A population is a group of the same species living in the same area. This framing lets us study how many individuals are present, how crowded they are, and how the group changes over time in that space. It’s different from an individual organism (one living thing) and from cells (the basic units of life). It’s also distinct from a microbiome, which is all the microorganisms in an environment and may include many different species rather than just one. For example, a school of fish or a stand of oak trees in a meadow each forms a population of that species in that area.

Impulse is the change in momentum, and when a net force is constant, it equals the force multiplied by the time it acts. With a 3 N force acting for 4 seconds, the impulse is 3 N × 4 s = 12 N·s. That impulse is the amount by which the object's momentum changes in the force’s direction. If the force weren’t constant, you’d add up F dt over the interval, but here multiplication suffices.

At the start of the lunar cycle, the Moon sits between the Earth and the Sun, so the side that is lit by the Sun faces away from us. That means nothing of the sunlight-facing side is visible from Earth, making the Moon effectively invisible in the sky. This phase is called the New Moon, and it’s why you don’t see the Moon at night during this time. As the Moon continues its orbit, we begin to see more of the illuminated side in the following phases, moving toward crescent, first quarter, and so on. By contrast, a Full Moon appears fully lit because the Earth is between the Sun and the Moon, and a Quarter Moon is a half-illuminated phase visible in the evening. “Moon Phase” is a general term for any phase, not a specific one.

When two or more populations rely on the same resource, they experience competition. Competition happens because the resource is limited, so each population tries to access what it needs, reducing the amount available for others. This interaction can slow growth, survival, or reproductive success for the populations involved. Over time, organisms may shift to reduce overlap in resource use (resource partitioning) or, if the resource remains scarce, one population may be dampened more than the other (competitive exclusion). For example, two herbivore species that eat the same grass will compete more as grass becomes scarce, since each species has to share a limited food supply. Energy, while crucial for life, describes the capacity to do work rather than how populations interact over a shared resource. A molecule is a chemical unit, and change is a broad process not specific to the interaction between populations over resources.

Energy in biology is stored in chemical bonds within molecules that organisms can tap to do work. The best term is energy storage molecule because it names a molecule that holds usable energy and can release it during metabolism to power cellular processes. Energy is a property, not a substance; a molecule is a general term and doesn’t specify the role of storing or releasing energy; change has no relevance to describing a molecule that provides energy. Classic examples include ATP, which stores energy in its phosphate bonds and releases it when a phosphate group is removed, and glucose, which stores chemical energy that is released when it is oxidized.