Gravity is just a theory. If the same pressure held down a truck as it would a person then we'd get squished. Helium is lighter than air so it floats above oxygen. Trucks are heavier than air and much heavier than people. It requires more energy to lift a truck into the air with momentum than it would a human. Why? Because humans are lighter then trucks, but heavier than air and that's also how we stay on the planet. And that's also why the weight of the ocean will eventually weigh down on you and crush you. Water is heavier than air, but can also become lighter than air by becoming a gas. Go Creation go! HalleluYah!