Introduction to Relative Motion PDF

INTRODUCTION to RELATIVE MOTION Domingo B. Bognalbal Assistant Professor  In 1678 Christian Huygens showed that the wave theory of light could be used to explain the laws of reflection and refraction – Wave Nature of light James Clerk Maxwell demonstrated that the speed...

INTRODUCTION to RELATIVE MOTION Domingo B. Bognalbal Assistant Professor  In 1678 Christian Huygens showed that the wave theory of light could be used to explain the laws of reflection and refraction – Wave Nature of light James Clerk Maxwell demonstrated that the speed of electromagnetic waves produced from an oscillating electric circuit is the same as the speed of light in vacuum. ROYGBIV = white light (3x10^8 m/s) Every wave motion has something for it to propagate Light is a transverse wave motion  Almost every phenomenon occurred under the sun is relative. What one observes usually depends on one’s viewpoint The study of physics ultimately deals with measurement and relativity deals with an analysis on how the measurements are made depending on the observer as well as what is being observed There is an intimate relationship between space and time, mass and energy. Relativity has as its basis The earliest description of the observation of the relative motion started with motion of a body by two Aristotle different observers in the earth was at absolute rest in relative motion to each the center of the universe other. and everything else moved This means subjective way relative to the earth of viewing an event; what you have perceived is different from others have perceived. The same phenomenon is viewed differently by different observers. It refers to disagreement. Galileo Galilee (1564-1642) The rock comes down to the same place on the boat only suggested that if you throw a rock straight because the boat is also upward in a boat that is moving at moving toward the right. constant velocity, then, as viewed from Hence, to the observer on the the boat, the rock goes straight up and boat, the rock went straight straight down, projectile motion is up and straight down and by observed from the shore, however, the Aristotle’s reasoning the rock is seen to be displaced to the right of boat must be at rest. the vertical path. But as the observer on the shore will clearly state, the boat was not at rest but moving with a velocity v. Thus, Aristotle’s argument is not valid The distinction between rest and motion ACTIVITY 1: Distinction between rest and at a constant velocity, is relative to the constant velocity observer Give examples showing situations of the distinction between example. rest and constant velocity relative to the observer You are inside the moving train playing with your yoyo and your friend is waiting a bus along the sidewalk. As an observer inside the moving train, the train is at rest and as an observer outside the train in the sidewalk, the train is in motion. t=0 t=5 s How do we distinguish between a state of We are capable of feeling forces and rest and a state of motion at constant accelerations but we do not feel velocity? Recall Newton’s 2nd Law (F=ma) motion at constant velocity, and rest is the special case of zero constant velocity If the unbalanced external force acting on the body is zero, then the acceleration is also zero. This means that there is no change in velocity of the body, and the velocity is constant. Let’s consider that you are at rest standing What is the apparent weight of the in a bathroom scale inside the elevator person standing inside the elevator when it starts moving upward or downward? First figure below shows that the elevator starts to move upward indicated by the acceleration is greater than zero (0) and the speed is greater than zero (0). The weight of the person is greater than 150 lbs. Second figure it shows that the elevator speed is greater than zero (0) but the acceleration is zero (0), this means weight is 150 lbs. Third figure shows the elevator is still moving upward but the speed decreases Second figure it shows that the elevator speed is greater than zero (0) but the acceleration is zero (0), this means weight is 150 lbs. Third figure shows the elevator is still moving upward but the speed decreases therefore acceleration is less than zero (0) and the weight of the person inside the elevator is less than 150 lbs. Third figure shows the elevator is still moving upward but the speed decreases therefore acceleration is less than zero (0) and the weight of the person inside the elevator is less than 150 lbs. Third figure shows the elevator is still moving upward but the speed decreases therefore acceleration is less than zero (0) and the weight of the person inside the elevator is less than 150 lbs. When the elevator is accelerating downward the speed is less than zero (0) and the acceleration is less than zero (0) too. In this case you would feel as if you are lighter. This means the weight is less than 150 lbs. the last figure acceleration is greater than zero because the speed slowing down. Therefore, the weight of the person is greater than 150 lbs. Thus, accelerations are easily felt but not constant velocities. Only if the last figure acceleration is greater than zero because the speed slowing down. Therefore, the weight of the person is greater than 150 lbs. Thus, accelerations are easily felt but not constant velocities. Only if the elevator accelerates can the passenger tell that he or she is in motion. EXAMPLE a person sits in a plane or a train moving at constant velocity, the motion is not sensed unless the person looks out the window. The person senses his or her motion only while the plane or train is accelerating. EXAMPLE (a) shows body 1 at rest while body 2 is moving with a velocity v to the right. From the point of view of body 2, he can say that he is at rest and body 1 is moving to the left with a velocity of –v. (b) above shows body 2 at rest while body 1 is moving with a velocity -v to the left. From the point of view of body 1, he can say that he is at rest and body 2 is moving to the right with a EXAMPLE (b) shows body 2 at rest while body 1 is moving with a velocity -v to the left. From the point of view of body 1, he can say that he is at rest and body 2 is moving to the right with a velocity of v. figure 1.5(c), and she will observe body 2 moving to the right with a velocity v/2 and body 1 moving to the left with a velocity of -v/2. EXAMPLE (c), and she will observe body 2 moving to the right with a velocity v/2 and body 1 moving to the left with a velocity of -v/2. CONCLUSION To describe the motion, we place a coordinate system at some point, either Therefore, we must conclude that, in the body or outside of it, and call this if a body in motion at constant coordinate system a frame of reference. The motion of any body is then made velocity is indistinguishable from a with respect to this frame of reference. body at rest, then there is no reason why a state of rest should A frame of reference that is either at be called a state of rest, or a state rest or moving at a constant velocity is of motion a state of motion. called an inertial frame of reference or an inertial coordinate system. Either body can be considered to be at rest while the other body is moving in the opposite direction with the speed v. Newton’s first law defines the inertial frame of reference. That is, Inertial frame is describe an object at rest when F = 0, and the body is either or moving with constant velocity at rest or moving uniformly in a Accelerated frame of reference or non- straight line, then the body is in an inertial frame of reference describes that a inertial frame. body is accelerating. There are an infinite number of inertial frames and Newton’s second law, in the form F = ma, holds in all these inertial frames ACTIVITY 2: Inertial Frame or Accelerated Frame DIRECTION: Newton’s Laws are 7.vehicle is moving in a straight line at 60 km/hr valid or Not? 8.A car is moving at constant speed 1.Train speeding at 100 Km/hr along a rotonda 2.Merry Go Round 9.The moon revolves around the earth 3.A car taking a turn 10.Running around the oval of Bicol University at constant speed 4.Sudden brake in a car 5.Hovering Balloon 6.The earth moving around the sun Assignment