Answers and Explanations

1. The answer is (D). Bacteria are often classified and named on the basis of their shape, of which there are three: spherical, rod-like, and helical (spiral). Spherical bacteria are known as cocci; rod-like bacteria are known as bacilli; and helical bacteria are known as spirochetes, or spirilla. Thus, if a sample from a CF patient infected with Staphylococcus were cultured and viewed under a microscope, the bacteria would appear spherical in shape.

2. The answer is (B). To solve this problem, apply Hooke's Law F = kx, where F is the force applied to the spring, x is the distance the spring stretches, and k is the spring constant. The forces applied in this case are weights. Since weight is proportional to mass, the distance the spring stretches is also proportional to the mass of the attached to the spring.

   The spring starts out with a length of 64 cm. When the 0.5 kg mass is attached, it stretches 8% longer or 64(.08) = 5.12 cm. Since the distance stretched and the mass attached are proportional, the ratio of the distance stretched to the mass attached is 5.12/(0.5 kg), which is approximately equal to 5/(0.5) = 10 cm/kg.

   In other words, each kilogram attached to the spring stretches it 10 cm. Thus, when the 0.4 kg mass is attached, the spring is stretched a distance (0.4 kg)(10 cm/kg) = 4 cm. Therefore, the total length of the spring is the original length plus the distance it stretches, or 64 + 4 = 68 cm.