Homework Help: Conservation of Momentum and energy problem help

1. Jan 25, 2006

kgbwolf

SOmeone explain to me this because im lost. i got 2 masses one velecity. the equation is like is mass times volocity + mass times velocity=0

122kg*22m/s+14kg*v=0 I made these numbers up what i do is multiply and add then move my anwser to the other side becoming negative but that doesnt seem to be the right anwser.

2. Jan 25, 2006

kgbwolf

ki even tried multipling then dividing by the remaining number still gives me the wrong anwser

3. Jan 25, 2006

Hootenanny

Staff Emeritus
Generally;
$$M_{1}V_{1} + M_{2}V_{2} = 0$$
Then Rearranging
$$V_{2} = - \frac{M_{1}V_{1}}{M_{2}}$$

4. Jan 25, 2006

kgbwolf

Ok i worked the first problem out now i have a nother problem

A 125kg pile driver falls from a hieght of 10m to hit a piling.

From this i see mass is 125kg and distance is 10m and it says find the speed it hits the piling? and the momentum how can i do this with just mass and distance or is distance really velocity?

5. Jan 25, 2006

Hootenanny

Staff Emeritus
Use conservation of energy. At 10m up the piledriver will have a potential energy of mgh. At the instant it hits the pile all the potential energy will be converted to kinetic $\frac{1}{2}mv^2$. You should be able to work out the velocity by rearranging and hence the momentum.

6. Jan 25, 2006

kgbwolf

i did 1/2*m moved it to the other side negative and ttok the square root and 7.91m/s

7. Jan 25, 2006

kgbwolf

and the right anwser is 14m/s

8. Jan 25, 2006

Hootenanny

Staff Emeritus
I got a different answer. What value did you get for the potential energy?

9. Jan 25, 2006

Hootenanny

Staff Emeritus
Yes, I got 14m/s.

10. Jan 25, 2006

kgbwolf

12,250Newtons

11. Jan 25, 2006

kgbwolf

how?................

12. Jan 25, 2006

Hootenanny

Staff Emeritus
Check you units. Newtons for energy? The value is correct. Now show me how you re-arrange the kinetic energy equation.

13. Jan 25, 2006

kgbwolf

do u have msn IM??

14. Jan 25, 2006

Hootenanny

Staff Emeritus
No..............

15. Jan 25, 2006

kgbwolf

1/2 times 125 = 62/1/2 and i took the square root gives me 7.9

16. Jan 25, 2006

Hootenanny

Staff Emeritus
$$E_{k} = \frac{1}{2} mv^2 \Rightarrow v = \sqrt{\frac{2E_{k}}{m}}$$

17. Jan 25, 2006

kgbwolf

Whats Ek........

18. Jan 25, 2006

kinetic energy

19. Jan 25, 2006

kgbwolf

potential got it

20. Jan 25, 2006

Hootenanny

Staff Emeritus
The potential energy you calculated it would all be converted into kinetic energy. The same equation could be written
$$v = \sqrt{\frac{2mgh}{m}} \Rightarrow v = \sqrt{2gh}$$