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The Dynamics of Rotational Motion

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More Lessons for High School Physics

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A series of free Online High School Physics Video Lessons.

In this lesson, we will learn

  • about torque
  • how to balance torques
  • how to calculate rotational inertia
  • about the law of the conservation of momentum

Torque, also called movement of force, measures how much a force rotates an object around an axis. Torque is the cross product of the force vector and the vector of the distance from the axis. The formula for rotational torque = radius × force × sin(angle between force and radius)

Understanding the concept of torque.
An introduction to torque

Balanced Torques
When the torques on either side of a fulcrum are equal, they are called balanced torques. A common example of balanced torques is two children on a see-saw. If the fulcrum is in the center of the see-saw, the two children must have equal mass for it to be balanced. If the fulcrum is not in the center, their masses must vary to create equal torques.
How to balance torques.
This video looks at how the concept of balancing Torque (rotational effects of forces) can be used to solve for unknown forces acting on a stationary structure.

This video looks at how the concept of balancing Torque (rotational effects of forces) can be used to solve for unknown forces acting on a stationary structure.
Rotational Inertia
Rotational inertia measures how much an object resists changing rotation. In linear motion, according to Newton's Second Law, we use mass to gauge an object's resistance to change but in rotational motion, rotational inertia serves the same purpose. Rotational inertia is a scalar, not a vector and is dependent upon the radius of rotation according to the formula, rotational inertia = mass × radius2.
Understanding rotational inertia.
Moment of Inertia: introduces the concept of rotational inertia or moment of inertia.

Conservation of Momentum
The law of conservation of momentum says that if two objects collide, the force lost by one object is equal to the force gained by the other object. The total momentum of the system is the same after the collision as before it as shown by the equation initial momentum = final momentum (where final momentum is the sum of all momentums present in the system). This principle is similar to the law of conservation of energy.
How momentum is conserved.
A brief introduction to the law of conservation of momentum and selected sample problems for algebra-based physics students.

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You can use the free Mathway calculator and problem solver below to practice Algebra or other math topics. Try the given examples, or type in your own problem and check your answer with the step-by-step explanations.

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