# Translational velocity and angular relationship The linear velocity has units of m/s, and its counterpart, the angular velocity, has Relationships between Rotational and Translational Motion. relationships among rotation angle, angular velocity, angular acceleration, and time Kinematics for rotational motion is completely analogous to translational. Consider an object that moves from point P to P' along a circular trajectory of radius r, as shown in Figure Definition: Tangential Speed. The average.

This arc length is just the distance the object has traveled and the time is the time that it took and distance per time is just speed. So this is the speed of the object. I'm gonna write that as v even though it's not velocity. This is not a vector and it's not velocity because think about it, this arc length isn't displacement. This was the distance the object traveled. Distance per time is the speed. Displacement per time is the velocity.

We didn't use displacement. Displacement was this weird one. We didn't wanna deal with that. So since we're choosing to deal with arc length which is distance, what we're gonna do is relate the angular velocity into the speed. Now we have that relationship. This is R the radius times the angular velocity equals the speed of the object. So this is the relationship between the angular velocity and the speed.

The speed of the object is gonna equal the radius of the circular path the object is traveling in times the angular velocity. I should box these. This arc length formula was how you relate the number of radians and object has rotated through to how much arc length it's traveled, i. In this formula down here relates the angular velocity omega, the number of radians per second something has rotating with to how many meters per second it's traveling.

In other words, how many meters per second it's tracing out along this arc length. So this is good. Now we know how to relate the angular displacement to the distance the object has traveled and we know how to relate the angular velocity to the speed of the object so you probably know what's coming next. We have to relate the angular acceleration to the regular acceleration. So we're called that the angular acceleration which we represented with a Greek letter alpha was defined to be the change in the angular velocity per times.

It's the rate at which your angular velocity was changing. So there's moving at the constant rate. You've got no angular acceleration because there's no change in omega. But if omega starts off slow and then it gets faster and faster, you do have angular acceleration. It's probably not a surprise that if you have angular acceleration, this ball is gonna have regular acceleration too because it's speeding up in its angular rotation.

It's gonna be changing its velocity as well. So how do we this? How do we relate the angular acceleration to the regular acceleration? Well the simplest thing to try is we go work down here. We multiply both sides of our equation by the radius and we found the relationship the related speed to angular velocity. So let's try it again.

### Relating angular and regular motion variables (video) | Khan Academy

Let's multiply both sides of this equation by radius and see what we get. On the left hand side, we can get the radius times the angular acceleration. That's gonna equal the radius times the change in angular velocity over the change in time. So all I've done here is multiply both sides of this equation. This definition of angular acceleration by the radius. So let's see what we get on the right hand side. We got R times delta omega. So this is really R times the change in omega. Well that's just omega final minus omega initial and then divide it by delta t so I can distribute this R and get that. This would equal R times omega final minus R times omega initial divided by the time that it took.

But now look what happens. We've got R times omega final and R times omega initial. We know what R times omega is. It's the speed, not the velocity, but the speed. So I could rewrite this. I could say that this is really the final speed minus the initial speed over the time that it took to change by that much speed.

So this is, if I just rewrite the left hand side, this is what R times alpha is equal to. Now if I were you, I'd be tempted to just be like, oh look, we did it. That's the acceleration which change in speed over time. But you gotta be careful. Acceleration, the true acceleration vector is the change in velocity per time, but these are not velocity vectors.

So this isn't the true acceleration vector. This is something different. This is the change in speed per time. So that's still an acceleration but it's not necessarily the entire acceleration because there's two ways to accelerate. You can change your speed or change your direction. Basically this acceleration we just found doesn't take into account any acceleration that's coming from changing your direction.

This is only the acceleration that's gonna be changing your speed. If I were you, I'd probably be confused at this point. So let me try to show you what this means.

### Relating Angular and Translational Quantities - Physics LibreTexts

If this ball is rotating in a circle just by the mere fact that the ball is rotating in a circle, it has to be accelerating even if the ball isn't speeding up or slowing down. There's got to be an acceleration because this ball is changing the direction of its velocity.

These are gonna be a force. It's centripetal force, in this case it would be the tension. There's gotta be a centripetal acceleration that's changing the direction of the velocity. That is not this acceleration over here. This is a different acceleration. We know the centripetal acceleration is directed inward. We already know how to find this centripetal acceleration. Remember the formula for centripetal acceleration is the speed squared divided by the radius. This component, this centripetal acceleration is the component of the acceleration that changes the direction of the velocity. So I'm gonna say that again because this is important. The centripetal acceleration which you can find with v squared over R is the component of the acceleration that changes the direction of the velocity. If something is going in a circle, it must have centripetal acceleration. But this acceleration component that we found down here is different. This is what's changing the speed. You don't have to have this if you're going in a circle.

You could imagine something going in a circle at a constant rate. If that's happening, it's got centripetal acceleration but it doesn't have this thing down here because this thing we found R times alpha is the change in the speed of the object per time.

## Relating angular and regular motion variables

How would I draw that up here if I wanted to represent this a that we found down here visually up here? I'd draw tangential to the direction of motion, i. You need a component of that acceleration that's either in the directional motion or opposite of the directional motion. If it was opposite the directional motion, the acceleration would be slowing the object down.

## Relationship of angular velocy and translational velocity in simple circular motion?

If the component of acceleration is in the direction of motion, then it's speeding the object up. Let us examine the equations of motion of a cylinder, of mass and radiusrolling down a rough slope without slipping.

As shown in Fig. Firstly, we have the cylinder's weight,which acts vertically downwards. Secondly, we have the reaction,of the slope, which acts normally outwards from the surface of the slope.

Finally, we have the frictional force,which acts up the slope, parallel to its surface. As we have already discussed, we can most easily describe the translational motion of an extended body by following the motion of its centre of mass. This motion is equivalent to that of a point particle, whose mass equals that of the body, which is subject to the same external forces as those that act on the body. Thus, applying the three forces, andto the cylinder's centre of mass, and resolving in the direction normal to the surface of the slope, we obtain Furthermore, Newton's second law, applied to the motion of the centre of mass parallel to the slope, yields where is the cylinder's translational acceleration down the slope.

• 10.3: Relating Angular and Translational Quantities

Let us, now, examine the cylinder's rotational equation of motion. First, we must evaluate the torques associated with the three forces acting on the cylinder.

Recall, that the torque associated with a given force is the product of the magnitude of that force and the length of the level arm--i. Now, by definition, the weight of an extended object acts at its centre of mass.

However, in this case, the axis of rotation passes through the centre of mass. Hence, the length of the lever arm associated with the weight is zero. It follows that the associated torque is also zero. It is clear, from Fig. Thus, the length of the lever arm associated with is zero, and so is the associated torque.

Finally, according to Fig. We conclude that the net torque acting on the cylinder is simply It follows that the rotational equation of motion of the cylinder takes the form, is its moment of inertia, and is its rotational acceleration. Now, if the cylinder rolls, without slipping, such that the constraint is satisfied at all times, then the time derivative of this constraint implies the following relationship between the cylinder's translational and rotational accelerations: It is clear from Eq.

Note, however, that the frictional force merely acts to convert translational kinetic energy into rotational kinetic energy, and does not dissipate energy. Now, in order for the slope to exert the frictional force specified in Eq. In other words, the condition for the cylinder to roll down the slope without slipping isor This condition is easily satisfied for gentle slopes, but may well be violated for extremely steep slopes depending on the size of.

Of course, the above condition is always violated for frictionless slopes, for which. Suppose, finally, that we place two cylinders, side by side and at rest, at the top of a frictional slope of inclination. Let the two cylinders possess the same mass,and the same radius. However, suppose that the first cylinder is uniform, whereas the second is a hollow shell. Which cylinder reaches the bottom of the slope first, assuming that they are both released simultaneously, and both roll without slipping?

The acceleration of each cylinder down the slope is given by Eq.