B The weight of one of three bricks

[Mike_bb]

Hello!

I have three bricks with masses $m$.
Am I right that the weight of first brick is $3mg-2mg=mg$?

Thanks.

 

[Lnewqban]

Which one is the first brick?
Each brick weights the same.

 

[Mike_bb]

Which one is the first brick?
Each brick weights the same.

number 1 on the picture

 

[Lnewqban]

Then, you are correct.
Its top surface is “feeling” a force equal to the weight of both bricks above it (2 mg).
Its bottom surface is “feeling” a force equal to the weight of the three bricks (3 mg).

 

[A.T.]

I have three bricks with masses $m$.
Am I right that the weight of first brick is $3mg-2mg=mg$?

If its mass is $m$ then its weight is $mg$ per definition. That has nothing to do with the other bricks or your picture.

 

[Herman Trivilino]

The weight of each brick is $mg$. You are confusing weight with normal force.

Take the middle brick as an example. There are three forces acting on it. There's the downward weight force of magnitude $mg$, a downward normal force of magnitude $mg$ due to contact with the brick above, and an upward normal force of magnitude $2mg$ due to contact with the lower brick.

And therefore, of course, the net force on the middle brick is zero. Indeed, the net force on each brick zero, as is the net force on the entire collection of bricks.

Edit: Draw three well-separated free body diagrams, one of each brick. Identify each Third Law pair of equal-but-opposite forces. The mental effort required will benefit you greatly.

 

[john1954]

Hello!

I have three bricks with masses $m$.
Am I right that the weight of first brick is $3mg-2mg=mg$?

View attachment 359686

Thanks.

each brick has a weight mg totally independent of the other bricks