Search

Consider a homogeneous magnetic field of induction $B_1$, which spans a half-space bounded by the plane of interface $y=0$, beyond which is an equally oriented, also homogeneous magnetic field of induction $B_2$. An electron flies out of the plane perpendicularly to it and the field lines (as in the figure) with velocity $v$. Determine the size and the direction of its average velocity parallel to the plane of the interface.

Bonus: Consider now that the magnitude of the field changes linearly as $B = B_0 \(1+\alpha y\)$ and its direction is in the positive direction of the $z$-axis. Again, determine the magnitude and direction of the average velocity of the electron parallel to the interface plane. The electron is initially emitted as in the previous case.

In the microworld of cells, there are two types of transport: transport by free diffusion, also known as Brownian motion where the motion uses the energy of the environment directly. The second type, so-called active transport, requires, among other things, a motor protein moving at a constant speed along the cytoskeletal filament. Consider the typical value of the diffusion constant $D \approx 10^{-9} cm^2.s^{-1}$ and the rate of active transport speed $u\approx 10^{-6} m.s^{-1}$. For which distances is the Brownian motion more time efficient than the active transport? Assume that the transport is happening in only one direction.

1. According to definitions by International System of Units, convert these into base units
   • pressure $1 \mathrm{psi}$,
   • energy $1 \mathrm{foot-pound}$,
   • force $1 \mathrm{dyn}$.
2. In the diffraction experiment, table salt's grating constant (edge length of the elementary cell) was measured as $563 \mathrm{pm}$. We also know its density as $2{,}16 \mathrm{g\cdot cm^{-3}}$, and that it crystallizes in a face-centered cubic lattice. Determine the value of the atomic mass unit.
3. A thin rod with a length $l$ and a linear density $\lambda $ lies on a cylinder with a radius $R$ perpendicular to its axis of symmetry. A weight with mass $m$ is placed at each end of the rod so that the rod is horizontal. We carefully increase the mass of one of the weights to $M$. What will be the angle between the rod and the horizontal direction? Assume that the rod does not slide off the cylinder.
4. How would you measure the mass of:
   • an astronaut on ISS,
   • a loaded oil tanker,
   • a small asteroid heading towards Earth?

When working out, we often come across workout machines that contain pulleys. Consider the machine in the following figure. What force must be applied on the rope if the velocity of the end of the rope at point A is $v = 0,4 \mathrm{m\cdot s^{-1}}$ and its direction is downwards? Each pulley has a radius $r = 15 \mathrm{cm}$ and mass $m = 15 \mathrm{kg}$. A weight of mass $M = 25 \mathrm{kg}$ hangs over the free pulley.

Jarda is standing at the end of the platform, waiting for his train to arrive. When the train's first carriage passes him, he discovers that this is the carriage where he has his seat ticket. At this point, the speed of the train is $8{,}5 \mathrm{m\cdot s^{-1}}$, and the train begins to slow down steadily until it stops in $28 \mathrm{s}$. Jarda immediately starts walking in the direction of his carriage, but because he has to push through the crowds of passengers, his speed is only $1 \mathrm{m\cdot s^{-1}}$. What is the shortest time the train must stay in the station for Jarda to board his carriage?

Measure the coefficient of static friction between two sheets of office paper.

Jarda found an apple in his backpack after the FYKOS camp, which was no longer in good condition. He threw it into a low kitchen trash can $1{,}0 \mathrm{m}$ away, and of course, he scored a hit. He threw the apple horizontally from a height $0{,}5 \mathrm{m}$, and it landed on the spot where the wall and the base of the trash can meets, where it smashed. The basket with a mass $910 \mathrm{g}$, was displaced by a distance of $5 \mathrm{cm}$ after the apple hit. What is the coefficient of friction between the floor and the basket? The apple has a mass of $230 \mathrm{g}$.

Every second, a material of mass $\mu $ falls vertically onto a moving horizontal conveyor belt and falls away at its end. A resistive force $F\_{odp}=kv$, which is directly proportional to the belt speed $v$ through the constant $k$, acts on the belt. At what speed does thevbelt move if

• a constant driving force $F$ acts on it?
• it is driven by a motor of constant output power $P$?

Karel uses an elevator in a building with a ground floor and $12$ floors above it, while the height between floors is $h=3{.}0 \mathrm{m}$. Consider that the elevator accelerates half the time and decelerates half the time at a constant acceleration of $a=1{.}0 \mathrm{m\cdot s^{-2}}$ and that there is a $50\mathrm{\%}$ probability that the elevator is stationary on the ground floor. The rest of the probability is evenly distributed among the other floors. What is the expected waiting time for the elevator on each floor of the building? Neglect the time needed for opening doors.

Bonus: Let us have $2$ elevators in a twelve-story building. One elevator is always recalled to the ground floor. To which floor should we send the second one to minimize the average waiting time? Similarly, assume that half of the rides will start on the ground floor and the other half, with equally distributed probability, will start on any other floor.

A once positively ionized xenon atom flew out from the center of a large cylindrical coil with velocity $v=7 \mathrm{m\cdot s^{-1}}$ and began to move through a homogeneous magnetic field, which is in a plane perpendicular to the magnetic lines of force. At a certain point the coil is disconnect from the source, thus its induction begins to decrease exponentially according to the following equation $\f {B}{t}=B_0\eu ^{-\Omega t}$, in which $B_0=1,1 \cdot 10^{-4} \mathrm{T}$ and $\Omega =600 \mathrm{s^{-1}}$. What is the deviation from the initial direction after the atom is stabilized?