C
The atomic mass is the sum of the number of neutrons and protons.
Answer:
15.75 cm
Explanation:
focal length of convex lens, f = + 9 cm
position of object, u = - 21 cm
Let the distance of image is v.
Use lens equation
1 / f = 1 / v - 1 / u
1 / 9 = 1 / v + 1 / 21
1 / v = 1 / 9 - 1 / 21 = (21 - 9) / (21 x 9)
1 / v = 12 / 189
v = 15.75 cm
Unbalanced forces is what they are called
