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Relative motion Relative motion The laws of physics which apply when you are at rest on the earth also apply in any reference frame moving at a constant velocity with respect to the earth. The bike racers here are in relative motion with respect to each other. Lets learn more about the concept of relative velocity as we run through this topic.

Learning Objectives

After completing the topic, the student will be able to:

  • Connect day–to–day experiences to linear motion.
  • Discriminate clearly between distance and displacement.
  • Define and distinguish between speed, average speed, instantaneous speed, velocity, acceleration and deceleration.
  • Differentiate between uniform and non–uniform motions.
  • Develop and apply equations of motion.
  • Understand the nature of acceleration due to gravity.
  • Develop and apply position and displacement vectors in two dimensions.
  • Apply equations of motion in X and Y directions.
  • Develop and apply equations of motion, time of flight and range of projectile for any projected object.
  • Apply changes to the equations of motion, time of flight, height reached by the projectile, range of projectile when a projectile is launched vertically upwards.
  • Understand and correlate the concept of relative velocity to daily life examples like moving boat in a river etc.,.
Tugboat Tugboat race The participants in the tugboat race maneuver to optimize their chances of winning. They do so by controlling the displacement, velocity and acceleration of their boats.

Kinematics is the branch of mechanics that describes the motion of objects without considering the cause of motion. It provides a foundation that will help us in all areas of physics. It is most intimately connected with dynamics (another branch of mechanics): while kinematics describes motion, dynamics explains the cause for this motion.

It involves the relationships between the quantities displacement (d), velocity (v), acceleration (a), and time (t). We develop a mathematical formalism that allows us to represent the position, velocity, and acceleration of moving objects, and to express how these quantities are related to each other with time.

Note: We will restrict our study to speeds that are slow compared to the speed of light and also to objects which are generally large, compared to atoms or molecules.

Reference frame Reference frame When we talk about the motion of an object, we first determine a reference frame and then according to this reference frame we discuss the motion of the object. With respect to Earth, the Sun appears to move across the sky, but it is the Earth which is actually spinning and causing that apparent motion.
Study of Motion

The world, and everything in it, moves. We see cars and trains moving, a person walking on the street, birds flying in the sky, a leaf floating in the breeze. Even seemingly stationary things such as buildings move with Earth′s rotation. Earth orbiting around the Sun, the Sun's orbit around the center of the Milky Way galaxy and the galaxy′s migration relative to other galaxies are all examples of such movements. In fact, nothing in the entire universe is completely stationary.

Reference Frame: All motions are relative to some frame of reference. An object is said to be in motion if the position of the object changes with respect to some reference frame. If an object is said to be at rest, then it means that it is being described with respect to a reference frame that is moving together with the body. The reference frame is called the 'observer' and is arbitrarily chosen. The example below illustrates the concept of an observer.

Look at a person standing in a playground. His position relative to the ground, the trees and the buildings in the background does not change. So it appears as if he is at rest. But he is standing on the ground i.e. on the Earth, which rotates on its axis revolving around the Sun. If we have a facility to stand on the Sun, we will feel that the person is moving! That is the person is rotating as well as moving forward, along with the Earth. Thus, the person is at rest (or is not moving) relative to the ground, the trees and the buildings. But, on the other hand, the person, along with the ground, the trees and the buildings, is moving with respect to the Sun.

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