Understanding Normal and Gravity Forces in Stacked Block Structures

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In summary, when stacking two blocks on top of each other, the normal force on the top block is equal to its weight, while the normal force on the bottom block is equal to the combined weight of both blocks. However, the gravitational force on each individual block is simply its own weight. The gravitational force is a force exerted by the earth on the block, while the normal force is a force exerted by another object on the block.
  • #1
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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  • #2
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!)
 
  • #3
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!
 
  • #4
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.)
 
  • #5
hey thanks doc! this makes a ton of sense. i appreciate your time here.
 

Related to Understanding Normal and Gravity Forces in Stacked Block Structures

1. What is the purpose of understanding normal and gravity forces in stacked block structures?

The purpose of understanding normal and gravity forces in stacked block structures is to ensure the safety and stability of the structure. By understanding these forces, engineers and architects can design and construct buildings that can withstand the weight and pressure of the stacked blocks without collapsing.

2. What is the difference between normal and gravity forces in stacked block structures?

Normal forces are perpendicular to the surface of the stacked blocks and are caused by the weight of the blocks above pushing down. Gravity forces, on the other hand, are vertical forces that pull the stacked blocks towards the earth's center. Both forces are essential in keeping the structure stable and balanced.

3. How do normal and gravity forces affect the stability of a stacked block structure?

Normal and gravity forces work together to keep a stacked block structure stable. The normal forces provide the necessary support to the blocks and prevent them from collapsing under their own weight. Gravity forces act as a downward force, keeping the blocks in place and preventing them from toppling over.

4. Can normal and gravity forces be altered in a stacked block structure?

Yes, normal and gravity forces can be altered in a stacked block structure. For example, adding additional blocks or redistributing the weight of the blocks can change the forces acting on the structure. However, it is crucial to consider the effects of these changes on the overall stability of the structure.

5. How do engineers and architects calculate and account for normal and gravity forces in stacked block structures?

Engineers and architects use mathematical equations and computer simulations to calculate and account for normal and gravity forces in stacked block structures. These calculations take into account the weight of the blocks, the angle of the structure, and other factors to ensure the stability and safety of the building.

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