### World Jump Day

You too can stop global warming! Just jump at the right time. Of course, I don't know how to judge the physics - fact or farrago? In any case it only caters to those in timezones from the International Date Line to +2 GMT (Istanbul), so those of us in the opposite part of the world will just brace ourselves for the impact.

Edit: welcome to all my Belgian visitors! Any chance of sending over mussels and beer?

Edit: welcome to all my Belgian visitors! Any chance of sending over mussels and beer?

## Comments

1. For a person to jump off the surface of the Earth, Newton's Third Law guarantees that the Earth will feel a reaction force equal and opposite to the force generated by jumping. This then to an excellent approximation cancels the impulse of the force generated on impact. The only effect of 6 billion jumping human beings will be lots of sore kneecaps and a few stress lines in concrete all over the world, and the earth's days changing ever so slightly in duration.

2. Imagine for a moment that Newton's Third Law is suspended. Then if the average person weighs 60 kg and manages to attain an impact velocity of 10 m/s in one jump, the collective momentum imparted by 6 billion jumping human beings is 3.6x10^12 J.s. This is enough momentum to change the velocity of the earth (with a mass of 6.0x10^24 kg) a grand total of 6x10^-11 m/s. Considering the orbital velocity of the earth is 30 km/s, this change is as good as nothing.

3. Celestial mechanics guarantees that if escape velocity is not attained, the earth will only be perturbed about its current orbit. The average distance from the earth to the sun will

notchange; instead the proposed actions (multiplied ten-quadrillion fold, i.e getting everyone to jump once a day at the right time for 27 billion years) will only succeed in wobbling the earth's orbit, the least of which would result in climatic chaos. The animation on the website is completely bogus.4. The only possible effect on the earth would be changing its angular momentum. Assuming the average human stays aloft for two seconds during the jump, the angular change of the earth due to rotation is 0.5'. So the maximum possible change in angular momentum due to World Jump Day is 1.3x10^8 J.s, which is enough to change the earth's angular momentum about its axis (which is 1.8x10^34 J.s) by a factor of 7x10^-27, which would reduce the length of the day by 6 x 10^-21 s.

Geez, and professional scientists wonder why the public continues to be ignorant of science.

I know it violates the laws of physics, but you see, I never studied law. [/bugsbunny]

i hope your bugs bunny imitation doesn't include nibbling on raw carrots; as far as i can tell, they don't make one any wheatier. ;)

Greatings

JP

This would make the co2 levels skyrocket, and i'm not sure what would happen first.

If we would suffocate from lack of oxygen, starve from lack of food, or incinerate from huge temps

You've made some interesting calculations, and I'm not going to try and argue that the collective force of a billion people jumping is going to have the slightest influence on the Earth's orbit- of course it won't. From a physics point of view, though, I have to question a couple of your assumptions - most notably that the influence on the Earth of pushing off and landing would not cancel each other out - in fact they reinforce each other. Both are downward impulses on the Earth's surface, so both influence it in the same direction.

In terms of celestial mechanics, while it's certainly true that orbits are a fairly stable equilibrium (and hence slight deviations tend to be self-correcting), they can be adjusted and still remain stable in their new configuration, even without acheiving escape velocity. The orbit represents a balance of forces between gravitational attraction (towards the centre of the orbit) and angular acceleration (at a tangent to the orbit). The gravitational force is related to the distance between the bodies, and the angular acceleration is related to the speed at which the orbiting body moves along the orbit path. Therefore it is theoretically possible to increase the distance from the gravitational centre and still have a stable orbit, provided that the speed of the orbiting body is reduced.

if you look on there the calculator would make everyone jump at the same time, which means everyone would cancel each other out

Oh, and to the previous commenter, I think only half the world is supposed to jump at one time...

Why dont we all blow upwards at the same time? It should change the Earth's orbit as well

Newton's third law says that for every action or force there is an equal and opposite reaction.

So if we all blow upwards at a certain point on the planet, the Earth showld move in the opposite direction.

LOL

May be we could all fart at the same time; but tell the indians and the chinesse to join, otherwise It's not gonna work. (we should eat lots of baked beans if we were gonna try this method)

LOL

So if a large mass of people jump at the same time it will create some kind of force so in turn which will give rise to a reaction.

So it is possible that the reaction can be in the way that the orbit can change...as the earth is a bit inclined....

If there is no external force the center of mass of earth moves with constant speed in a stright line: in this case no internal force (like those from the jump)can change the motion of the centre of mass.

But this is not the case : here we are in presence of external forces.

My doubt is : can internal forces (and mass distribution variation) cause a change in external forces ? If yes the orbit could be changed in principle. Surely the rotation period of the earth can be changed by redistribution of mass !

Giuseppe

1. Newton´s third law is not correctly mentioned here, because there are two different events: on the 1st moment you jump, and you apply a force on earth´s surface. On the second moment, you fall down and apply a second force on earths surface. Any force is cancelled here.

2. The speed generated by the impact, calculated by our friend as being 6x10^-11 m/s, is orthogonal to the speed of rotation (30 km/s), and so has no influence to it.

3. It´s right that the average distance from the sun will not change, but the trajectory will.

4. About angular momentum, will there be a change since the average ratio of the orbit will be higher, and the shape of earth will not change?

I agree that most of people are ignorant of science, but I would like to see here if someone can explain why wjd can not work.

I will give the right answer on july 19th.

Thanks.

I think I'll just stick to SPF 45+ please :p