ACT Science Practice Test 43

Students conducted experiments to determine what factors affect the period of a pendulum. A pendulum is a swinging weight, or bob, attached to a string (see Figure 1). When released from an angle, a pendulum will move back and forth. One complete back-and-forth movement is called a period.

Figure 1

Three variables may affect the period of a pendulum: swing amplitude, measured as the angle at which the bob is released; the length of the string; and the mass of the bob. Students created pendulums by attaching a string to the ceiling and then tying a bob onto the other end of the string. They conducted three experiments. In each experiment all variables except the one tested were kept constant.

Experiment 1

In this experiment students varied the mass of the bob. Bobs of 20g, 100g, and 200g were tested for three trials each. The string length was 1m and the amplitude was 45 degrees. The results are shown in Table 1.

Table 1

Experiment 2

The second variable tested was amplitude. Bobs were dropped from 15-, 45-, and 75-degree angles for three trials each. The string length was 1 m and the bobs used were 100 g. Figure 2 displays the average results.

Figure 2

Experiment 3

Next the students tested the effect of varying the string length. Strings of 1 m, 0.6 m, and 0.3 m were tested in three trials each; 100 g bobs were used and the amplitude was 45 degrees. The results are displayed in Table 2 and Figure 3.

Table 2

Figure 3

Scientists conducted a series of experiments to determine the effect of changes in temperature on the intertidal rocky shore crab, Petrolisthes granulsus (P. granulosus). Several fitness-related traits, such as body size and reproductive capacity in P. granulosus individuals from three sites were compared after the crabs were exposed to various temperatures. In addition, metabolic rate experiments were conducted to determine the energetic cost associated with crab exposure to high temperatures.

Table 1 displays the temperatures (°C) used in the experiments. Thermal category TC1 refers to the maximum temperature registered at each site, while TC2 refers to the average of maximum temperatures recorded every day. The Control thermal category is defined as the average temperature experienced by the crabs during the acclimation period.

Table 1

Figure 1 displays the average size (cephalothorax length) of P. granulosus in the 3 populations, while Figure 2 displays the average egg volume in the 3 populations.

Figure 1

Figure 2

Figure 3 shows the average standard metabolic rates (SMR) of male crabs from the 3 populations exposed to 3 thermal categories.

Figure 3

When introduced into H₂O, many solid substances are able to dissolve, or disperse evenly throughout the solvent. Salts have been found to dissolve easily when introduced into H₂O, since they readily dissociate to yield ions that may interact directly with H₂O. Molecular compounds, on the other hand, do not dissolve as easily, since their interactions with water typically do not permit ionization, the physical process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. Two experiments were conducted to better understand the solubility of salts and molecules in water at various temperatures. The solubility, S, was measured as follows:

S = (m_sub) / (m_H20)

where m_sub was the mass of the substance dissolved in water, and m_H20 was the mass of the water itself. △S, or the change in solubility (from 0℃), was calculated in the experiments for three salts and three molecules with increasing temperature. The mass of water was held constant at 100g for each of these experiments.

Figure 1 shows the results of comparing the solubilities of three salts with increasing temperature, while Figure 2 shows the results of comparing the solubilities of three molecules with increasing temperature. Molecular masses (MM) are shown for each substance.

Figure 1

Figure 2

Recombination of genes is usually associated with the sexual reproduction of cells, or meiosis. However, it can also occur when cells that undergo asexual reproduction, or mitosis, need to be repaired, such as after radiation exposure. This repair process, known as homologous recombination, aligns two copies of the same double strand of DNA, one with the error and one without. As seen in Figure 1, correct genes are transplanted from the correct strand to the one with errors (genes with errors are represented with a *).

Figure 1

The activities of some genes have been found to promote homologous recombination (HR). In an experiment to quantify the genetic control over HR, 4 scientists measured the frequency of HR per hour over a 24-hour period in isolated connective tissue cells from rats placed in growth media. They then lysed the cells, separated out the entire protein content, and used gel electrophoresis to count the amount of protein present in the cells (see Figure 2).

Figure 2

These scientists noticed that only a few specific proteins appeared to be responsible for promoting HR, and labeled the genes encoding them as W, X, Y, and Z. They engineered cells to express combinations of two active genes and recorded the HR. They then analyzed the DNA content of the lysed cells and calculated distances between four genes that encoded the relevant proteins (see Table 1).

Each of the 4 scientists then proposed individual models for the positions of the genes they studied, taking into account the findings in Table 1. Each model shows where genes may be located along a strand of DNA (see Figure 3). Each model correctly assumes that the lengths of the genes are insignificant compared to the length of the DNA.

Figure 3

A final experiment showed that rat connective tissue cells in which genes W and Y were active had an HR frequency of 45 times per hour.