amount of electricity when 1kW of electricity is consumed for 1 hour, and can be expressed as follows.



＜Unit＞
1J =1Ws = 0.239[cal]
1Wh = 3600[J]

■Difference between work and energy

Work and energy both have the same unit [J], but they are different; energy is the ability to do work. For example, if an object with 100 J of energy does 10 J of work, the energy remaining in the object will be 90 J. The reverse is also true; if you do an additional 10 J of work on an object with an energy of 90 J, the energy of the object becomes 100 J.

List of related articles TOP MENU Mechanics ・Force,torque,work,power ・Acceleration ・Mass-spring-damper ・Moment of inertia ・Inverted pendulum ・Differential eq. for motors ・Slope angle ・Frictional force