# Motion help

1. Apr 20, 2010

### fcb

1. The problem statement, all variables and given/known data

A rocket moving towards Alpha Centuari at 1000ms^-1 fires its rockets, accelerating backwards at a constant rate for 5 minutes. During this 5 minutes, the rocket moved 240km close to the star. What was the acceleration?

2. Relevant equations

I have no idea what formula you should use
Maybe:
r= ut + 1/2at^2

3. The attempt at a solution

Hmm , i'll tell you my attempt , but srsly this question is hard

r = ut + 1/2at^2

r = 240 km = 240000 m
u = 1000m/s
t = 5 mins = 300s
a = ?

240000 = 1000 x 300 + 1/2 x a x (300)^2
240000 = 300000 + 45000a

= - 6000 = 45000a

a = -6000/45000

a = -0.133m/s/s towards the star ...

2. Apr 20, 2010

### MalachiK

I think your answer is sound, except that your final result is out by a factor of ten. Maybe you should check the numbers in the last few lines of your calculation.

In future, you might find it easier to move the symbols around first and then leave putting the numbers in until the very end. You could take the equation that you started with and rearrange it to get a = ... and THEN sub in the values from the question.

3. Apr 20, 2010

### fcb

Sorry, with the equation, I don't seem to know where I went wrong. I think it's the right formula but

4. Apr 20, 2010

### fcb

I am so confused ?!?! looking at that equation does my head in.

5. Apr 20, 2010

### MalachiK

Yes, the formula is right, and all of your working is fine until you get to the last few lines

240000 = 300000 + 45000a

= - 6000 = 45000a

Check the numbers and you should find that you're missing a zero somewhere.

6. Apr 20, 2010

### fcb

Ohh I see what you did. Thanks