Given a scientific passage and an answer, generate a question for the given answer.

Passage: Figure 2.43 Positions and velocities of a metal ball released from rest when air resistance is negligible. Velocity is seen to increase linearly with time while displacement increases with time squared. Acceleration is a constant and is equal to gravitational acceleration. Answer: acceleration
If a ball is released from rest when air resistance is negligible, velocity is seen to increase linearly, while what related property is a constant?

Passage: Physics is the study of energy, matter, and their interactions. It is concerned with matter and energy at all levelsfrom the most fundamental particles of matter to the entire universe. Answer: physics
Which branch of science is the study of energy, matter, and their interactions?

Passage: A fully developed placenta, like the one in Figure below , is made up of a large mass of blood vessels from both mother and fetus. The maternal and fetal vessels are close together but separated by tiny spaces. This allows the mother's and fetus's blood to exchange substances across their capillary walls without the blood actually mixing. Answer: blood vessels
A fully developed placenta is made up of a large mass of what?