Questions and Answers
What is rigid body dynamics?
The study of the motion of rigid objects that do not change their shape or size during motion.
Which of the following is NOT an assumption of rigid body dynamics?
What type of motion involves every point on the object moving in the same direction and by the same amount?
What is the definition of angular momentum?
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According to Newton's Second Law, $F = _", where F is the net force, m is the ______ss, and a is the acceleration.
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The total angular momentum of a closed system changes when acted upon by an external torque.
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Name one application of rigid body dynamics.
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Study Notes
Problem 1: Beam with Uniform Loading
- A beam with a uniform loading of 'w' kg/m is supported at one end (A) and 2m from the other end (B).
- The reaction at support A is [wx^2]/[2(x-2)] kg.
Problem 2: Balance on a See-Saw
- When one boy sits 1.2m from the center, another boy must sit 1.5m from the center to balance.
- If the first boy carries an additional 14kg weight and sits 1.8m from the center, the second boy must move to 3m from the center.
- The weight of the heavier boy is 42kg.
Problem 3: Block on an Inclined Plane
- A 40kg block rests on an inclined plane at 20° from the horizontal with a coefficient of friction of 0.60.
- The force parallel to the incline required to cause impending motion down the plane is 77.
Problem 4: Block on an Inclined Plane with Friction
- A 250lb block is on a flat surface inclined at 30° with coefficients of kinetic and static friction of 0.30 and 0.40, respectively.
- The force required to start the block moving up the plane is 125lb.
Problem 5: Block on an Inclined Plane with Friction and Horizontal Force
- A 600N block rests on a surface inclined at 30° with an angle of friction of 15°.
- The horizontal force P required to prevent the block from sliding down is 164.60N.
Problem 6: Resultant Force Vector
- Three force vectors F1 = 4i + 2j + 5k, F2 = -2i + 7j - 3k, and F3 = 2i - j + 6k intersect at a single point.
- The magnitude of the resultant force vector R is 13.
Problem 7: Cable Suspension
- A cable is suspended between two supports at the same elevation, 500ft apart, with a load of 500lbs per horizontal foot, including the weight of the cable.
- The sag of the cable is 30ft, and the total length of the cable is 503.76ft.
Problem 8: Cable with Unit Weight
- A cable supported at two points of the same level has a unit weight of 0.02 kg per meter of horizontal distance.
Problem 1: Beam with Uniform Loading
- A beam with a uniform loading of 'w' kg/m is supported at one end (A) and 2m from the other end (B).
- The reaction at support A is [wx^2]/[2(x-2)] kg.
Problem 2: Balance on a See-Saw
- When one boy sits 1.2m from the center, another boy must sit 1.5m from the center to balance.
- If the first boy carries an additional 14kg weight and sits 1.8m from the center, the second boy must move to 3m from the center.
- The weight of the heavier boy is 42kg.
Problem 3: Block on an Inclined Plane
- A 40kg block rests on an inclined plane at 20° from the horizontal with a coefficient of friction of 0.60.
- The force parallel to the incline required to cause impending motion down the plane is 77.
Problem 4: Block on an Inclined Plane with Friction
- A 250lb block is on a flat surface inclined at 30° with coefficients of kinetic and static friction of 0.30 and 0.40, respectively.
- The force required to start the block moving up the plane is 125lb.
Problem 5: Block on an Inclined Plane with Friction and Horizontal Force
- A 600N block rests on a surface inclined at 30° with an angle of friction of 15°.
- The horizontal force P required to prevent the block from sliding down is 164.60N.
Problem 6: Resultant Force Vector
- Three force vectors F1 = 4i + 2j + 5k, F2 = -2i + 7j - 3k, and F3 = 2i - j + 6k intersect at a single point.
- The magnitude of the resultant force vector R is 13.
Problem 7: Cable Suspension
- A cable is suspended between two supports at the same elevation, 500ft apart, with a load of 500lbs per horizontal foot, including the weight of the cable.
- The sag of the cable is 30ft, and the total length of the cable is 503.76ft.
Problem 8: Cable with Unit Weight
- A cable supported at two points of the same level has a unit weight of 0.02 kg per meter of horizontal distance.
Definition of Rigid Body Dynamics
- Rigid body dynamics studies the motion of rigid objects, which maintain their shape and size during motion.
Assumptions of Rigid Body Dynamics
- Objects are continuous and unbroken.
- Objects have a fixed shape and size.
- Objects are not subject to deformation or change in shape.
Types of Motion in Rigid Body Dynamics
Translation
- Motion of the object as a whole, where every point moves in the same direction and by the same amount.
Rotation
- Motion of the object around a fixed axis, where every point moves in a circular path.
Kinematics of Rigid Body Motion
Describing Motion
- Position: described by the position of a reference point (e.g., center of mass).
- Orientation: described by the orientation of the object in space (e.g., using Euler angles or quaternions).
- Velocity: described by linear velocity of the reference point and angular velocity around the axis of rotation.
- Acceleration: described by linear acceleration of the reference point and angular acceleration around the axis of rotation.
Kinetics of Rigid Body Motion
Forces and Torques
- Force: causes linear acceleration of the object.
- Torque: causes angular acceleration of the object.
Newton's Second Law and Euler's Equations
- Newton's Second Law: F = ma, where F is the net force, m is the mass, and a is the acceleration.
- Euler's Equations: describe the rotation of a rigid body in terms of torque and angular velocity.
Angular Momentum
Definition and Conservation
- Angular momentum: product of moment of inertia, angular velocity, and distance from the axis of rotation.
- Conservation: total angular momentum of a closed system remains constant, unless acted upon by an external torque.
Energy of Rigid Body Motion
Kinetic and Potential Energy
- Kinetic energy: energy of motion, described by linear and angular velocity.
- Potential energy: energy of position, described by height or configuration of the object.
Applications of Rigid Body Dynamics
Fields of Application
- Robotics: models and controls motion of robots.
- Computer Graphics: simulates motion of objects in virtual environments.
- Mechanical Engineering: designs and analyzes mechanical systems, such as gears and linkages.
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