What if everyone in India jumped at the same moment?
1.4 billion people. One enormous jump.
Jump Day
An asteroid is coming. The government has a plan: everyone jumps at once.
1. The Broadcast
The announcement came on every screen at once. An asteroid, codenamed Kali-7, was on a path that might hit Earth in twenty years. The new Ministry of Momentum had a solution. At exactly noon on 1 January, all 1.4 billion people in India would jump. 'Together,' said the minister, 'we will push our planet out of danger.'
There were posters, songs and jumping drills at school. Rohan's friends practised every morning. Rohan, who had paid attention in physics, felt sick.
2. The Countdown
At 11:59, the whole country went quiet. Trains stopped. Farmers stood in fields. Rohan's class lined up in the school yard. Ten, nine, eight… at zero, 1.4 billion people bent their knees and sprang about half a metre into the air.
For a moment, about 84 billion kilograms of humanity was off the ground. Then gravity pulled everyone back. A great thud rolled across the country.
3. The Measurement
The Ministry's seismographs did pick something up: a faint tremor, smaller than a distant lorry. The scientists who worked out how far Earth had moved got a number of about 0.000000000000007 metres. Less than the width of the nucleus of a single atom.
And as soon as everyone landed, Earth moved back. The asteroid's path hadn't changed by a millimetre. The minister announced that Jump Day would now happen every year 'for good luck'.
4. Real Pushing
Rohan wrote a letter to the newspaper. You can't move a planet by pushing on it from the inside, he explained, just as you can't move a boat by pushing on its mast while you stand on its deck. Earth and everyone on it is one system, and its centre doesn't budge.
Then he wrote about the thing that had actually worked. In 2022 NASA's DART spacecraft had crashed into a small asteroid called Dimorphos and changed its orbit by about half an hour. 'If you want to move a rock in space,' he finished, 'you have to go and hit it.' To his surprise, they printed it.
The end. Now see the science that the story is built on.
See it happen
What would happen, step by step
1.4 billion people at an average of about 60 kg weigh around 84 billion kg together. They all jump about half a metre at once.
The science behind it
Action and reaction
m_people × v_people = M_Earth × v_Earth
When you jump, you push down on Earth and Earth pushes up on you with an equal force (Newton's third law). The momentum you gain upwards equals the momentum Earth gains downwards. Earth is about 70 trillion times heavier than everyone in India combined, so it moves about 70 trillion times less.
Internal forces can't move a system
Earth plus everyone on it is one system. Pushes inside a system always come in equal and opposite pairs, so its centre of mass stays exactly where it was. When everyone lands, Earth moves back. To change a system's motion you need an outside force.
How we really move asteroids
NASA's DART mission flew a car-sized spacecraft into the asteroid Dimorphos in September 2022, at about 6 km/s. It shortened the asteroid's 11 hour 55 minute orbit round its partner by about 32 minutes. It was the first time humans had changed the motion of an object in space on purpose.
In one line: Momentum is conserved: everyone's upward push equals Earth's push the other way. With mass ratios this huge, Earth's movement is far too small to feel.
Forces and Newton's laws →Think about it
If you stand on a skateboard and throw a heavy ball forwards, which way do you roll? How is that different from Jump Day?