Newton's Laws of Motion Sample Problem Answers

1. An object dropped near the earth's surface falls 4.9m in the first second. If the moon's gravitational pull is about 1/6 that of the earth's, how long would it take the same object to fall the 4.9m?

This problem is actually much simpler than it sounds. Since the gravitational force of the moon is 1/6 that of the earth, an object would fall 1/6 as fast. All you need to do is multiply the original time by 6.

2. A 5kg block is pulled westward across a frictionless horizontal surface by a 30N force applied at 87° angle above the horizontal?
a. What is the block's acceleration?
b. What is the normal force?

a. 0.314m/s^2
b. 19.04N

First, we find the components of the force, and then we use Newton's second law.

30N cos 87° = 1.57N
30N sin 87° = 29.96N
1.57N = 5kg * a
a = 0.314m/s^2

For part b, we will simply subtract the y component of the force from gravity to find the normal force.

FG = 9.8m/s^2 * 5kg
FG = 49N
49N = FN + 29.96N
FN = 19.04N