Why is the friction force considered to be a contact force?

a.) friction affects only solid objects.

b.) friction is produced when objects slide past one another.

c.) friction always acts in the same direction.

d.) friction acts in a direction opposite to the direction of an object's motion.

Astudent wants to test the hypothesis that fertilizer improves the growth rate of grass seeds. to test this hypothesis, the student put the same amount of grass seed in 3 containers with the same type of soil. the student measured the growth at the end of the week. all plants received equal amounts of water and sunlight. if you were asked to graph this data, what would you place on the x-axis? a. fertilizer b. water c. plant growth d. week one