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Last Word is Â鶹´«Ã½â€™s long-running series in which readers give scientific answers to each other’s questions, ranging from the minutiae of everyday life to absurd astronomical hypotheticals. To answer a question or ask a new one, email lastword@newscientist.com
If Earth turns at (say) 1000 kilometres per hour at London’s latitude, when wanting to travel, why not just go straight up and wait for your destination to rotate around until it is beneath you? (Continued)
Mel Earp
Macclesfield, Cheshire, UK
Imagine a small quadcopter-style drone, much used by film crews these days. Set it on the ground and fly it vertically upwards, then wait. Given calm conditions, the drone will just hover there with only very minor sideways force needed to keep it in the same position relative to the ground it rose from. This is because the air around you and the drone is moving with the spin of Earth. It will take additional force to keep the drone stationary inside the spinning atmosphere of Earth. This force must move the drone through the air.
In essence, this is what an aircraft wishing to travel due west from London tries to do. Aircraft like the Airbus A380 and Boeing 747-8 can cruise at 1000 km/h, but usually travel at less than that. At this top speed, they would be moving relative to London, but stationary with respect to the fixed frame of reference within which Earth is spinning. They have to use their engines to create the thrust necessary to overcome the forces exerted by the air moving with the spin of Earth.
Ron Dippold
San Diego, California, US
The problem is that London is spinning at 1050 km/h west to east, but so are you and the air around you. Stopping relative to that is actually a lot of work! Try jumping straight up – thankfully, you won’t get splattered against the walls of a building, which are also moving at 1050 km/h – but this indicates why it isn’t so easy.
This is Isaac Newton’s first law of motion at work: “A body remains at rest, or in motion at a constant speed in a straight line, unless it is acted upon by a force.” If you are moving, you have inertia; you will keep moving in that direction unless pushed otherwise.
If we were to ignore Earth’s rotation for a moment, what if a stuntperson were standing on the hood of a car going 100 km/h and jumped another 2 metres upwards while the car kept going? If you ignore the air, there is nothing to slow them down, so they spend about 1.3 seconds aloft and land on the hood about 36 m down the road, still going at 100 km/h. If you add in air resistance, they land about 4 m behind the car. But if the car had a 100 km/h tailwind, there would be no air resistance, and the stuntperson would land on the hood again. This is much like you in London, if the car is Earth!
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If you were in Hyde Park and jumped 1 kilometre high in a vacuum, you’d land about 1.3 metres west of where you started
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Other than gravity, there are two major factors to consider. First, the higher you go, the larger the circumference you have to travel. It takes twice as long to go around a 2 km-radius circle as a 1 km-radius circle at the same speed, and even going from 6371 km (Earth’s radius) to 6372 km (1 km above the surface) is a small but noticeable 0.02 per cent difference. If you were in London’s Hyde Park and jumped 1 km high (leg day!) in a vacuum, you’d land about 1.3 m west of where you started.
Then there’s the (on average) 1050 km/h air dragging you eastwards. Since the air is typically moving with the planet’s surface, if you ignore air currents like the jet stream, there is no difference from this factor whether you are going west to east or east to west.
Of course, aircraft do go higher to be in thinner air and to make use of the 200 km/h boost of jet streams, but there is a form of transportation that can break free of the air’s tyranny: rockets! They can go into low orbit, hang around, then come down anywhere along their orbit they want. But for now, they are hardly a bargain.
Guy Cox
Sydney, Australia
If the questioner is standing on the ground, they are also moving at 1000 km/h. In a simplified scenario, when they rise up in the air, they will still be going that speed and will therefore come down on the same spot. That is, unless the wind has blown them somewhere.
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