Short conceptual question on Work, Energy and Forces

[devilish_wit]

Homework Statement

(Please check the attached image)

2. Homework Equations

The Attempt at a Solution

Those are my answers. I thought the magnitude of the applied force would be bigger because that would be responsible of keeping the block on that spot without having it pulled back by the spring force. I'm also sure that the applied force is directed to the left but please do correct me if it's otherwise.

 

[phinds]

So the spring pulls on the block to the left, and the block pushes the spring to the left? You might want to rethink that.

 

[Celso]

Third Newton's law states that anybody that applies a force receives back another force with the same magnitude but with oposite direction.
Now there are many reasons for which two bodies interacting with each other don't simply move away

 

[Celso]

So in any situation where something applies a force to another thing, there are actually two forces to be considered. The block does exert a force on the spring opposite to the force that it undergoes (in this case, the spring pulls the block to the left,so the block pulls the spring to the right).
Your argument to justify that the force the block exerts is greater than the force it receives is that the block keeps on the same spot, but the problem says nothing about wheter the block will keep in the same place or not. In a scenario where it would keep on the same spot, a reason for which it would hapen could be friction or simply its weight (heavy things stand still when they there's no force strong enough to move them).

 

[gneill]

In a scenario where it would keep on the same spot, a reason for which it would hapen could be friction or simply its weight (heavy things stand still when they there's no force strong enough to move them).

That would still be friction. There's nothing magic about "heavy things" in Newton's laws.

 

[ehild]

(heavy things stand still when they there's no force strong enough to move them).

What do you think can 1 N force move an object of 10000000000 kg mass?

 

[Celso]

In practice? It's more of a philosophical matter. If we consider exclusively the system 1 person - Earth, (let's consider a person with 100kg of mass) mathematically Earth would suffer an acceleration of $a \approx \frac{10^3}{10^{24}} = 0.000000000000000000001\frac{m}{s^2}$ (Newton's model). Does this number mean anything in reality?

 

[gneill]

Yes, well this is not a philosophy forum. Nor is it a practical rule-of-thumb, measure by eye sort of place. Whether the numbers are of any practical import does not matter so long as strict adherence to the physical laws involved is upheld.