How do I calculate the normal force for equilibrium lab?

[Noman Rasheed]

I did torque and equilibrium lab report, where I had to hang some weights on a meter stick, which was placed on a fulcrum. There were 3 parts for the lab, with different settings. I am stuck at the last part, where I hang the meter stick with a string over a pulley, and there were 4 weights hanging on the meter stick with a string for each at different displacement.
Like this:

At last, book says,

How do I calculate the normal force?
I have also attached my excel sheet for the review too.
https://drive.google.com/file/d/1AvY50hWh1eajAc74uUHKNaF9xTlTLyLH/view

Thank you!

 

[berkeman]

I did torque and equilibrium lab report, where I had to hang some weights on a meter stick, which was placed on a fulcrum. There were 3 parts for the lab, with different settings. I am stuck at the last part, where I hang the meter stick with a string over a pulley, and there were 4 weights hanging on the meter stick with a string for each at different displacement.
Like this:

At last, book says,
View attachment 214745

How do I calculate the normal force?
I have also attached my excel sheet for the review too.
https://drive.google.com/file/d/1AvY50hWh1eajAc74uUHKNaF9xTlTLyLH/view

Thank you!

They ask for the normal force at the fulcrum, which is just the total weight of the stick and weights, no?

 

[Noman Rasheed]

They ask for the normal force at the fulcrum, which is just the total weight of the stick and weights, no?

I added the mass of hanging weights and the weight of the stick, and multiplied the weight sum by 9.8, which gave me 4.31 N approximately. Do you think it is right?

 

[berkeman]

I added the mass of hanging weights and the weight of the stick, and multiplied the weight sum by 9.8, which gave me 4.31 N approximately. Do you think it is right?

You are mixing terminology there. F=ma, so the normal force is the sum of the masses multiplied by g. You don't multiply a weight by g...

 

[berkeman]

Can you show your calculation of the normal force?

 

[Noman Rasheed]

Can you show your calculation of the normal force?

Here it is:

 

[haruspex]

Here it is:

View attachment 214747

Try adding those masses again. You have two digits swapped.

 

[Noman Rasheed]

Try adding those masses again. You have two digits swapped.

∑F_N = m*g*cosΘ , where Θ = 0°
∑m = 0.1 + 0.07 + 0.075 + 0.06 + 0.1342 = 0.4392 kg
∑F_N = 0.4392 * 9.8 = 4.30 N

This time I used the experimental mass of the stick, my bad for the last time.
Shouldn't the normal force be zero actually?

 

[haruspex]

Shouldn't the normal force be zero actually?

Why?

 

[Noman Rasheed]

Why?

Because sum of F and T should be 0 for being in equilibrium. Right?

 

[haruspex]

Because sum of F and T should be 0 for being in equilibrium. Right?

What are F and T? You cannot expect me to guess.

 

[Noman Rasheed]

What are F and T? You cannot expect me to guess.

Force and Torque. Apologize!

 

[haruspex]

Force and Torque. Apologize!

You cannot add a force to a torque.
Do you mean the sum of forces is zero and the sum of torques is zero? If so, yes, but how does that lead to the normal force being zero?

 

[Noman Rasheed]

You cannot add a force to a torque.
Do you mean the sum of forces is zero and the sum of torques is zero? If so, yes, but how does that lead to the normal force being zero?

Yes, that's what I was saying that sum of T and sum of F (separately) should be zero. If the normal force can't be, then what is that 4.31 N representing? Does it say that "4.31 N" of weight is acting downward (due to gravity)?

 

[haruspex]

Does it say that "4.31 N" of weight is acting downward (due to gravity)

You obtained it by adding the masses and multiplying by g, which is equivalent to multiplying each mass by g then adding the resulting weights. Therefore what you have found is the total force due to gravity on the stick+masses system. What other force(s) act on that system?

 

[Noman Rasheed]

You obtained it by adding the masses and multiplying by g, which is equivalent to multiplying each mass by g then adding the resulting weights. Therefore what you have found is the total force due to gravity on the stick+masses system. What other force(s) act on that system?

There is pressure, and I guess torque too, which was calculated as 0 ± 0.003 Nm.

 

[berkeman]

There is pressure, and I guess torque too, which was calculated as 0 ± 0.003 Nm.

Sorry, that makes no sense to me.

houldn't the normal force be zero actually?

The normal force is what holds the setup up, and keeps it from falling. That is either supplied by the fulcrum pushing up on the stick, or in the later configuration it is supplied by the rope pulling up at the same point on the stick as the fulcrum was pushing up before.

When I stand on the floor, the normal force pushes up on my feet to keep me at that height. If there were no floor pushing up on me, I would fall in the downward direction.

Does that help?

 

[haruspex]

There is pressure, and I guess torque too, which was calculated as 0 ± 0.003 Nm.

Pressure is not force; torque is not force.
The force of gravity acts to pull the system down. What force stops it moving down?

 

[Noman Rasheed]

Sorry, that makes no sense to me.

The normal force is what holds the setup up, and keeps it from falling. That is either supplied by the fulcrum pushing up on the stick, or in the later configuration it is supplied by the rope pulling up at the same point on the stick as the fulcrum was pushing up before.

When I stand on the floor, the normal force pushes up on my feet to keep me at that height. If there were no floor pushing up on me, I would fall in the downward direction.

Does that help?

I got it. Thank you so much!

Pressure is not force; torque is not force.
The force of gravity acts to pull the system down. What force stops it moving down?

Oh!
That mean's a normal force of 4.30 N which is acting in the upward direction stops the meter stick, which is carrying masses, from falling downward. Am I right?

 

[haruspex]

That mean's a normal force of 4.91 N which is acting in the upward direction stops the meter stick, which is carrying masses, from falling downward.

Where did 4.91N come from? In post #8 it was 4.30N. Other than that, yes.

 

[Noman Rasheed]

Where did 4.91N come from? In post #8 it was 4.30N. Other than that, yes.

Typo. I got it. Thank you so much!