A problem with Newton's mechanics

[m0urazz]

[Mod note: Thread moved from forum Classical Physics so no template shown]

This set is in equilibrium and I can't find out what will the scale write down in Newtons, any help?

 

[Doc Al]

What do you think? To start with, what's the weight of those masses?

 

[m0urazz]

Everyone weights 49N

 

[Doc Al]

Everyone weights 49N

Good. So what must be the tension in the rope?

 

[scarpma]

The tension T is T=49 N

 

[PeroK]

The tension T is T=49 N

Yes. The tension is the same whether there are two counter balanced masses or one mass and one end of the scale tied off to a large static object.

If there was only one mass, the mass would simply fall, pulling the scale with it and there would be effectively no tension.

 

[m0urazz]

Good. So what must be the tension in the rope?

In every rope, there's also a tension which is equal to 49N

 

[Doc Al]

In every rope, there's also a tension which is equal to 49N

Exactly. So what must the scale read?

 

[m0urazz]

Exactly. So what must the scale read?

I know what you mean, but there are two masses, if we simply put them at one end it'd read 98 N, and now by putting one at every end it will read 48 N, I just can't understand why :/

 

[Doc Al]

if we simply put them at one end it'd read 98 N,

Careful here. If you put both masses at the the same end of the scale, the scale would go flying.

What you need to understand is that the scale (assuming it's not accelerating) must be in balance. Equal forces pulling it in both directions. And the scale simply reads the tension with which it is being pulled.

Try these. Say two people were playing tug of war with ropes attached to each end of the scale. Say they each pull with a force of 49 N. What will the scale read? Now have one of the ropes tied to a tree. If one person pulls on the other rope with a force of 49 N, what force must the tree be exerting? What will the scale read?

 

[m0urazz]

If one person pulls on the other rope with a force of 49 N, what force must the tree be exerting? What will the scale read?

The tree will exert 49 N just to balance the force exerted

 

[Doc Al]

The tree will exert 49 N just to balance the force exerted

Right. And the tension in the ropes will be 49 N. Which is what the scale will read.

 

[m0urazz]

Oh, that was easy I just couldn't see that, thanks Doc!