# How fast could the person have to travel?

1. Apr 18, 2007

### jenita

a person whose mass is 48 kg wishes to gain 12 kg relativistically with respect to another reference frame. How fast could the person have to travel?????

the formula is as follows....
i Think M is 48 kg
mo is 12 kg
v=?????? can anyone help me to do this problem

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2. Apr 18, 2007

### hage567

Mo will be 48 kg. M will not be 12 kg, since the question states that the person wishes to gain 12 kg (so the mass will increase by this much).

I can't see the picture yet. Does it show where you are getting stuck?

3. Apr 18, 2007

### jenita

sorry i dont no why the pic is not showing up...its the formula in the pic.... i hope u no the formula.....if mo is 48 then what is 12kg

4. Apr 18, 2007

### jenita

is 12 kg M or do we have to find M first

5. Apr 18, 2007

### jenita

oh ya by the way c is 3.8 * 10^8

6. Apr 18, 2007

### jenita

7. Apr 18, 2007

### hage567

Is this the equation you have?

$$M = \frac{M_o}{\sqrt{1 - v^2/c^2}}$$

You want to solve for v. Don't worry about the actual value of c (you've got it wrong by the way, it's 3x10^8 m/s), just leave it as "c" in your work. At the end, you can worry about converting it.

You are given Mo. You can figure out M because you know the gain in mass is 12 kg. So, what is M, then?

Last edited: Apr 18, 2007
8. Apr 18, 2007

### jenita

but to find M dont u need V

9. Apr 18, 2007

### hage567

10. Apr 18, 2007

### jenita

how can i solve it if i dont have M, v, and c...dont i need to have 1

11. Apr 18, 2007

### hage567

You are trying to solve for v. You must rearrange that equation to get v in terms of the masses. It IS the equation you are using, right?

You have the information to get both masses without solving anything!

12. Apr 18, 2007

### jenita

75000000...i got this by inserting 12 for M, 48 for mo and 3.08*10^8 for c....
i divided 12 from 48 which i got.25 and the squared it and got.0625.... i squared 1 and c and then multiplied by c and 1 to the .0625...... i was left with 5.625e15=v^2 and i squared it and got the answer..... sorry it looks confusing

13. Apr 18, 2007

### jenita

and yes that is the equation we are using

14. Apr 18, 2007

### hage567

Like I've already said, M is not 12 kg. 12 kg is the change in the mass. So if the mass started at 48 kg and 12 kg is added relativistically, what is the new mass (M)?

15. Apr 18, 2007

### jenita

oh lol....its 60 kg

16. Apr 18, 2007

### jenita

so u insert 60 for m

17. Apr 18, 2007

### jenita

so for M u insert 60 and then solve

18. Apr 18, 2007

### hage567

There you go! Let's see what answer you get now.

19. Apr 18, 2007

### jenita

i got 375000000...i did the same way like i did to get 75000000

20. Apr 18, 2007

### hage567

Well that can't be right since that is faster than the speed of light. Are you using 3 x 10^8 m/s for c?