A force acts on an object which is free to move. If we know the magnitude of the force on the object and the mass of the object, Newton's 2nd law of motion enables us to determine the object's :
An object of mass 1 kg was moving at a speed of 10 m/s. A constant force acts on the object for 4s and the object moved with a speed of 2 m/s in opposite direction. The force acting on the object was ___ .