A little girl sits in a car at a traffic light holding a helium-filled balloon. 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. Explain why.
Solution 26E Step 1 In this problem, when the car is moving forward, the girl is moving backward due to the inertia while changing the state. So, if the car moves forward, the girl will resist the change of state from rest to motion. Hence, she will move backward. The balloon in her hand will try to move towards the girl when the hand is moving towards the girl. But, there will be an inertia for the balloon too since it is an object with mass. So, it will try to move away from the girl’s hand since it is changing its state from rest to motion. Conclusion The girl will move backward and the balloon will move forward with respect to the direction of motion of the car. It is due to the inertia acting upon the girl and the balloon.