Note: the application uses an acceleration sensor (not a gyroscope), because is not available for devices without it.
Enter the weight of punching bag, to which you have attached the smartphone. Appendix multiply acceleration on weight and you will learn the force of impact. The app will save your seven best records. Keep in mind that the results depend on the properties of a punching bag (softness, stiffness attachment). With this application you can get accurate relative results, if they are carried out on the same sports equipment.
Disclaimer. The developer is not liable for the consequences of using an android device as a measuring device.