Reference Frames To describe a physical event, we need to establish a 3-dimensional coordinate system associated with measurement. Let’s consider two reference frames. One, called S, is shown at left. The other, called S’, is shown at right. Let’s imagine a meatball which is moving with velocity ‘v’ within reference frame S. Reference frame S’ may be moving with respect to frame S, so the perceived velocity of the meatball in S’ may be different than an observer in S would measure. In fact, v’ could be zero. So the value of the y’ coordinate may also differ in the two reference frames. However, as drawn, the x (x’) and z (z’) coordinates would be the same in the two reference frames. Inertial Reference Frames Inertia: An object moves at constant velocity unless acted upon by an external force. Given the concept of inertia, we find it useful to talk about 'inertial reference frames' which are three-dimensional coordinate systems which travel at constant velocity. In such a frame, an object is observed to have no acceleration when no forces are acting on it. If a reference frame moves with constant velocity relative to an inertial reference frame, it also is an inertial reference frame. There is no absolute inertial reference frame, meaning that there is no state of velocity which is special in the universe. All inertial reference frames are equivalent. One can only detect the relative motion of one inertial reference frame to another. Principle of Galilean Relativity "Laws of mechanics must be same in all inertial frames of reference" Frames of Reference y x z velocity: v  S z’ y’ x’ Velocity: v’ S’Galilean Transformations Consider a meatball in frame S moving with velocity, v, within that frame, and S' is moving with velocity V' relative to frame S. This is shown in the following Figure. We want to know how to determine the coordinates in S’ when we know them in frame S. In the picture above, in the S frame the meatball is moving and the {x,y,z} axes are fixed. When we transform to the S’ coordinate system (so that {x’,y’,z’} are at rest), it now looks like the meatball has velocity v’ & that the old axes {x,y,z} are moving with velocity v in the NEGATIVE x’ direction. "v" then appears as the relative velocity of the PRIMED coordinate system {x’,y’,z’}, S’, compared to the UNPRIMED coordinate system {x,y,z} or S. To determine the coordinates of the meatball in one frame, S’, when we know it's coordinates in another frame, S, we employ the Galilean space and time transformations. Galilean space-time transformations: If S’ has a velocity relative to S so that v’ = 0, then we have x' = x+vt y'=y z'=z t'=t (Note: remember, v or v’ are vectors, so they have a sign.) The time interval between any two events is the same in any frame of reference. Galilean velocity transformation We also want to know the velocity of the meatball in frame S’. From the above situation, frame S’ is moving with V’ relative to S, and the meatball has velocity v in frame S. So the velocity within frame S’ would be v’ = V’ + v. This is the Galilean velocity transformation. Frames of Reference x y z velocity: v  S z’ x’ y’ Velocity: v’ S’Energy & Momentum Classically, several forms of energy are recognized. - energy of motion (‘kinetic energy’) KE - energy from forces (‘potential energy’) PE or binding energy - heat It was recognized that energy can change form from one to the another. Kinetic energy can be converted into heat, for instance. If one considers a swing, then when the swing is at the bottom of its trajectory, it has a maximum velocity (and so maximum KE), and a minimum potential energy. At the highest points of its path, its velocity goes to zero (no KE), but its potential energy is a maximum. However, it was always observed that the total amount of energy (=KE+PE+heat) in a closed system was constant. This is known as ‘conservation of energy’: Energy may not be created nor destroyed in a closed system. A related concept concerns the momentum of a particle, or series of particles in a closed system. In general, momentum (p) = mv and it preserves the direction of the velocity (unlike energy). As is indicated by the Law of inertia, the magnitude and direction of an object’s velocity does not change unless it is acted upon by an external force. This is what F = ma means. Since acceleration is a change in velocity per time, it equals Δv/Δt. This means that force can also be expressed as a change in momentum per time, F = Δp/Δt. The net momentum of a closed system cannot change, so when two particles collide in the same way, the sum of their outgoing momenta will equal the sum of their incoming momenta.light Observer O Observer O’ Traveling with some velocity (v) relative to Observer O. Speed of Light Recall that the speed of light was predicted by Maxwell’s equations to be the enormous value of c=3x108 m/s. Questions immediately arise about what this velocity is to be measured with respect to. Also, recall that Galilean relativity only encompassed mechanical phenomena. Does Galilean relativity apply to electricy, magnetism, & optics? First, we need to establish what light is. From Maxwell’s equations, we know that it is an electromagnetic wave. But what is waving? It turns out something called electric and magnetic fields are waving. These fields are related to the electrical and magnetic forces associated with particular charged objects. But in general, there is always a medium that a wave phenomenon is waving ‘in’. So what is light waving ‘in’? Maxwell postulated that there is an omnipresent ‘ether’ which is the medium of propagation. The ether would represent a special, absolute reference frame. We on Earth, in our laboratory, can have a net velocity relative to this ether. Observers with different motion should observe different apparent values of c. If reference frame O corresponds to the ether reference frame, then the velocity of light within this frame is ‘c’. The Galilean velocity tranformations should hold for observers of light in any frame moving at a speed v relative to absolute ether. If an observer O’ moves at a velocity v (as pictured), then she should measure vlight in her rreference frame to be c-v.L12111 L22 Michelson-Morley Experiment So how do we detect the