Understanding Normal and Gravity Forces in Stacked Block Structures

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Discussion Overview

The discussion revolves around understanding the normal and gravitational forces acting on stacked blocks, specifically addressing the differences in forces experienced by the top and bottom blocks in a stacked configuration. Participants explore concepts related to equilibrium, free body diagrams, and the nature of gravitational versus normal forces.

Discussion Character

  • Conceptual clarification
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions why the top block experiences a normal force equal to its weight (M_t) while the bottom block's normal force accounts for both blocks (M_t + M_b), despite the gravitational force on the bottom block being M_b * g.
  • Another participant explains that the normal force on the top block only needs to balance its weight, while the normal force on the bottom block must balance both its own weight and the force exerted by the top block.
  • A participant seeks clarification on why the gravitational force on the bottom block is M_b * g rather than (M_b + M_t) * g, indicating confusion about the distinction between gravitational and normal forces.
  • Another response reiterates that gravitational force is defined by the weight of the block itself, and emphasizes that normal forces are interactions between the blocks, not gravitational forces.

Areas of Agreement / Disagreement

Participants express differing levels of understanding regarding the distinction between gravitational and normal forces, with some clarifications provided but no consensus reached on the conceptual confusion surrounding these forces.

Contextual Notes

Participants highlight the importance of distinguishing between gravitational forces, which depend solely on the mass of the individual blocks, and normal forces, which are influenced by the interactions between stacked blocks.

member 428835
why is it when you stack two blocks on each other that the top box with mass [itex]M_t[/itex] has a normal force of [itex]M_t[/itex] and a gravity force [itex]M_t*g[/itex]where as the bottom box has a normal force of [itex]M_t+M_b[/itex] where [itex]M_b[/itex] is the mass of the bottom box yet gravity is [itex]M_b*g[/itex]?

i guess i don't get why gravity is the same for each box the the normal force considers all the weight stacked on top (why doesn't gravity?)

thanks for your help!
 
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joshmccraney said:
why is it when you stack two blocks on each other that the top box with mass [itex]M_t[/itex] has a normal force of [itex]M_t[/itex] and a gravity force [itex]M_t*g[/itex]where as the bottom box has a normal force of [itex]M_t+M_b[/itex] where [itex]M_b[/itex] is the mass of the bottom box yet gravity is [itex]M_b*g[/itex]?

i guess i don't get why gravity is the same for each box the the normal force considers all the weight stacked on top (why doesn't gravity?)

thanks for your help!
The normal force is whatever is needed to keep the blocks in equilibrium. The normal force exerted by the bottom block on the top block need only balance the weight of the top block.

But the normal force from the floor on the bottom block must balance both the weight of the bottom block and the downward normal force that the top block exerts on the bottom block. Those two forces add up to equal the weight of both blocks.

(Be careful not to use the same symbol for mass and normal force!)
 
i appreciate your reply! can you explain why the gravity force acting on the bottom block (when considering the free body diagram) is simply [itex]M_b*g[/itex] rather than [itex](M_b+M_t)*g[/itex]. this concept is killing me.

i appreciate your time!
 
joshmccraney said:
i appreciate your reply! can you explain why the gravity force acting on the bottom block (when considering the free body diagram) is simply [itex]M_b*g[/itex] rather than [itex](M_b+M_t)*g[/itex]. this concept is killing me.
The gravitational force on anything is simply its weight. The gravitational force on the top block acts on the top block, not on the bottom block. (Of course the downward normal force that the top block exerts on the bottom block happens to equal the weight of the top block.)

So regardless of whatever else is going on between them, the gravitational force on the top block is Mt*g and the gravitational force on the bottom block is Mb*g.

Remember: The gravitational force on a block is a force exerted by the earth on the block. The forces that the blocks exert on each other are normal forces, not gravitational forces. (They may well be equal in magnitude to the weight, but they are not gravitational forces.)
 
hey thanks doc! this makes a ton of sense. i appreciate your time here.
 

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