ACT Science Practice Test 5

Vasoconstriction involves a narrowing of blood vessels that could lead to poor blood flow in the body if it persists over a long time. Ergotamine is a substance that can cause vasoconstriction. When ergotamine is injected into a normal blood vessel, vasoconstriction occurs quickly at the site of the injection (see Figure 1).

The diameter of the blood vessel at the site of vasoconstriction is less than the diameter of the normal blood vessel, so blood flow has a higher velocity through this narrow site. As a result, the blood pressure in the site of vasoconstriction is less than the blood pressure in the normal blood vessel. Moreover, the higher the velocity of the blood flow through the site of vasoconstriction, the lower the blood pressure at that site.

The percent change in blood pressure (%BP) can be defined as:

Blood vessel sections of similar diameters were isolated from laboratory rats and %BP was measured over three experiments. When the researchers needed to create a site of vasoconstriction for some of the experimental trials, they would inject ergotamine to induce vasoconstriction within the blood vessel section.

Experiment 1

An artificial heart, which mimics a human's heartbeat, is used to move a constant volume of 500 mL of blood with each beat through four blood vessel sections. These four blood vessel sections were injected with the same amount of ergotamine, leading to sites of vasoconstriction of the same diameter. The rate at which the blood is pumped was varied for the four different blood vessel sections, and the %BP values that resulted were measured.

Table 1
Rate of artificial heart beat (beats per minute)	%BP
60	1.2
90	9.3
120	22.3
150	45.1

Experiment 2

The artificial heart used in Experiment 1 was then used to pump a constant volume of 500 mL of blood with each beat at a constant rate of 90 beats per minute through five other blood vessel sections. These blood vessel sections were injected with different amounts of ergotamine, resulting in sites of vasoconstriction with different diameters. The %BP values were then measured.

Table 2
Diameter of site of vasoconstriction (cm)	%BP
0.4	40.3
0.6	18.6
0.8	9.3
1.0	4.6
1.2	2.5

Experiment 3

The artificial heart used in Experiment 1 was used to pump different volumes of blood at a constant rate of 90 beats per minute through five blood vessel sections with the same diameter at the site of vasoconstriction. The %BP values were then measured.

Table 3
Volume of blood pumped (mL)	%BP
400	8.4
450	8.8
500	9.3
550	9.7
600	10.2

As the pressure on a gas is increased, the volume of that gas is expected to decrease by an inversely proportional amount. For example, if pressure is doubled the volume is halved. Under certain conditions, the volume of the gas will change by an amount that deviates from an inverse proportion. Various 10.00 L samples of gas were subjected to increases in pressure. Table 1 shows the resulting volume changes at 300°C, while Tables 2 and 3 show the volume changes at 25°C and -200°C, respectively. All pressures are measured in atmospheres (atm).

There are four planets in our solar system called gas giants: Jupiter, Saturn, Uranus, and Neptune. They are so named because they are composed largely of gases rather than solids. Figure 1 shows how temperatures of the atmospheres of Jupiter, Neptune, and Saturn vary with altitude above the cloud tops. Table 1 gives the composition of the planets in both relative abundance of gases and the altitude at which those gases are most abundant. Table 2 gives what the temperature at the cloud tops would be without greenhouse warming.

Figure 1

Nuclear fission occurs when the nucleus (central core) of an atom splits into multiple parts. This splitting is accompanied by the release of a large amount of energy, as in nuclear weapons and nuclear power plants.

A chemical element is said to be radioactive if it is prone to fission. Fission is often the result of the nucleus of a radioactive atom absorbing a free neutron (an uncharged nuclear particle). When a fission event occurs, the nucleus often splits into two new nuclei and produces free neutrons. This process generates the possibility of a chain reaction. If, on average, a fission event produces one neutron and that neutron causes another nucleus to fission, the reaction is said to be critical; that is, it will sustain itself, but not increase in magnitude. If one fission event releases more free neutrons than are required to initiate another fission event, the reaction is said to be supercritical; that is, it will sustain and increase in magnitude. If more neutrons are required to initiate a fission event than are released in fission, the reaction is said to be subcritical: the reaction will not sustain itself.

Many factors affect how many neutrons from each fission event will trigger another fission event. The most important factor is the mass (m) of the substance. The criticality of a substance also depends on the substance's purity, shape, density, temperature, and whether or not it is surrounded by a material that reflects neutrons.

In a nuclear weapon, a radioactive substance is made highly supercritical. One of the primary challenges in building a nuclear weapon is keeping the radioactive material subcritical prior to detonation, then upon detonation, keeping it supercritical for a long enough period of time for all of the material to fission before it is blown apart by the energy of the blast. A fizzle occurs when a nuclear weapon achieves supercriticality but is blown apart before all of the radioactive material fissions.

The first nuclear weapons were made of enriched uranium, or U-235. The density (ρ) of U-235 under normal conditions is 19.1 g/cm³. For U-235 to attain a supercritical state, the product of its mass and density must exceed 10⁶ g²/cm³. If it is assembled over too long a time (t), it will achieve slight supercriticality and then fizzle. Therefore, the speed of assembly (measured as t divided by ρ), must be less than 10^-5 sec × cm³/g (Michelson's Criterion).

Two schemes for the assembly of a supercritical amount of U-235 that avoid fizzle are discussed below.

Gun-Type Weapon

At one end of a tube, similar to a gun barrel, is a hollow, subcritical cylinder of U-235 with a mass of 48 kg; on the other end is a subcritical pellet of U-235 with a mass of 12 kg. The pellet is propelled by a small explosion down the tube and into the cylinder of U-235. The combined mass of the two pieces of U-235 is great enough to induce a supercritical state. Since the combined cylinder of U-235 is at or near normal density, the assembly process must be completed in less than 2 × 10^-4sec to meet Michelson's Criterion.

Implosion-Type Weapon

A 15-kg sphere of U-235 is surrounded by explosives. When the explosives are simultaneously detonated, the U-235 is compressed in order to achieve supercriticality. The explosives are designed to compress the U-235 to a density of approximately 70 g/cm³ in less than 10^-7 sec.