David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.
After watching this lesson, you'll be able to explain the difference between translational and rotational motion, explain how translational quantities are replaced by rotational ones in physics and apply Newton's First Law to rotation. A short quiz follows.
Translational vs. Rotational Motion
Why does a tornado spin so fast or a hanging mobile take so long to stop moving? To answer these questions we need to talk about rotational motion.
In other video lessons, we've discussed statics, kinematics and forces, but always in relation to translational motion. Translational motion is motion that involves the sliding of an object in one or more of the three dimensions: x, y or z. But an object can still be moving even when it's just sitting at a particular x-, y- and z-coordinate; it can still spin.
Rotational motion is where an object spins around an internal axis in a continuous way. An ice-skater can do this by spinning on the spot. She will give herself rotational energy. And because energy is always conserved and a smaller object must spin faster to have the same energy, when she moves her arms in towards her body, her rotation speed will increase - the spinning will get faster and faster.
This is also the reason that tornadoes spin so fast. Before the tornado forms, the air, in general, is rotating at a large radius. But if that radius decreases, the spinning gets faster, until you have a storm with incredible power.
When moving from translational to rotational motion, many of the concepts don't really change at all. You just replace translational quantities with rotational ones.
For example, Newton's First Law says that a body in motion stays in motion, and a body at rest stays at rest unless acted upon by an unbalanced force. That law is true for rotation, too! But instead of a linear force, we have a rotational torque. Newton's First Law thus becomes: a spinning body will stay spinning, and a non-spinning body won't start spinning unless acted upon by an unbalanced torque. A torque is just a force that acts off-center and causes an object to spin.
Practically every quantity in translational motion has a rotational equivalent. Instead of linear acceleration, we have rotational (or angular) acceleration. Instead of forces, we have torques. Instead of momentum, we have angular momentum. Instead of velocity, we have angular velocity. And instead of mass, we have the moment of inertia.
We'll go through all of these things individually in other lessons, but for now, we'll just say that rotational motion is separate from translational. But we still have velocities, accelerations, forces and masses. The basic principles are exactly the same.
Spinning Top Example
Let's take a look at Newton's First Law again. One day you decide to play with a spinning top. This one has a rainbow of colors painted on it, and it makes a very satisfying sound when it spins. After spinning it on a table, it will eventually stop moving and fall over. But here's a question for you: what torque or torques cause it to do that?
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Well, first of all, we have to remember that torque is just the rotational version of force. So the question is asking what forces stop it moving. Imagining rotational quantities as linear ones can help a lot if you understand some linear physics. Newton's First Law tells us that for motion to increase or decrease, you need an unbalanced force. So what force stops the spinning top?
The answer is friction and air resistance - friction between the spinning top and the table and air resistance as it spins through the air. Which means that if it wasn't for friction and air resistance, the spinning top would keep spinning forever!
Translational motion is motion that involves the sliding of an object in one or more of the three dimensions: x, y or z. But an object can still be moving even when it's just sitting at a particular x-, y- and z-coordinate, because it can spin. Rotational motion is where an object spins around an internal axis in a continuous way. These two types of motion are independent, but follow a lot of the same laws.
For example, Newton's First Law says that a body in motion stays in motion, and a body at rest stays at rest unless acted upon by an unbalanced force. For rotation, we have the same law, just with torques instead of forces. Newton's First Law for rotation says that a spinning body will stay spinning, and a non-spinning body won't start spinning unless acted upon by an unbalanced torque. A torque is just a force that acts off-center and causes an object to spin.
Rotational motion explains why ice-skaters get faster when they bring their arms closer to their body, why tornadoes spin so fast and why a spinning top will eventually fall to the table.
After reviewing this lesson, you should have the ability to:
Describe translational motion and rotational motion
Explain how Newton's First Law applies to rotational motion
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