What change the state of object is called “force”. We mean by saying state, shape or position of the object. You can kick the ball and apply a force then you give speed to the ball and changes its position or you can press the spring apply a force and give potential energy to it, we can increase the number of example. In summary, you observe and apply force every day. Force is a vector quantity having both magnitude and direction. We understand that cause of motion is “force”. The unit of force is Newton or kg.m/s².