Finding Torque for Angular Momentum Parking Gate

[Goofball Randy]

Homework Statement

Homework Equations

Torque = r X F

The Attempt at a Solution

r is 2.5m since that's the length of the red (minus 0.5), and F should be the weight, so ma? That gives something like (5 * 2.5/3) * 9.8 * 2.5, which doesn't give me an answer at all :(

 

[haruspex]

You've included the weight of the short end, but as though it is at the mass centre of the long end.

 

[Goofball Randy]

You've included the weight of the short end, but as though it is at the mass centre of the long end.

Do I need to calculate them individually? The correct answer here is 49, and I think my approach as a whole is wrong.

 

[haruspex]

Do I need to calculate them individually? The correct answer here is 49, and I think my approach as a whole is wrong.

Yes, your error is more fundamental, I realized just after posting. You seem to be calculating moment of inertia, which is not what is asked for.

 

[Goofball Randy]

Yes, your error is more fundamental, I realized just after posting. You seem to be calculating moment of inertia, which is not what is asked for.

Can you point out where? I thought I was calculating torque.

 

[haruspex]

Can you point out where? I thought I was calculating torque.

You squared the distance and divided by 3. That happens the formula for the moment of inertia of a bar, but it is not in the (correct) equation you quoted.

 

[Goofball Randy]

You squared the distance and divided by 3. That happens the formula for the moment of inertia of a bar, but it is not in the (correct) equation you quoted.

I'm sorry, I don't understand. Are you saying I SHOULD be using inertia of a bar, I = 1/3 M r^2 ? Because right now I am literally just doing f = ma.
And if I need to use inertia, should I use the torque = Inertia * angular acceleration equation?

 

[haruspex]

I'm sorry, I don't understand. Are you saying I SHOULD be using inertia of a bar, I = 1/3 M r^2 ? Because right now I am literally just doing f = ma.
And if I need to use inertia, should I use the torque = Inertia * angular acceleration equation?

No, I said the equation you quoted is correct:

Homework Equations

Torque = r X F

But that's not what you did here:

like (5 * 2.5/3) * 9.8 * 2.5, which doesn't give me an answer at all :(

Yiu have two factors of 2.5 and a divisor of 3. That looks like a moment of inertia calculation to me.

 

[Goofball Randy]

No, I said the equation you quoted is correct:

But that's not what you did here:

Yiu have two factors of 2.5 and a divisor of 3. That looks like a moment of inertia calculation to me.

Oh!

It wasn't, that was a straight up r x F calculation. See, I was confused by the part where the bar is attached "0.5m from the end". So I figured that the actual mass would not be 5kg, but rather 5 * (2.5/3), since that would be the fraction of the bar that I'm actually looking at.

 

[haruspex]

Oh!

It wasn't, that was a straight up r x F calculation. See, I was confused by the part where the bar is attached "0.5m from the end". So I figured that the actual mass would not be 5kg, but rather 5 * (2.5/3), since that would be the fraction of the bar that I'm actually looking at.

Ok. In that case, your error is that you have treated the short end as simply not being there. It will produce a counter torque.
The easiest way is just to treat all the mass as being at the mass centre (which is valid for finding the effective weight).

 

[Goofball Randy]

Ok. In that case, your error is that you have treated the short end as simply not being there. It will produce a counter torque.
The easiest way is just to treat all the mass as being at the mass centre (which is valid for finding the effective weight).

I see, how clever!

The mass center would be 1m away from the pole-thingy.

So torque = 1 * 5 * 9.8

= 49.

Thanks so much!