9
lesson pretestMultiple Choice

Pretest

Question 9 • Physical Science

According to Newton’s first law of motion, when will an object at rest begin to move?

Answer
A
when the action and reaction forces are equal
B
when an unbalanced force acts upon it
C
when two equal and opposite forces act upon it
D
when its inertia decreases to zero

Explanation

Newton’s first law says an object at rest stays at rest unless a net, or unbalanced, force acts on it. Equal and opposite forces cancel, so they do not change its motion; inertia does not decrease to zero. Therefore, the object begins to move when an unbalanced force acts upon it.

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