Rolling and Static Equilibrium

Rolling motion represents the general plane motion of a rigid body It can be considered as a combination of pure translational motion parallel to a fixed plane plus a pure rotational motion about an axis that is perpendicular to that plane. The axis of rotation usually passes through the center of mass. In Sect. 6.4, we’ve seen that the motion of an object (or a system of particles) can always be considered as a combination of the motion of the object relative to its center of mass plus the motion of its center of mass relative to some origin O. From Sect. 6.4.3, the kinetic energy of an object relative to the origin is K = 1 2 ∑


Rolling Motion
Rolling motion represents the general plane motion of a rigid body It can be considered as a combination of pure translational motion parallel to a fixed plane plus a pure rotational motion about an axis that is perpendicular to that plane. The axis of rotation usually passes through the center of mass. In Sect. 6.4, we've seen that the motion of an object (or a system of particles) can always be considered as a combination of the motion of the object relative to its center of mass plus the motion of its center of mass relative to some origin O. From Sect. 6.4.3, the kinetic energy of an object relative to the origin is where v cm is the velocity of the center of mass of the object relative to the origin O, m i is the mass of the ith particle and v i is the linear velocity of the ith particle relative to the center of mass. In the case of the general plane motion of a rigid body, the motion can be considered as a combination of pure translational motion of the center of mass plus pure rotational motion about an axis passing through the center of mass and perpendicular to the plane of motion. Therefore, the first term in Eq. 8.1 can be written as where r i is the perpendicular distance from the ith particle to the center of mass axis. Hence Thus, the total kinetic energy of a rolling object is the sum of the translational kinetic energy of its center of mass and the rotational kinetic energy about its center of mass.

Rolling Without Slipping
An important special case of the general plane motion is rolling without slipping. Such motion occurs if a perfectly rigid body rolls on a perfectly rigid surface. As the object rolls without slipping, the instantaneous s point of contact between the object and the surface is at rest relative to the surface since there is no slipping. Now, consider a wheel of radius R rolling without slipping along the straight track shown in Fig. 8.1. The center of mass of the wheel moves along a straight line, while a point on the rim such as P moves in a cycloid path. As the wheel rotates through an angle θ , its center of mass moves through a distance equal to the arc length s (see Fig. 8.2) given by The acceleration of the center of mass is given by The combination of pure rotational and translational motions is viewed in Fig. 8.3. In the pure translational motion (see Fig. 8.3 part a) every particle in the wheel moves with the velocity v cm . In pure rotational motion (see Fig. 8.3 part b), each particle moves with an angular speed ω about the center of mass axis and the linear speed of any particle at the rim is The resulting motion of these two combined motions is shown in Fig. 8.3 part c, where the linear velocity of each particle is the vector sum of its linear velocity in pure translational motion and its linear velocity in pure rotational motion. Therefore, the instantaneous velocity of the point of contact is equal to zero (v 1 = 0) and of a point at the top of the wheel is equal to twice the velocity of the center of mass (v 2 = 2v cm ). Note that Eq. 8.2 is valid only in the special case of rolling without slipping; in the general rolling motion this equation does not hold. The total kinetic energy of a rigid object rolling without slipping is therefore given by Another way to view rolling without slipping is to consider the wheel to be in pure rotational motion about an instantaneous axis that passes through the point of contact P (see Fig. 8.4).
In that case, the velocity of the point of contact P is zero and the velocity of the center of mass is v cm = Rω (since it is at a distance R from the axis of rotation) and the velocity of a point at the top is v t = 2Rω = 2v cm . Note that the angular velocity ω of the wheel is the same as its angular velocity if the axis of rotation is at the center of mass. For simplicity, only homogeneous symmetrical objects will be considered here such as hoops, cylinders, and spheres. When a rigid body rolls without slipping with a constant speed, there will be no frictional force acting on the body at the instantaneous point of contact. However, if the object is accelerating, then a statistical frictional force acts on it at the instantaneous point of contact producing a torque about the center (see Fig. 8.5). This will cause the object to rotate about its center of mass. The direction of the statistical force opposes the tendency of the object to slide. For example, if a wheel is rolling down an incline, the direction of the frictional force will be opposing the downward motion.
In most situations, the body and the surface are not perfectly rigid. As a result, the normal force would not be a single force; rather it would be a number of forces that are distributed over the area of contact (see Fig. 8.6). Therefore, each normal force will exert an opposing torque since its line of action will If the body and the surface are not perfectly, the normal force would not be a single force; rather it would be a number of forces that are distributed over the area of contact not pass through the center of mass. Furthermore, as the object rolls over the surface, both the object and the surface undergo deformation resulting in a loss in the mechanical energy. Solution 8.1 (a) the total energy is given by If the hoop slides without rolling its total kinetic energy is 1 2 Mv 2 cm , that is, its value is half of that if the hoop were to roll without slipping. ( Example 8.2 A uniform solid cylinder, sphere, and hoop roll without slipping from rest at the top of an incline (see Fig. 8.7). Find out which object would reach the bottom first. Fig. 8.7 A uniform solid cylinder, sphere and hoop roll without slipping from rest at the top of an incline

Solution 8.2 For each object, we have
Hence, the speed of the center of mass of any object at the bottom of the incline does not depend on its mass or size; it depends only on its shape. Therefore, all objects of the same shape such as spheres (of any mass or size) have the same speed at the bottom. That is, the smaller the ratio I cm /M R 2 the faster the object moves since less of its energy goes to rotational kinetic energy and more goes to translational kinetic energy The ratio I cm /M R 2 is equal to 0.4, 0.5, and 1 for a sphere, cylinder, and hoop, respectively Therefore, these objects will finish in the order of any sphere, any cylinder, and any hoop.
Hence, the coefficient of static friction must be at least as great as μ s = 2 7 tan θ in order for the ball not to slip.
Example 8.4 A string is wrapped around a uniform solid cylinder of radius of R and mass of M as in Fig. 8.9. If the cylinder is released from rest while the string is fixed in place and assuming that the string does not slip at the cylinder's surface, find: (a) the acceleration of the center of mass using Newton's laws (b) the acceleration of the center of mass using energy methods if the cylinder descends a distance h(c) the tension in the string.
Solution 8.4 (a) Applying Newton's second law in both the linear and angular form gives Substituting Eq. 8.6 into Eq. 8.5 gives Example 8.5 A uniform solid sphere of radius R and mass M is released from rest at the top of an incline at a distance h above the ground. If it rolls without slipping, find the speed of the center of mass at the bottom of the incline.

Solution 8.5
That gives v cm = 10 7 gh Example 8.6 A block of mass m is attached to a light string that passes over a light pulley and is connected to a uniform solid sphere of radius R and mass M as in Fig. 8.10. Show that the acceleration of the system is a = g 1 + 7/5(M /m ) when the block is released from rest. Since the block and the sphere are connected, they have the same speed, therefore Therefore, the speed of the system when the block is at the bottom of the incline is The acceleration of the system is

Static Equilibrium
An extended object is said to be in equilibrium if two conditions are satisfied. First, the net external force acting on the object must be equal to zero. Second, the net external torque on the object about any origin must also be equal to zero. In other words, an object is in equilibrium if its total linear momentum and its total angular momentum (about any origin) are constants. Only the first condition is necessary if the object can be treated as a particle. Thus, the conditions of equilibrium may be written as In terms of components, we may write An object is said to be in static equilibrium if it is at rest (there isn't any kind of motion with respect to our inertial frame of reference). Now consider the case in which all external forces acting on the object lie in the same plane (for example the x-y plane). Such forces are called coplanar forces. The net external torque due to these forces is then perpendicular to the x-y plane and parallel to the z-axis. Equations 8.9 and 8.10 are, therefore, reduced to Next, we will prove that if the object is in translational equilibrium where (ΣF = 0) and the net external torque on the object is equal to zero about some origin, it is also equal to zero about any other origin. Note that the origin may be chosen anywhere inside or outside the object. Suppose that a number of forces F 1 , F 2 , F 3 , . . . F n are acting on a rigid object at different points (see Fig. 8.11) and that the object is in translational equilibrium. The point of application of F 1 relative to O is r 1 and of F 2 is r 2 and so on. The net external torque about O is given by τ 0 = τ 1 +τ 2 +· · ·+τ n = r 1 × F 1 +r 2 × F 2 + + · · · r n × F n The net external torque about O (see Fig. 8.12) is Since ΣF = 0 we have

The Center of Gravity
The resultant gravitational force acting on an object is the resultant of the individual gravitational forces acting on different mass elements of the object (see Fig. 8.13), i.e., This force can be replaced by a single force that is equal to the weight of the object (Mg) and that acts at a single point called the center of gravity Now consider an object that is near the earth's surface where the force of gravity is assumed to be constant over that range. Equation 8.11 becomes To locate the center of gravity, let us calculate the net torque acting on an object about an origin due to gravity This torque is the vector sum of the individual torques acting on different mass elements. That is, Fig. 8.13 The resultant gravitational force acting on an object is the resultant of the individual gravitational forces acting on different mass elements of the object Therefore, we conclude that if the gravitational field (g) is constant over the body, the center of gravity of the object coincides with its center of mass.
Example 8.7 Two blocks of masses m 2 = 20 kg and m 1 = 10 kg are supported by a uniform horizontal beam of length L = 1.5m and mass M = 6 kg (see Fig. 8.14). Find: (a) the normal force exerted by the fulcrum (supporting point) on the beam if it is placed under the center of gravity of the beam; (b) the distance x in which m 2 must be placed in order for the system to be balanced. If the ladder is at the verge of slipping the statistical frictional force is maximum f s = μ s n 1 . From Eq. 8.12, we have The resultant of the contact forces on the ladder at the base is The free-body diagram is shown in Fig. 8.15. From the equilibrium condition, we have Hence x = 0.54 L Example 8.9 A uniform beam of weight w and length L is held by two supports as in Fig. 8.16. A block of weight w 1 is resting on the beam at a distance of L/6 from the center of gravity of the beam. Find the magnitude of the forces exerted by the supports on the beam.

Solution 8.9
The free-body diagram of the system is shown in Fig. 8.16. Because the beam has a uniform density its cen-ter of mass and gravity are located at its geometrical center. Applying Newton's second law gives Taking the torque about an axis passing through one end (at F 1 ) gives From Eqs. 8.13 and 8.14 we have Example 8.10 A man of mass of 80 kg is standing at the end of a uniform beam of mass of 30 kg and length of 12 m as shown in Fig. 8.17. Find the tension in the rope and the reaction force exerted by the hinge on the beam. (The center of mass is at L/3 from one end).

Solution 8.11
The free-body diagram is shown in Fig. 8.18. Applying Newton's second law to the beam gives Example 8.12 A solid sphere of mass of 12 kg is in static equilibrium inside the wedge shown in Fig. 8.19. If the surface of the wedge is frictionless, find the forces that the wedge exerts on the sphere.   3. A wheel of mass 2 kg and radius of 0.05 m rolls without slipping with an angular speed of 3 rad/s on a horizontal surface. How much work is required to accelerate the wheel to an angular speed of 15 rad/s. 4. A block weighing 1000 N is held by a cable that is attached to a uniform rod of weight 500 N (see Fig. 8.20). Find (a) the tension in the cable, (b) the horizontal and vertical components of the force exerted on the base of the rod. 5. A uniform sphere of radius r and mass m is held by a light string and leans on a frictionless wall as in Fig. 8.21. If the string is attached a distance d above the center of the sphere, find (a) the tension in the string, (b) the reaction force exerted by the wall on the sphere. 6. Find the minimum force applied at the top of a wheel of mass M and radius R to raise it over a step of height h as in Fig. 8.22. Assume that the wheel does not slip on the step. Open Access This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made. The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.
7. Three identical uniform blocks each of length L are on top of each other as in Fig. 8.23. Find the maximum value of h in order for the stack to be in equilibrium.