Chapter 14

Rotational Motion

How can we describe the rotational motion of the Earth and how can we calculate the velocity of a point on the surface of the Earth due to its rotation?

We can move a rigid body about by moving it, through translation, and by rotating it, through rotation. Up to now we have only discussed translational motion. In this chapter we will introduce the tools to describe rotations.

14.1 Rotational State—Angle of Rotation

How can we describe the motion of the rod shown in Fig. 14.1? We would like to seperate the translational and rotational motion of the rod. In this case, for a rod thrown across the room, the rod rotates around its center of mass. We can therefore use the center of mass, R(t ), to describe the translational motion of the rod. This is a good choice, since the motion of the center of mass is determined from the external forces acting on the rod—we could therefore find the motion of the center of mass by solving the equations of motion. If we study the motion of the rod relative to the center of mass, we get the bottom-right part of Fig. 14.1. How can we describe the rotational state of this system? By the angle θ it has rotated around the center of mass!

Angle and Axis of Rotation

While a freely moving object (such as a rod thrown across the room) usually rotates around its center of mass,1 objects can also rotate around other points. We could for example nail the rod to the wall in any point along the rod, and the rod would be

1You will learn more about conditions for this later, when we discuss the physics of rotational motion.

© Springer International Publishing Switzerland 2015

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A. Malthe-Sørenssen, Elementary Mechanics Using Python,

Undergraduate Lecture Notes in Physics, DOI 10.1007/978-3-319-19596-4_14

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14 Rotational Motion

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Fig. 14.1 The motion of a rotating rod thrown through the air

forced to rotate around this attachment point. The rotational configuration of the rod is often called the rotational state of the rod. In order to uniquely define the rotational state of the rod we need to specify both the attachment point O and the angle θ the rod forms with the horizontal. But if we only specify the point O , we do not really know how the object rotates around this point. We need to specify the rotational axis as well as a point on the axis. For rotations in the x y-plane, we say that the rotational axis is normal to this plane, that is, along the z-axis. This description holds for rotations in two dimensions. We describe the three-dimensional case in Sect. 14.6.

In two dimensions, the rotational configuration of an object is described by: the angle θ ; the point O it is rotating around; and the direction of the rotational axis, k.

How do we describe the positive rotational direction? This is customarily determined by the right hand rule. Figure 14.2 shows how the direction of the positive z-axis is determined from the directions of the x - and y-axes. We can also use this rule

14.1 Rotational State—Angle of Rotation	439
Fig. 14.2 Illustration of the	Z
right-hand rule	X

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backwards: Given the direction of an axis, such as the z-axis, we can find the positive rotational direction by pointing the right thumb in the direction of the axis: the positive rotational direction is then in the direction your remaining fingers are curling: from the x - towards the y-axis. In this direction θ increases, in the opposite direction the angle decreases.

A Point on a Rotating Object

Given the angle θ and the rotation axis (including both a point on the axis and the positive direction along the axis), we can unqiuely determine the orientation of a rotating object. But how do we find the position of a particular point on a rotating object from this?

Figure 14.3 shows the motion of a point P on an object rotating around a fixed axis. We describe the position of P using a coordinate system that rotates along with the object. The rotating coordinate system has to unit vectors that rotate with the object: the unit vector uˆr , which is directed radially outwards from the rotation axis, and an axis normal to the radial direction with unit vector uˆn . A point on the rod can be described in this coordinate system by:

p = pr uˆr + pn uˆn .

(14.1)

When the object has rotated an angle θ both unit vectors have also rotated. The radial unit vector now forms angle θ with the horizontal, and is given as:

uˆr = cos θ i + sin θ j ,	(14.2)
as illustrated in Fig. 14.3. The normal vector, uˆn , is obtained by rotating uˆr	90◦ in
the positive direction:
uˆn = − sin θ i + cos θ j .	(14.3)

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Fig. 14.3 Illustration of the unit vector uˆ P , and the position of the point P during rotation of a rod around an axis through the origin

If the object is rotating around a fixed axis, this gives the position of any point on the object. If the object is rotating around a moving axis, such as the rotating rod in Fig. 14.1, we also need to add the position of the axis—here given as the position of the center of mass:

p = R + pr uˆr + pn uˆn .

(14.4)

The attentive reader may recognize the decomposition using the unit vector uˆr and uˆn as polar-coordinates. This is indeed correct.

Rotational Motion

We can describe the rotational motion of the rod from Fig. 14.1 by a motion diagram for the rod. We have illustrated one such diagram in the bottom-right part of Fig. 14.1, where we show the position of the rod at various times, ti , taken at constant time intervals t . This plot looks like a movie of the motion of the rod, where all the images have been superimposed into one image. A better way to visualize the rotational motion of the rod is to plot the time evolution of the angle, θ (t ). Figure 14.4 shows θ (t ) for the rotational motion in Fig. 14.1.

Test your understanding: Can you sketch two other motion diagrams for a rod that is rotating in the negative direction and for a rod that is rotating faster and faster in the positive direction. Sketch the corresponding diagrams for θ (t ).