A group of students is investigating whether the smoothness of a surface affects the force of friction. Each student pushes a book on two different surfaces. One surface is ice and the other is concrete. The distance traveled by the book on each surface is shown. Experimental Observations
Student 1 Student 2 Student 3 Student 4
Distance traveled by book on ice 23 cm 21 cm 24 cm 22 cm
Distance traveled by book on concrete 15 cm 18 cm 16 cm 17 cm

Based on the table, which conclusion is correct about the average distance traveled?
A. It is less on concrete.
B. It is greater on concrete.
C. It is less when the book is pushed harder.
D. It is independent of the type of surface.

their crystal patterns have been disrupted by stress

a lot of it means it can be "burned" for energy and sold at one price. if it is scarce, the price will go up

air exerts forces on falling objects near earth’s surface.

explanation:

the term 'free falls' refer to object that are falling towards the earth, in a situation in which gravity is the only force acting on the object.

for an object in free fall, the acceleration is constant (g, the gravitational acceleration), and so the velocity of the object constantly increases.

for objects near earth's surface, there is always another force acting on the object: the resistance due to the air. this force is opposed to the direction of fall of the object, and it is proportional to the velocity of the object, so as the object falls down, this force increases up to a point when it balances the force of gravity, and the object continues its motion at constant velocity (called terminal velocity). since in this case gravity is not the only force acting on the object, we are not in a situation of free fall.

i don't understand. explain it clearly