An electric vehicle assembly includes an electric vehicle and a processing device. The electric vehicle includes a wheel, and an electric motor configured to drive the wheel to rotate and to output a value of torque force applied on the wheel and a speed measurement of the electric vehicle. The processing device stores a vehicle weight, and receives a value of torque force and two speed measurements of the electric vehicle from the electric motor, obtains an acceleration based on the speed measurements, obtains a current user weight based on the value of torque force, the acceleration and the vehicle weight, and outputs a notification when the current user weight is lower than an initial user weight by a predetermined ratio.