Home >> Science & Nature >> Science

What Is the Meaning of Rotational Motion?

Physicists have to do everything their own way. They can't just say something like, "I'm studying the way this dumbbell moves." They say things like, "I'm evaluating the translation and rotation of this object." Of course, there is a reason. Physics, like all sciences, strives for precision, and saying something is "moving," is not nearly precise enough. Rotation has its own unique properties.

Translation
- When an object moves from one spot to another, that's called translation. Some quantities associated with translation come in handy for understanding rotational motion. A moving object has a velocity, which is a vector quantity. A vector has a size and a direction. So a car driving north at 60 mph has a velocity vector that's 60 mph in length and is pointed straight north. If a force is applied to an object, the velocity can change in size or direction. The change of velocity is called acceleration, and it's also a vector. A moving object has energy, and the amount of energy is equal to one-half times the mass of the object times the magnitude of the velocity squared.
Rotation
- Scientists have developed a set of mathematical constructions to characterize rotation, because it influences translational motion.
  
  Translation is something like a hockey puck sliding along. The puck moves from one spot to another. Rotation is what happens when you take the hockey puck, put a hole in the center of it, put a pencil point in the puck, and spin the puck around the center. The puck doesn't move anywhere, but the edge of the puck moves in a circular pattern around the center. This is rotation. If you sit in an office chair and spin, you aren't going anywhere, but you and the chair are rotating about the axis of the chair. If you remove a bicycle wheel and roll it down the street, it exhibits both translation and rotation.
Rotational Quantities
- In the same way translation has some useful quantities associated with it, you can also define useful quantities for rotational motion. Although they're useful, they aren't necessarily as intuitive as their corresponding quantities for translation. For example, a spinning object has an angular velocity. The angular velocity is a vector, and the size is proportional to the rotational speed of the object. The direction of the angular velocity vector is a little unusual, though. If you curl the fingers of your right hand in the same direction as the rotation and stick out your thumb, lined up with the center of the object, that's the direction of the angular velocity. A torque, or moment of force, applied to the object will change the velocity with another vector, called the angular acceleration. A spinning object also has energy, equal to one-half the moment of inertia times the angular velocity squared.
Demonstration
- The angular velocity, the angular acceleration, the torque --- those are all vector quantities, but they are not pointed along the direction that anything is moving. Because of that, they are called pseudovectors. So why go to all that trouble to define these strange quantities? Because a spinning object behaves much differently than a nonspinning one.
  
  You can demonstrate this quite easily with a can containing solid contents. Hold the can flat in your hand, and toss it in the air. Then do the same thing, but give the can a spin as you're tossing it. The can behaves completely different when it's rotating. The mathematics of the special quantities for rotational motion describe and predict the motion of the spinning can.

Science