# Tag Info

Accepted

### How does massless particles arise in string theory when it starts with the assumption that all strings are composed of massive little pieces?

Susskind corrects himself at 1:00:08 where he says that the "mass density" coefficient in front of the $\dot{X}^2$ term is a non-relativistic analogue; it is not actually a mass-density of ...
• 167k

### How does massless particles arise in string theory when it starts with the assumption that all strings are composed of massive little pieces?

Here is a somewhat hand waving explanation that I think is appropriate given the nature of the question. The equations for the positions of the little masses along the string is essentially a wave ...
• 7,531

### Determine which ropes are under tension in pulley systems

In all of the cases the tension is the same in all parts of the rope. If the weight of the box is in each case $Mg$, then you can equate the weight to the tension in the ropes supporting the box to ...
• 41.3k

### How do I find the constraint relation in this question?

The speed of the connecting belt must be twice the downward speed of the center of the lower pulley. This means that $α_1 = 2 α_2$. Then: $mg – T – t = ma = mr α_2$ where, $T$, is the connecting ...
• 11.5k
1 vote

### Cosmological expansion around ropes between galaxies

You're mistaken. Mass doesn't mysteriously turn off expansion. There's just a distance regime (for a given mass) for which gravity dominates the combined effect of gravity and expansion. Looking at ...
• 8,643
1 vote

### Hooke's law and modulus of elasticity

It is not the usual way to deal with the notion of modulus of elasticity. But by the context, it is clear that the relevant equation is: $$F = E\frac{\Delta h}{h}$$ In that way, $E$ has units of force....
1 vote

### Hooke's law and modulus of elasticity

They use "modulus of elasticity" which is not the same as "spring constant". The spring constant is in N/m and is the quantity most used in physics. But for a given type of spring (...
• 6,345

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