rotational motion class 11 - rotational mechanics

Introduction to rotational motion

  • In this chapter, we will study about the special kind of motion of a system of particles that is rotation
  • We see examples of rotational motion in our everyday life for example rotation of earth about its own axis create the cycle of day and night . Motion of wheel, gears ,motors, planet ,blades of the helicopter etc are all the example of rotational motion
  • To understand the rotational motion as a whole we are first required to understand the concept of angular position, velocity, acceleration, centripetal acceleration
  • Till now in our study of dynamics we have always analysed motion of an object by considering it as a particle even when the size of the object is not negligible. Here in this chapter we will study rotational mechanics.
  • In this process we represent object under consideration as a point mass and shape and size of the object remains irrelevant while discussing the particular problem under consideration
  • But this point mass or point particle model is inadequate for problems involving rigid body motion i.e. rigid body undergoing both translational and rotational motion

  • As an example consider the motion of a wheel, we cannot consider a wheel as a single particle because different parts of the wheel in motion has different velocities and acceleration
  • Here in this chapter we will consider rigid bodies having definite shape and size and are capable of having both rotational and translational motion
  • The rigid body is a body with a perfectly defined and unchanging shape that is no matter how the body moves, the distance between any two particles within the body remains constant
  • Although the way we define rigid body gives us the definition of an idealized rigid body and real materials always deforms on the application of force and this idealized rigid body assumption can be used freely for the substances where deformation is negligibly small and can be neglected
  • Motion of a rigid body in general can be considered to consist of a translation of center of mass of the body plus rotation of the body about an axis through the center of mass as shown below in the figure

    Motion of a rigid body having transnational motion and rotational motion about center of mass

  • In this chapter we will concentrate on the simplest kind of rotation that is the rotation of the rigid body about the fixed axis

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