The acceleration due to gravity at the surface of the moon is 1.67 ms^-2. If the radius of the moon is 1.74 × 10⁶ m, find the mass of the moon. Given G = 6.67 × 10^-11 Nm² kg^-2.

16 If a ball is thrown horizontally, what is
(i) the state of motion in that direction and
(ii) the force acting on the ball in the vertical direction? 17 What happens to the plants in the absence of gravity? 19 An astronaut on the moon measures the acceleration due to gravity and finds it to be 1.67 ms^-2. He knows that the earth is about 80 times more massive than the moon. What is his estimate of the ratio of the radius of the earth to that of the moon? 20 Why is the gravitational force of attraction between two bodies of ordinary mass not noticeable in everyday life?