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Kinetic Energy of an object at the speed of light 
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#1
Sep1011, 07:41 PM

P: 9

If you could fire a piece of lead at the earth that was 1cm^{3}, at close to the speed of light, what would be the amount of enery released? What would be the effect on the planet with and without an atmosphere?
thanks guys GRB 


#2
Sep1011, 07:50 PM

P: 174

http://hyperphysics.phyastr.gsu.edu...iv/releng.html
You can input your own values into the topic titled "Relativistic Kinetic Energy" to find out. As for your 2nd bit, no idea. Probably would either melt or oxidise. 


#3
Sep1011, 08:11 PM

P: 9

I was hoping someone could do the calculations for me, and then translate that into something that I could grasp with my limited intellect... I really have no idea what a joule even is. I'm not a student, and the only reason for this question is personal curiosity. If the earth had no atmosphere, and the object hit the surface, what do you educated guys think would happen? Any guesses? 


#4
Sep1011, 09:37 PM

P: 400

Kinetic Energy of an object at the speed of light



#5
Sep1011, 10:21 PM

Sci Advisor
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P: 1,841

Kinetic energy is[tex]
E = mc^2 \left( \frac{1}{\sqrt{1\frac{v^2}{c^2}}}  1 \right) [/tex] where m is the mass of the lead, and v is its speed, and c is the speed of light. So it depends how fast you go: the closer to the speed of light, the larger the energy gets. If it were possible to reach the speed of light (it isn't), the energy would be infinite. 


#6
Sep1011, 10:32 PM

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P: 11,473

Well, for a 1 kg mass travelling at 0.9 c, 90% the speed of light, it would have kinetic energy equal to 1.163 x 10^17 joules. 2.1 x 10^17 joules is roughly what was released by the 50 megaton Tsar Bomba nuclear weapon. So 1kg of mass travelling at 0.9 c would have about 25 megatons worth of energy.
0.9 c is about equal to 269,000,000 meters per second, or 269,000 km per second...so its REALLY fast. 


#7
Sep1011, 11:40 PM

P: 9

I guess another one of the things i was curious about was if a very tiny object could destroy the planet, if it was moving fast enough. Someone said earlier that at the speed of light, the object's KE would be infinite. So that means that any amount of mass could destroy the earth, if it were travelling at 1c? I realize this is impossible, but theoretically? 


#8
Sep1111, 05:01 AM

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P: 5,400




#9
Sep1111, 07:01 AM

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P: 11,473




#10
Sep1111, 11:17 AM

P: 693

There is also the issue of energy transfer. While a 1 cc object may have enough kinetic energy to destroy the earth that doesn't mean that an impact will destroy the earth. It may just punch a hole straight through and keep going on the other side.
I think determining what would actually happen would require either massive amounts of programing and computer simulation or building a gun that can fire a round a .99 c (pick your own velocity). 


#11
Sep1111, 12:31 PM

P: 9

Thanks for all of your replies, Ill do my best to ask questions that make sense. I'm just a normal guy that has an appreciation for science, even if I don't understand most of it. GRB 


#12
Sep1111, 01:46 PM

P: 158

@ GammaR4yBurs : Why don't you just get into something real like start learning about physics and astronomy. Your curiousness about astronomy will help you very much.
Its never late in life 


#13
Sep1111, 02:14 PM

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P: 5,400

"What do the laws of physics say about X (where X is something the breaks the laws of physics)". Don't worry, everyone learns this at some point. 


#14
Sep1111, 03:17 PM

P: 9

GRB 


#15
Sep1111, 06:54 PM

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P: 11,473




#16
Sep1211, 10:23 AM

P: 5,632

And with enough radiation energy earth could be pushed out of it's orbit if that energy exceeded gravitational attraction between earth and moon or earth and sun. Don't worry about feeling dopey here: we all learn from others and feel "dopey" from time to time...feeling dopey is also called "growth" if taken constuctively....and growth is usually uncomfortable. 


#17
Sep1211, 11:57 AM

P: 96

Isn't there an outside chance that a big glob of material ejected from a black hole could be going 99.999999% relative to earth, and impact hard enough to break the planet up?



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