You might have seen sunglasses that change to a color when exposed to light, while revert to transpaparency when the light is blocked or dimmed. Thematerical used to make the glasses is called photochromic material that has light sensitive components inside. The technology is similar to that used to make a photographic film.

(a) Explain, using necessary chemical formula, the working principle of photochromic sunglasses.

(b) The color of the photochromic sunglasses is reversible, however, the image on a photographic film once exposed and developed, is fixed and becomes irreversible. Why?

The ground temperature a few meters below the surface is fairly constant throughout the year, and is near the average value of the air temperature. In areas in which the air temperature drops very low in the winter, the exterior unit of a heat pump designed for heating is sometimes buried underground in order to use the earth as a thermal reservoir. Why is it worthwhile to bury the heat exchanger, even if the underground unit costs more to purchase and install than one above ground?

A 0.18 kg puck is initially stationary on an ice surface with negligible friction. At time t = 0, a horizontal force begins to move the puck. The force is given by F→=(15.0-3.94t2)î, with F→ in newtons and t in seconds, and it acts until its magnitude is zero. (a) What is the magnitude of the impulse on the puck from the force between t = 0.560 s and t = 1.79 s? (b) What is the change in momentum of the puck between t = 0 and the instant at which F = 0?

A 0.00618-kg bullet is fired straight up at a falling wooden block that has a mass of 1.52 kg. The bullet has a speed of 616 m/s when it strikes the block. The block originally was dropped from rest from the top of a building and had been falling for a time t when the collision with the bullet occurs. As a result of the collision, the block (with the bullet in it) reverses direction, rises, and comes to a momentary halt at the top of the building. Find the time t.

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WHAT IS UNUSUAL ABOUT THE DISCOVERY OF URANUS ?

A truck accelerates uniformly from rest to 19.5 m/s in 5.3 s along a level stretch of road. Determine the average power required to accelerate the truck for the following values of the weight (ignore friction).

(a) 1.00x10^4 N

(b) 1.20 x10^4 N

A student stands several metres in front of a smooth reflecting wall, holding a board on which a wire is fixed at each end. The wire, vibrating in its third harmonic, is 75.0 cm long, has a mass of 2.25 g, and is under a tension of 400 N. A second student, moving towards the wall, hears 8.30 beats per second. Determine:

a. frequency of sound produced by the wire;

b. the speed at which the second student approaches the wall.

Obtain the wavelengths of a photon and an electron that have the same energy of 3.0eV.

A) Find the time tH it takes the projectile to reach its maximum height H.

B) Find tR, the time at which the projectile hits the ground after having traveled through a horizontal distance R.

C) Find H, the maximum height attained by the projectile.

D) Find the total distance R (often called the range) traveled in the x direction; see the figure in the problem introduction.

A 0.280-kg piece of aluminum that has a temperature of -166

A car is moving at 55 miles per hour. The kinetic energy of that car is 5 × 105 J.How much energy does the same car have when it moves at 98 miles per hour? Answer in units of J

12) Two identical particles of charge 6 μC and mass 4 μg are initially at rest and held 4 cm apart. How fast will the particles move when they are allowed to repel and separate to very large (essentially infinite) distance? Answer: Last Answer

Hint: The particles are identical, so you can assume that the scenario is symmetrical. Use energy conservation: What type of energy is stored in the system when the particles are near each other and at rest? How does the voltage of a particle allow you to compute this energy? What about when the particles have flown away from each other? What is the relationship between these energies, according to energy conservation? (Also, remember that the SI unit for mass is the *kilo*gram and you are given the particle masses in *micro*grams.)

Now suppose that the two particles have the same charges from the previous problem, but their masses are different. One particle has mass 4 μg as before, but the other one is heavier, with a mass of 28 μg. Their initial separation is the same as before. How fast are the particles moving when they are very far apart? [Enter the heavier particle's final speed in the first box and the lighter particle's final speed in the second box.]

Answer 1 of 2:

Answer 2 of 2:

Hint: Notice that the asymmetry in the particle masses means that you cannot assume that they end up with the same speeds after separating. Use momentum conservation to help solve this problem.

A uniform stick 1.3 m long with a total mass of 270 g is pivoted at its center. A 3.8-g bullet is shot through the stick midway between the pivot and one end. The bullet approaches at 250 m/s and leaves at 140 m/s. With what angular speed is the stick spinning after the collision?