Two point masses, m, = 1 kg and m, =2 kg, are suspended by a massless rod of total length {= 1 m. The rod is pivoted a distance d= 0.25 m from the left end as shown. (a) What is the moment of inertia of the system about the pivot point? [Hint: the rotational inertia of a single point mass is Imau = mr, where r is the distance from the mass to the axis of rotation. To find the total rotational inertia of a system, you simply add the individual rotational inertias. This is just as you would find total mass of a system by adding the individual masses.] (b) Draw a free body force diagram for the rod. Remember that it is now important to draw the forces on the body where they actually act. (c) Convince yourself (and the others in your group) that the torque about the pivot point due to gravity acting on m, is out of the page, while that due to gravity acting on m, is into the page. Find numerically the torque due to each, taking g = 9.8 m/s. (d) Find the net torque about the pivot point at the instant shown. ej Find the angular acceleration about `the pivot point at the instant shown. H The rod is released from rest from the position shown. Through what angle does the rod rotate in the next 0.001 s? Assume that the angular acceleration is constant through this short time interval. CAns: (a) I, = 1.19 kg m²; (d) Et,= 12.25 N m, in which direction?; (e) a, = 10.3 rad/s², in which direction?: (f) 5.2 x 10ʻrad
Rigid Body
A rigid body is an object which does not change its shape or undergo any significant deformation due to an external force or movement. Mathematically speaking, the distance between any two points inside the body doesn't change in any situation.
Rigid Body Dynamics
Rigid bodies are defined as inelastic shapes with negligible deformation, giving them an unchanging center of mass. It is also generally assumed that the mass of a rigid body is uniformly distributed. This property of rigid bodies comes in handy when we deal with concepts like momentum, angular momentum, force and torque. The study of these properties – viz., force, torque, momentum, and angular momentum – of a rigid body, is collectively known as rigid body dynamics (RBD).
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