at the local county fair, you watch as a blacksmith drops a 0.50kg iron horseshoe into a bucket containing 25kg of water. If the initial temperature of the horseshoe is 450C and the initial temp of water is 23C, what is the equilibrium temp of the system?

Do equipotential lines get closer together or further apart at a sharp corner of a cconductor? what about the electric field lines? What does this indicate about the electric field strength in this region?

A spherical weather balloon is filled with hydrogen until its radius is 3.40 m. Its total mass including the instruments it carries is 11.0 kg.

(a) Find the buoyant force acting on the balloon, assuming the density of air is 1.29 kg/m³.
_________N

(b) What is the net force acting on the balloon and its instruments after the balloon is released from the ground?
_________N

Two point charges lie on the x axis. A charge of -2.3 x 10-6 C is at x=-5 cm, and a charge of +9.2 x 10-6 C is at x=12 cm. At what position x would a third positive charge be in equilibrium?

The greater the speed of a satellite, the shorter its orbital period. T/F

Reading this PE question can-we-transport-energy-over-infinite-distances-through-vacuum-using-light, a related question arises naturally:

Is energy transported (by light)? -- (I did believed in this answer until now) or energy is already 'in site' (vacuum) just expecting to be excited by the photons?

This news insinuated the doubt anti-laser(1)

The antilaser does the reverse: Two perfect beams of laser light go in, and are completely absorbed.

If vacuum is able to absorb energy then it can do the reverse, and supply energy. We are already prepared to accept that vacuum has energy.

I am inclined to accept that energy do not travel at all. What is travelling is the excitation of vacuum, and we call this: photons. It may appear a question of semantics, but I think that the explicit reconaissance of this notion can be helpful.

(1) The two rays entering the slab are in phase opposition when they met, and cancel. The nature of cancelation was obscure to me, until now.

added:

I googled this: "where goes the energy in a destructive interference" and followed past answers to this question. Someone answered "into the surrounding environment." We are in minority;) Most of the times they said that the total extinguishing is impossible. This anti-laser experiment shows that energy is destroyed.

We see the same effect with sound cancelation , with boat wake (trailing waves) cancelation (by double/triple hull or when they sail in formation), and now with light.

added after 2 answers
image from astro-canada.ca
What is amazing is that this fact is inside the theory since the begining, quoting from there:

In 1801, the British physicist Thomas Young demonstrated that light propagates as waves, like waves on the surface of water. Young understood that when two light waves meet, they interact with each other. Scientists call this

A conduction electron is confined to a metal wire of length 13.2 cm. By treating the conduction electron as a particle confined to a one-dimensional box of the same length, find the energy spacing between the ground state and the first excited state. Give your answer in eV.

With a plane wave, I always took the direction of the wavevector, k, as the direction of propogation (magnitude proportional to the inverse wavelength). Alternatively, it could represent the momentum (minus a factor ?) of a particle.

However inside a crystal, the electron wavevector and the electron velocity are not necessarily in the same direction. I'm thinking here of a 2D material with a cylindrical Fermi surface where the momentum may have a z component, but the Fermi velocity does not. In everyday cases you would expect momentum and velocity to be in the same direction, moreover I considered the propogation of the wave to be in the same direction as its particle analogue.

I realise that inside a crystal the electrons are no longer simple plane waves, but what then does the k vector mean?

A series RLC circuit consists of a 58.0 ? resistor, a 2.50 mH inductor, and a 450 nF capacitor. It is connected to a 3.0 kHz oscillator with a peak voltage of 5.60 V. What is the instantaneous emf E when i = I ? What is the instantaneous emf E when i = 0A and is decreasing? What is the instantaneous emf E when i = - I ?

A river has a steady speed of 0.650 m/s. A student swims upstream a distance of 1.00 km and swims back to the starting point.

(a) If the student can swim at a speed of 1.15 m/s in still water, how long does the trip take? s

(b) How much time is required in still water for the same length swim? s

(c) Intuitively, why does the swim take longer when there is a current?

You hold a ruler that has a charge on its tip 4 cm above a small piece of tissue paper to see if it can be picked up. The ruler has −14 µC  of charge. The tissue has 1 g of mass. What is the minimum charge required to pick up the tissue paper?

Answer: ___ µC

The drawing shows a square, each side of which has a length of L = 0.250 m. Two different positive charges q1 and q2 are fixed at the corners of the square. Find the electric potential energy of a third charge q3 = -3.00 x 10-9 C placed at corner A and then at corner B.

Olive and her friend Wellington are playing down by Captain Don's docks when they find an old chain. The old chain has only three links. By measuring with an old fish scale which is a permanent feature of the dock area, they determine that the total mass of the chain is 2.55  kg (the scale reads in newtons, but Olive knows how to calculate the mass of the chain from its weight). While playing with the scale and the chain (the chain is hanging vertically from the end of the scale, and Olive is holding on to the top of the scale with both hands, either moving the entire system upwards or downwards), Olive notices that if she is accelerating the chain either upwards or downwards, the scale no longer accurately reads the weight of the chain.

When the Scale Reading is Larger than the Weight of the Chain: At one point in their experiments with the chain and the scale, Wellington observes that the scale reads 42.00  N

A: When the scale reads 42.00  N, what is the tension in the chain at the point where the lowest two links connect?

B: When the scale reads 42.00  N, what is the tension in the chain at the point where the topmost two links connect?

C: Consider all the downward forces which are acting on the middle link. When the scale reads 42.00  N, what is the magnitude of the sum of all those downward forces?

When the Scale Reading is Smaller than the Weight of the Chain: At another point in their experiments, Wellington observes that the scale reads only 11.50  N. (We could ask the same questions for this case as were asked for the case when the scale read 42.00  N. Instead, to further test your understanding, we will ask a slightly different set of questions.)

D: When the scale reads 11.50  N what is the Up-Down component of the acceleration of the old chain? Let Up be positive.

E: When the scale reads 11.50  N what is the Up-Down component of the force on the lowest link by the middle link? Let Up be positive.

F: Consider all the downward forces which are acting on the middle link. When the scale reads 11.50  N, what is the magnitude of the sum of all those downward forces?

Using the Chain to Tow a Sled: Old Captain Don is overjoyed when he sees the kids playing with the chain, because he had lost the chain sometime last winter. It turns out that Captain Don uses the chain to connect his snowmobile to his homemade sled in which he tows loads of fish, or ice, or supplies, etc. back and forth from his nearby house and office to his dock in the wintertime. Assume, for the purposes of this problem, that the little homemade sled slips across the winter snows with little friction of any kind. Suppose the mass of the sled and its load is 33.00  kg, and suppose that Captain Don is speeding up on level ground at a rate of 39.00  cm/s2 cm/s2 as he is just starting out on a towing trip. Also, assume that the chain stretches horizontally from a hook on the snowmobile to a hook on the homemade sled, i.e. ignore any sagging in the chain which is connecting the snowmobile to the homemade sled.

G: What is the tension in the chain at the meeting of links which is closest to the sled?

H: What is the tension in the chain at the meeting of links which is closest to the snowmobile?