Kaplan Nursing Entrance Practice Exam

Why does a handgun recoil slightly when fired?

1. It loses momentum

2. For every action, there is an equal but opposite reaction

3. It is affected by air resistance

4. It generates heat

The correct answer is: For every action, there is an equal but opposite reaction

A handgun recoils slightly when fired due to the principle of action and reaction, famously described by Newton's Third Law of Motion, which states that for every action, there is an equal but opposite reaction. When the gun is fired, the bullet moves forward at high speed. As a result, the gun experiences an equal force in the opposite direction, causing it to recoil backward. This phenomenon can be observed in various other scenarios involving forces. For instance, when a swimmer pushes against the pool wall, they propel themselves forward while simultaneously moving the wall slightly in the opposite direction. Similarly, a rocket launches into space by expelling gas downward, which propels the rocket upward due to the same principle. In contrast, the other factors listed, such as losing momentum, being affected by air resistance, or generating heat, do not directly cause the recoil of the handgun. Momentum loss does not apply here because momentum is conserved in this closed system; air resistance affects flight paths rather than initial recoil; and heat generation is a byproduct of the firing process, not the cause of recoil. Understanding this core principle clarifies the mechanics of why a gun recoils when fired.