Just as good as another as far as Newtonian mechanics is concerned.īut what happens if the second frame of reference accelerates with Motion is relative-hence, the name ``relativity'' for Einstein's theory. In fact, there is no absolute standard of rest, which implies that all However, Einstein showed that this is not the case. Newton thought that one of these inertial frames was special andĭefined an absolute standard of rest: that is, a static object in this frame was in a state of absolute rest. We conclude that the second frame of reference is also an inertial frame.Ī simple extension of the preceding argument allows us to conclude that thereĮxists an infinite number of different inertial frames moving with constant Moves in a (different) straight line with a (different) constant speed Object moves in a straight line with a constant speed in our original Object in the two reference frames satisfyĪccording to Equations ( 2.7) and ( 2.11), if an It is evident, from Figure 2.1, that at any given time, (See Figure 2.1.) Suppose that the position vector To the corresponding axes in the first frame, thatĪnd, finally, that the origins of the two frames instantaneously coincide at We can suppose that the Cartesian axes in the second frame are parallel ![]() With respect to the origin of the coordinate system, as a function of time,Ĭonsider a second frame of reference moving with some Of a point object can now be specified by giving its position vector, Set up a Cartesian coordinate system in this frame. Suppose that we have found an inertial frame of reference. Net external force moves in a straight line with constant speed. Thus, an inertial frame of reference is one in which a point object subject to zero Indeed, we can think of Newton'sįirst law as the definition of an inertial frame. However, this is only true in special frames of reference called inertial frames. In a straight line with a constant speed (i.e., it does not accelerate). Newton's first law of motion essentially states that a point object The diagram shows an example of a situation where three forces sum to zero it illustrates a vehicle parked on a hill at rest and a vector triangle that shows the forces.Ī normal reaction force acts on all four wheels the arrow shown on the diagram represents the sum of these forces.Next: Newton's second law of Up: Newtonian mechanics Previous: Newton's laws of motion You have already studied examples of a vehicle moving at constant velocity where the driving force and resistive force are equal in size and act in opposite directions. The closed figure is a triangle if there are three forces acting a quadrilateral for four forces, etc. If there are three or more forces acting then the vector diagram is a closed figure. ![]() This means that where there are two forces acting on an object that satisfies these conditions, they must be equal in size and opposite in direction. If an object is at rest or moving at constant velocity the resultant force on it is zero The converse of Newton’s first law is also true: The nearest that we can get to modelling motion with no resistive forces is to study motion on ice or an air track. Motion without resistive forces is difficult to achieve on Earth: there is always air resistance or friction from a surface that a moving object rests on. The phrase ‘uniform motion’ means moving in a straight line at constant speed i.e. Newton realised that in this case the unseen resistive forces of friction and air resistance together act in opposition to the motion as there is no longer a driving force after the object has been pushed, there is a resultant backwards-directed force acting on it. ![]() This is not ‘continuing in a state of uniform motion’. ![]() Give an object a push and it slows down before coming to rest. An object maintains its state of rest or uniform motion unless there is a resultant force acting on it.Īt first, this seems contrary to everyday experience.
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