# Homework Help: College phys neeed help

1. Sep 5, 2009

### efaizan

college phys 1111... neeed help!!

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

A space probe is desperately trying to escape the gravitational pull of a massive planet. After a final blast of its thrusters, the probe has a velocity of 2940 m/s away from the planet. Later, the probe is 176.4 km closer to the planet than it originally was, with a velocity of 8820 m/s toward the planet.

You are to find (assume constant acceleration):
a) the acceleration (magnitude and direction)
b) How far beyond its starting point the probe got.
c) How much time elapsed between initial and final positions.

I don’t understand how to approach this problem…. How do I find acceleration when I don’t know the time?

2. Sep 6, 2009

### ideasrule

Re: college phys 1111... neeed help!!

You know that the probe's velocity was originally 2940 m/s, but that after falling 176.4 km towards the planet (and losing a corresponding amount of potential energy), its velocity became 8820 m/s. You can use the conservation of energy to calculate g, the acceleration due to gravity.

For b, you can use the conservation of energy again. c should be easy once you get the answer for a.

3. Sep 6, 2009

### efaizan

Re: college phys 1111... neeed help!!

i am sorry i still dont understand... =[

4. Sep 6, 2009

### efaizan

Re: college phys 1111... neeed help!!

i mean i know (Velocity final- Velocity initial)/time = acceleration but they dont tell the time and that is what is throwing me off...

5. Sep 6, 2009

### Redbelly98

Staff Emeritus
Re: college phys 1111... neeed help!!

Look at the kinematic equations for motion with constant acceleration. Look for the one equation that does not contain time t.

6. Sep 6, 2009

### efaizan

Re: college phys 1111... neeed help!!

v^2=vo^2+2a(x-xo) or

7. Sep 6, 2009

### efaizan

Re: college phys 1111... neeed help!!

v^2-vo^2=2a(x-xo)

8. Sep 6, 2009

### efaizan

Re: college phys 1111... neeed help!!

2940^2-8820^2=2a(x-176.4) and solve for a?

9. Sep 6, 2009

### Redbelly98

Staff Emeritus
Re: college phys 1111... neeed help!!

Yes. Um, you do know what the value of x is, right?

10. Sep 6, 2009

### efaizan

Re: college phys 1111... neeed help!!

ummm 0?

11. Sep 6, 2009

### ideasrule

Re: college phys 1111... neeed help!!

Yup, exactly. But you used 176.4, which is in kilometers, whereas all your speeds are in m/s. You'll have to make the units agree.

12. Sep 6, 2009

### efaizan

Re: college phys 1111... neeed help!!

ok so 2940^2-8820^2=2a(0-109.6) i got a= 315459.85...... that doesnt sound right does it?

13. Sep 6, 2009

### efaizan

Re: college phys 1111... neeed help!!

thank you guys btw for ur help!!

14. Sep 6, 2009

### Jebus_Chris

Re: college phys 1111... neeed help!!

Well first convert x into meters, there are 1000 meters in 1km. Know you just plug and chug. And for some odd reason you have Vo as 8820 and V as 2940.

15. Sep 6, 2009

### efaizan

Re: college phys 1111... neeed help!!

ohhhh oops i think i converted the 176.4 km to miles..... soooo i should flip flop the v's? and 176.4 km to meters would be 176400?

16. Sep 6, 2009

### efaizan

Re: college phys 1111... neeed help!!

ok so i did it again and i got 196 for a now... that sounds kinda right. right?

17. Sep 6, 2009

### Jebus_Chris

Re: college phys 1111... neeed help!!

Looks good.