# Work and Energy Theorem and Kinetic Energy

1. Oct 29, 2006

### rmarkatos

A 5.0x10^4kg space probe is travelling at a speed of 11,000m/s through deep space. Retrorockets are fired along the line of motion to reduce the probe's speed. The retrorockets generate a force of 4.0x10^5N over a distance of 2500km. What is the final speed of the probe.

For this problem i know exactly what to do but i know i am getting the wrong answer because I have odd answer selections.

I used W=Fd=1/2m(vf^2)-1/2m(vi^2). I know this is correct because when i plug in the correct answer for vf=9,000 m/s it equals the other side of the equation so i know i am right but for some reason in the process of solving i am not sure what i am doing wrong because i keep getting the wrong answer for vf. It is kind of strange because my units in every part of solving the equation come out correctly.

Any suggestions?

2. Oct 29, 2006

### Hootenanny

Staff Emeritus
Are you manipulating the equality correctly? Perhaps if you were to show your steps we may be able to comment further.

3. Oct 29, 2006

### rmarkatos

Yes, sorry about that i should of.

First i converted km to meters because we work in the mks system

(4x10^5N)(2.5X10^6m)=w
=1x10^12J

1x10^12J=1/2(5x10^4kg)(vf^2)-1/2(5x10^4kg)(11,000m/s)^2

1x10^12J=1/2(5x10^4kg)(vf^2)-3.025J

from here i tried different ways to solve the equation but each way i still get the wrong answer. I added the 3.025J to both sides and then multiplied the 1/2(5x10^4). Then i divided the 4.025J by the 25,000kg and then solved for vf by taking the square root of both sides but for some reason my answer does not really come out relativlely close 9000m/s

4. Oct 29, 2006

### Hootenanny

Staff Emeritus
Do you not think this number looks a little small?

5. Oct 29, 2006

### rmarkatos

Oh sorry a bit of typo there. It is supposed to be-3.025x10^12. Sorry i forgot to type that in there but when i was doing the problem i did have it correctly written.