Which best explains why a wood- burning fireplace represents an open system?
a.)because they are all made of brick
b.)because the ashes have to be swept away
c.)because they can be used for cooking
d.)because a fire requires oxygen from the atmosphere to burn

Point charges q1=+2.00μc and q2=−2.00μc are placed at adjacent corners of a square for which the length of each side is 1.50 cm . point a is at the center of the square, and point b is at the empty corner closest to q2. take the electric potential to be zero at a distance far from both charges. part a what is the electric potential at point a due to q1 and q2? express your answer with the appropriate units.

Researchers conducted an experiment to identify the effects of three different hand sanitizers on the growth of bacteria. the researchers grew one type of bacteria in twenty-four identical covered petri dishes. then, they opened eighteen petri dishes and applied the three hand sanitizers to six petri dishes each. they left the remaining six petri dishes closed as a control group. when the researchers measured the growth of bacteria two days later, the six dishes that had no hand sanitizer showed the least amount of bacterial growth. the researchers concluded that not using any hand sanitizer would lead to the least bacterial growth. which statement best explains why the researchers’ conclusion was not justified? the experiment tested too many variables. the control group was not treated the same as the experimental group. the sample size of twenty-four petri dishes is not large enough. the single type of bacteria is not adequate to make any conclusions.

Aheavy boy and a lightweight girl are balanced on a massless seesaw. the boy moves backward, increasing his distance from the pivot point. what happens to the seesaw? it is impossible to predict without knowing additional information. the side the boy is sitting on will tilt downward. the side the girl is sitting on will tilt downward. nothing; the seesaw will remain balanced.