A little girl sits in a car at a traffic light holding a helium-filled balloon as shown in the illustration below. The windows are closed and the car is relatively airtight. When the light turns green and the car accelerates forward, her head pitches backward but the balloon pitches forward. Why?
A)This demonstration will only work with a helium filled mylar balloon since the mylar is unaffected by the motion of the air moving backwards inside the car.
B)Since helium is lighter than air, it will follow the path of the car and not the direction of movement of the air within it.
C)Since the car is relatively airtight and the balloon is not anchored to the car, the balloon will be unaffected by the acceleration of the car.
D)The air inside the car has more inertia than the helium in the helium balloon. Therefore, although the air pitches backwards, the lighter helium moves forward.

Question

A little girl sits in a car at a traffic light holding a helium-filled balloon as shown in the illustration below. The windows are closed and the car is relatively airtight. When the light turns green and the car accelerates forward, her head pitches backward but the balloon pitches forward. Why? 
A)This demonstration will only work with a helium filled mylar balloon since the mylar is unaffected by the motion of the air moving backwards inside the car. 
B)Since helium is lighter than air, it will follow the path of the car and not the direction of movement of the air within it. 
C)Since the car is relatively airtight and the balloon is not anchored to the car, the balloon will be unaffected by the acceleration of the car. 
D)The air inside the car has more inertia than the helium in the helium balloon. Therefore, although the air pitches backwards, the lighter helium moves forward.

Quizplus · Accepted Answer

According to Newton's first law of motion, an object in motion will remain in motion in a straight line unless acted upon by an external force. In this case, when the car accelerates forward, the air in the car also moves forward, but due to its higher inertia or resistance to change in motion, it tends to stay in its original position. However, since the helium in the balloon is lighter and has lower inertia, it moves forward with the car. Hence, the balloon pitches forward while the girl's head pitches backward.

A Little Girl Sits in a Car at a Traffic