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$\bullet$ A certain drawbridge can be modeled as a uniform $15,000 \mathrm{N}$ bar, 12.0 $\mathrm{m}$ long, pivoted about its lower end. When this bridge is raised to an angle of $60.0^{\circ}$ above the horizontal, the cable holding it suddenly breaks, allowing the bridge to fall. At the instant after the cable breaks, (a) what is the torque on this bridge about the pivot and (b) at what rate is its angularmomentum changing?

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a) 45000 $\mathrm{N.m}$b) 45000 $\mathrm{kg} \cdot \mathrm{m}^{2} / \mathrm{s}$

Physics 101 Mechanics

Chapter 10

Dynamics of Rotational Motion

Newton's Laws of Motion

Rotation of Rigid Bodies

Equilibrium and Elasticity

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02:34

In physics, a rigid body is an object that is not deformed by the stress of external forces. The term "rigid body" is used in the context of classical mechanics, where it refers to a body that has no degrees of freedom and is completely described by its position and the forces applied to it. A rigid body is a special case of a solid body, and is one type of spatial body. The term "rigid body" is also used in the context of continuum mechanics, where it refers to a solid body that is deformed by external forces, but does not change in volume. In continuum mechanics, a rigid body is a continuous body that has no internal degrees of freedom. The term "rigid body" is also used in the context of quantum mechanics, where it refers to a body that cannot be squeezed into a smaller volume without changing its shape.

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In physics, rotational dynamics is the study of the kinematics and kinetics of rotational motion, the motion of rigid bodies, and the about axes of the body. It can be divided into the study of torque and the study of angular velocity.

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that problem. 27 talks about a drawbridge that were modeling is a uniform 15,000 Newton bar, which means it has 15,000 Newton wait, located in the center of this 12 meter long bar. So okay. Did six meters from the pivot point. Um, we're looking that the bridges is raised up 60 which makes this angle here 30 um, and cable holding it suddenly breaks, allowing the bridge to fall down. What is the torque on the bridge at this point? And at what rate is its angular momentum changing? So let's first look at our torque at this point. Now, our torque is gonna be due to that weight times the liver arm now for a liver arm. We have angles involved. We're looking at this perpendicular distance to the force, so our torque would be this 1500 news times six is perpendicular distance will be six. Sign of 30 populated out. That makes the tour 45 1000 noon leaders. No. Now point B part B asks read of angular momentum change, but that a raid it's only means is overtime. So wants to know what is Delta. Change of momentum over Delta T That is actually defined as the sum of all torques on the bar at that time. What? We only have one torque. The only gonna be that 45,000 Newton meters. Thank you for learning with me.

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