Newton’s first law of motion states that an object at rest will stay at rest, while an object in motion will stay in motion, unless acted upon by an unbalanced force. An easy way to demonstrate this is in the car. When the car accelerates, you (and all other objects in the car) appear to be pushed backwards because the car is moving faster than you were as a resting object. A sudden deceleration makes you fly forward in your seat, because your body was now traveling at a particular speed and was going to keep going at that rate. This is why seat belts are important.
Smarter Every Day host Destin explores these ideas in his minivan (accompanied by two adorable and incredibly polite little science helpers), first by using a pendulum, then by using a helium balloon. The pendulum acts completely predictably, swinging back when the van accelerates and forward when it slows to a stop. The helium balloon, however, doesn’t act like Newton said it would at all. That’s because in the case of the lighter-than-air helium balloon, Newtonian physics are out and fluid dynamics are in.
Check out the video for yourself and find out why that happens:
Hat tip: i09