Rigid Bodies: Translational Motion And Rotational Motion

Translational Motion

Let us understand translational motion with the help of examples. Let’s imagine a rectangular block placed on the slanting edge of a right angled triangle. If the block is assumed to slide down this edge without any side movement, every point in the rectangular block experiences the same displacement and more importantly, the distance between the points is also maintained. In pure translational motion, every point in the body experiences the same velocity be it at any instant of time. Both the points, P1 and P2 undergo the exact same motions. A car moving in a straight line, path of a bullet out of a gun etc are examples of translational motion.

Translational Motion

Rotational Motion

Now let us imagine a circular block going down the edge of the right angled triangle. Examining the location and orientation of different points on the cylindrical block will tell us something new. The points on the cylindrical body experience something much different than the rectangular block. As shown by the arrows in the diagram representing the velocity, each point experiences a different magnitude of velocity in a different direction. Here the points are arranged with respect to an axis of rotation. Rotation is what you achieve when you constrain a body and fix it along a straight line. This means that the body can only turn around the line, which is defined as rotational motion. Ceiling fan, a potter’s wheel, a vehicle’s wheel are all examples of rotational motion.

Say you go to a bowling alley, and throw the bowling ball towards the pins. If you notice closely, you will see that the ball is not just moving forwards i.e performing translational motion but it is also spinning on itself because of which you can spin and curve the entry of the ball; this motion is categorized as a rotational motion. The motion of a rigid body which is not fixed or pivoted is either a pure translational motion or a combination of translational and rotational motion. Rigid bodies which are fixed/pivoted experience motion which is rotational.

This is an article on the basics of motion in rigid bodies. Check out center of mass calculations in rigid bodies in subsequent articles only on BYJU’s.

Practise This Question

A particle travels in a circle of radius 20 cm at a speed that uniformly increases. If the speed changes form 5.0 ms to 6.0 ms in 2.0 s, find the angular acceleration.