# CI2 - Typeset by: CEPHA Imaging Pvt. Ltd., INDIA I.2 I.2.1...

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I.2 Fundamentals I.2.1 Degrees of Freedom and Motion The number of degrees of freedom (DOF) of a system is equal to the number of independent parameters (measurements) that are needed to uniquely de±ne its position in space at any instant of time. The number of DOF is de±ned with respect to a reference frame. Figure I.2.1 shows a rigid body (RB) lying in a plane. The rigid body is assumed to be incapable of deformation and the distance between two particles on the rigid body is constant at any time. If this rigid body always remains in the plane, three parameters (three DOF) are required to completely de±ne its position: two linear coordinates ( x , y ) to de±ne the position of any one point on the rigid body, and one angular coordinate θ to de±ne the angle of the body with respect to the axes. The minimum number of measurements needed to de±ne its position are shown in the ±gure as x , y , and θ . A rigid body in a plane then has three degrees of freedom. Note that the particular parameters chosen to de±ne its position are not unique. Any alternative set of three parameters could be used. There is an in±nity of sets of parameters possible, but in this case there must always be three parameters per set, such as two lengths and an angle, to de±ne the position because a rigid body in plane motion has three DOF. Six parameters are needed to de±ne the position of a free rigid body in a three-dimensional (3-D) space. One possible set of parameters which could be used are three lengths, ( x , y , z ), plus three angles ( θ x , θ y , θ z ). Any free rigid body in 3-D space has six degrees of freedom. A rigid body free to move in a reference frame will, in the general case, have complex motion, which is simultaneously a combination of rotation and translation. For simplicity, only the two-dimensional (2-D) or planar case will be presented. For planar motion the following terms will be de±ned, Figure I.2.2: Pure rotation in which the body possesses one point (center of rotation) which has no motion with respect to a “±xed” reference frame [Fig. I.2.2(a)]. All other points on the body describe arcs about that center. Pure translation in which all points on the body describe parallel paths [Fig. I.2.2(b)]. 51

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rigid body (RB) x y θ X Y Z FIGURE I.2.1 Rigid body in planar motion with three DOF: translation along the x axis, translation along the y axis, and rotation, θ , about the z. Complex motion that exhibits a simultaneous combination of rotation and translation [Fig. I.2.2(c)]. With general plane motion, points on the body will travel nonparallel paths, and there will be, at every instant, a center of rotation, which will continuously change location. Translation and rotation represent independent motions of the body. Each can exist without the other. For a 2-D coordinate system, as shown in Figure I.2.1, the x and y terms represent the translation components of motion, and the θ term represents the rotation component.
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## This note was uploaded on 08/29/2011 for the course MECH 2120 taught by Professor Gabale,a during the Summer '08 term at Auburn University.

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CI2 - Typeset by: CEPHA Imaging Pvt. Ltd., INDIA I.2 I.2.1...

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