16. Angular Momentum

A solid disc of mass M = 40 kg and radius R = 2 m is free to rotate about a fixed, frictionless, perpendicular axis through its center. You apply a constant, tangential force on the disc's surface (as shown), to get it to spin. Calculate the magnitude of the force needed to get the disc to 100 rad/s in just one minute.

A thin string is wrapped around a cylindrical hoop of radius R and mass M (Fig. 11–46). One end of the string is fixed, and the hoop is allowed to fall vertically, starting from rest, as the string unwinds.

(a) Determine the angular momentum of the hoop about its cm as a function of time.

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A thin string is wrapped around a cylindrical hoop of radius R and mass M (Fig. 11–46). One end of the string is fixed, and the hoop is allowed to fall vertically, starting from rest, as the string unwinds.

(b) What is the tension in the string as a function of time?

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"A radio transmission tower has a mass of 76 kg and is 12 m high. The tower is anchored to the ground by a flexible joint at its base, but it is secured by three cables 120° apart (Fig. 11–52). In an analysis of a potential failure, a mechanical engineer needs to determine the behavior of the tower if one of the cables breaks. The tower would fall away from the broken cable, rotating about its base. Determine the speed of the top of the tower as a function of the rotation angle θ. Start your analysis with the rotational dynamics equation of motion dL→/dt =τ→ₑₓₜ . Approximate the tower as a tall thin rod.

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A baseball bat has a sweet spot where a ball can be hit with almost effortless transmission of energy. A careful analysis of baseball dynamics shows that this special spot is located at the point where an applied force would result in pure rotation of the bat about the handle grip. Determine the location of the sweet spot, xₛ , of the bat shown in Fig. 11–53. The linear mass density of the bat is given roughly by (0.61 + 3.3x²) kg/m , where x is in meters measured from the end of the handle. The entire bat is 0.84 m long. The desired rotation point should be 5.0 cm from the thin end where the bat is held. [Hint: Where is the cm of the bat?]

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