# I Confusing speed of man in moon question

1. Dec 6, 2016

### thedudereturns

Consider a man having superpower,weighing 70 kg jumping from moon to reach earth in 10sec..what's the speed at which he travels..considering gravity etc...

2. Dec 6, 2016

### A.T.

Gravity won't do much at that speed over this short period.

3. Dec 6, 2016

### thedudereturns

So is it simple d/t or do we have to consider anything else ..he is jumping...to earth from moon

4. Dec 6, 2016

### jbriggs444

Ignoring all of the impossibilities, yes. It's simple d/t. He's moving at something like 15% of the speed of light.

5. Dec 6, 2016

### CWatters

and over 10s gravity adds about 14m/s. You get more error by not specifying which bit of the earth he lands on.

6. Dec 6, 2016

### thedudereturns

Pardon me ..didn't quite understood what you said...please do explain

7. Dec 6, 2016

Understand*

8. Dec 6, 2016

### jbriggs444

The man is travelling in the neighborhood of 40 million meters per second. Most of that time will be spent far from the earth and far from the moon where the acceleration from gravity is low. @CWatters has done some work and obtained an estimate of 14 meters/sec2 for average acceleration over the ten second interval.

As a crude estimate, that acceleration would amount to about a one part per million discrepancy in velocity. Maybe three parts per million if you are careless in the estimate. Three parts per million of the 400,000 km distance to the moon is about 1.2 km. There are mountains on the earth taller than that. So the error from neglecting the acceleration is smaller than the error in neglecting to specify a landing spot.

[Note that the radius of the earth is 6000 km. It's not just how high a mountain you land on. Whether you land dead center or nearer the horizon is even more important].

9. Dec 6, 2016

### Staff: Mentor

Which is nothing compared to the variation in the distance between the Earth and the Moon.

10. Dec 6, 2016

### Charles Kottler

The location of the moon in its' orbit makes even more difference - about 45,000km between apogee and perigee...

11. Dec 6, 2016

### Cutter Ketch

So this superman accelerates from 0 to 4e7m/s in the distance of extending his legs and body. Say his center of mass moves 1m. The acceleration lasts 50ns. Let's say his mass is 100kg. That requires a force of 8e16N imparting an energy of 8e16J. That assumes the moon outweighs him by a very large margin (it does) and that the moons surface can take the massive impact (it can't!)

Rather than accelerating toward the earth most of superman's energy will go into putting a very large crater in the moon. How big? Let's picture an asteroid colliding at the much more usual collision speed of 10 m/s. For the collision to have the same energy the asteroid would have to weigh 1.6e15 kg which is the mass of an asteroid about 20km in diameter. Good thing superman attempted this jump from the moon. If he tried it on earth we'd all go the way of the dinosaurs!!!

12. Dec 6, 2016

### PeroK

This reminds me of "The man in the moon came down too soon and asked the way to Norwich ...".

13. Dec 6, 2016

### CWatters

The 14m/s figure I mentioned isn't really valid. It's the velocity you reach if you fall, starting from rest and accelerating at 9.8m/s/s, for 10s. So it's the most that gravity could add (I think).

14. Dec 6, 2016

### rootone

You turn right off the M11 motorway and drive about 1 hr in NE direction if you started in London
It doesn't rhyme though.