# Equal to zero if angular velocity is constant

darkmagic

## Homework Statement

Which is equal to zero if angular velocity is constant, normal or tangential acceleration?
The one with zero angular velocity is a crank. The crank moves at a fixed point.

Homework Helper

## The Attempt at a Solution

So which do you think it is and why?

darkmagic

I think it is the tangential one because that means it is a uniform motion but continuously changing speed? is that correct?

Homework Helper

I think it is the tangential one because that means it is a uniform motion but continuously changing speed? is that correct?

But it is not changing speed, the angular velocity is the same.

At different points in the circular motion, if you draw a tangent to the circle, you will get the direction of velocity right? Does this tangent always point in the same direction?

darkmagic

sorry it should be constant velocity. changing direction but constant speed, right?

Homework Helper

sorry it should be constant velocity. changing direction but constant speed, right?

Yes. So would the tangential velocity be changing is α is defined as α=dω/dt ? Since the direction of the tangential velocity is changing what does that say about acceleration?

darkmagic

velocity is changing. If that is changing, acceleration is also changing?

Homework Helper

velocity is changing. If that is changing, acceleration is also changing?

Right so if you were to draw the two velocity tangents and then find the change in velocity, it would point towards the center of rotation. So which acceleration is zero?

darkmagic

Normal acceleration is always towards the fixed point. The normal one?

Homework Helper

Normal acceleration is always towards the fixed point. The normal one?

That would be correct.

darkmagic

Can you give a summary on how to determine which is zero among the two in different situations?

How can I determine the tangential acceleration if I have given the angular velocity?

Homework Helper

Can you give a summary on how to determine which is zero among the two in different situations?

How can I determine the tangential acceleration if I have given the angular velocity?

tangential acceleration is related to α by atangential=αr

darkmagic

so, if normal is zero, then tangential equals the angular acceleration. I only depend on the tangential and normal formula.

Homework Helper

so, if normal is zero, then tangential equals the angular acceleration. I only depend on the tangential and normal formula.

no no, the only way for your normal acceleration to be zero is if the object is not spinning. Remember a=ω2r

darkmagic

so normal is zero in a sliding block?

Homework Helper

so normal is zero in a sliding block?

You mean a sliding block on a curved surface? No, the normal is not zero. In that case there is still a normal acceleration.

darkmagic

i mean in a mechanism, in a flat surface. the sliding block has zero normal? I'll be back.

Homework Helper

i mean in a mechanism, in a flat surface. the sliding block has zero normal? I'll be back.

Once your object is basically rotating with respect to some point, there will be a normal acceleration.

darkmagic

I mean in a mechanism, the sliding block, the normal in a flat surface is zero?

Homework Helper

I mean in a mechanism, the sliding block, the normal in a flat surface is zero?

ok, what sort of mechanism are you talking about?

darkmagic

its only a sample, only a schematic drawing.

Homework Helper

its only a sample, only a schematic drawing.

Can you post a picture of the schematic?

darkmagic

its only a sketch angular velocity at 2 is constant

#### Attachments

• sample.JPG
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Homework Helper

its only a sketch angular velocity at 2 is constant

If your surface is flat in that manner, then there is no normal acceleration.

darkmagic

what is the normal acceleration doing?