The stiffness coefficient of the spring is k = 20n / cm, with which a 20kg object is pulled to move on the horizontal plane. When the elongation of the spring is 4cm, the object just moves in a uniform straight line on the horizontal plane, and the speed is 4m / s. when the elongation of the spring is 6cm, the speed of the object moves for 2S, and then the distance that the pulling object can continue to slide is removed

The stiffness coefficient of the spring is k = 20n / cm, with which a 20kg object is pulled to move on the horizontal plane. When the elongation of the spring is 4cm, the object just moves in a uniform straight line on the horizontal plane, and the speed is 4m / s. when the elongation of the spring is 6cm, the speed of the object moves for 2S, and then the distance that the pulling object can continue to slide is removed

When the elongation is 4cm, the pulling force is t = k * x = 20 * 4 = 80N, and the object moves in a uniform straight line, which indicates that the friction force between the object and the ground is f = 80N. When the elongation is 6cm, the pulling force is t = k * x = 120N, and the object is subject to the pulling force of 120N and the friction force of 80N, so the combined external force F = 120-80 = 40n is calculated