4. Newton’s laws of motion I4.1ForceForces are what cause any change in the velocity of an object, according to Newton’s definition, aforce is that which causes an acceleration. There are two classes of forces, contact forces and fieldforces. Contact forces involve physical contact between two objects (for example, friction). Fieldforces act through empty space, and no physical contact is required (for example, gravitationalforces). Forces are vectors, so you must use the rules for vector addition to find the net force actingon an object. The forces are applied perpendicularly to each other, and the resultant (or net) force isthe hypotenuse.Figure 4.1: Vector nature of forces.4.2Newton’s first law and inertial framesAny reference frame that moves with constant velocity relative to an inertial frame is itself aninertial frame. A reference frame that moves with constant velocity relative to the distant stars isthe best approximation of an inertial frame. We can consider the Earth to be such an inertial frame,although it has a small centripetal acceleration associated with its motion.Newton’s first law states that in the absence of external forces, when viewed from an inertialreference frame, an object at rest remains at rest and an object in motion continues in motion with aconstant velocity. Newton’s First Law describes what happens in the absence of a force. It also tellsus that when no net force acts on an object, the acceleration of the object is zero.