# Homework Help: 2d Momentum Problem Physics 30

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1. Sep 18, 2016

### Walter White

1. The problem statement, all variables and given/known data
Question is on the attachment photo.

2. Relevant equations

I tried to create an equation but I didn't know the momentum for each of the people in the scenario because none of them have a given speed.

3. The attempt at a solution

I just need the velocity of Superman to solve the problem. How do we find the speed?

#### Attached Files:

• ###### IMG_20160918_132550.jpg
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2. Sep 18, 2016

### haruspex

How far will Lois have fallen when she is caught?

3. Sep 18, 2016

### Walter White

It has to be the difference between the 2 heights. 13.2 - 6.8.

4. Sep 18, 2016

### haruspex

So what is Lois' velocity when they meet?

5. Sep 18, 2016

### Walter White

I don't know, I need time to figure that out

6. Sep 18, 2016

### haruspex

When you say you need time, do you mean you think you need to know how much time elapsed, or merely that you cannot work it out immediately?

7. Sep 18, 2016

### Walter White

Like I need the time to figure out the velocity from the formula d/t

8. Sep 18, 2016

### Walter White

Yes how much time was elapsed because there's no mention of time in the problem

9. Sep 18, 2016

### haruspex

For motion under constant acceleration, there are five standard variables (usually known as SUVAT, s for displacement, u for initial speed, v for final speed, a for acceleration, t for time). There are correspondingly five equations. Each equation omits one of the five variables. Are you familiar with these?

10. Sep 18, 2016

### Walter White

Actually no, sorry I haven't learned these

11. Sep 18, 2016

### Walter White

Oh wait, Lois is falling with gravity, so we know acceleration. Now I can figure out time

12. Sep 18, 2016

### Walter White

I'll try to figure it out and get back to you thanks

13. Sep 18, 2016

### haruspex

Right. I suggest you Google SUVAT. Plenty of references. Or, in this case, you can use work conservation.

Oh, and you do not need to find the time. You can get the speed directly.

14. Sep 18, 2016

### Walter White

Oh alright. Thanks

15. Sep 18, 2016

### Walter White

Hi, I got 16.1 m/s for Lois velocity. How do I find superman's initial velocity?

16. Sep 18, 2016

### haruspex

Stick with vertical motion for now. You know the vertical speed of each just before they meet. Can you find their vertical speed just afterwards?

17. Sep 18, 2016

### Walter White

Yes I will try

18. Sep 18, 2016

### Walter White

Hi again. I tried my best to solve it but I'm not getting the right answer. Can you possibly give it a shot yourself? Or just tell me another hint on how to solve it. Thanks

19. Sep 18, 2016

### haruspex

Please post your working.

20. Sep 18, 2016

### Walter White

21. Sep 18, 2016

### Walter White

First I found Louiss velocity using this formula in the above image

22. Sep 18, 2016

### Walter White

Then I found superman's velocity using the time for Louiss to reach down to Superman and the length of the pit

23. Sep 18, 2016

### Walter White

Then I put it into a momentum equation initial and final, solved for vx and vy and total velocity

24. Sep 18, 2016

### Walter White

Answer I got was 7.37m/s

25. Sep 18, 2016

### haruspex

I cannot follow what you did there, not only because it was upside down and hard to read. (The posting of images is intended for diagrams and textbook extracts. Please take the trouble to type in your working. This makes it both easier to read and easier to make specific comments on.)
What is going on in that first line? Why are you multiplying Superman's mass by 9.8 in a line titled X:?
In which line are you applying conservation of vertical momentum?

By the way, in calculating Lois' vertical speed you had a negative value inside the square root, so you just ignored the minus sign. To be consistent with 'positive up' you should have set Lois' displacement as -6.4m. Then you would have had a positive value to square root.

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