2. Apply Newton’s second law to calculate the frictional force needed to hold a 15 g pen in your fingers and keep it from falling.

Weight is a force, so we use newtons second law which states F=ma. So the weight of a watermelon on earth is 2kg x 9.81m/s², which is 19.62N (newtons). On the moon it weighs 2 x 1.63 which is 3.26N.

The weight of the pen is 0.02 x 9.81, which is 0.1962N. If the pen is not to fall the frictional force needs to be equal to this force, so the answer is 0.1962N.

You would need to apply a force of at least 15 grams to hold up the pen