How Do You Determine Superman's Initial Velocity in a 2D Momentum Problem?

[Walter White]

Homework Statement

Question is on the attachment photo.

Homework 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.[/B]

The Attempt at a Solution

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

 

[haruspex]

I didn't know the momentum for each of the people

How far will Lois have fallen when she is caught?

 

[Walter White]

How far will Lois have fallen when she is caught?

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

 

[haruspex]

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

So what is Lois' velocity when they meet?

 

[Walter White]

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

 

[haruspex]

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

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?

 

[Walter White]

So what is Lois' velocity when they meet?

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

 

[Walter White]

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?

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

 

[haruspex]

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

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?

 

[Walter White]

Actually no, sorry I haven't learned these

 

[Walter White]

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

 

[Walter White]

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

 

[haruspex]

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

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.

 

[Walter White]

Oh alright. Thanks

 

[Walter White]

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

 

[haruspex]

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

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?

 

[Walter White]

Yes I will try

 

[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

 

[haruspex]

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

Please post your working.

 

[Walter White]

 

[Walter White]

View attachment 106220

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

 

[Walter White]

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

 

[Walter White]

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

 

[Walter White]

Answer I got was 7.37m/s

 

[haruspex]

Then I put it into a momentum equation initial and final, solved for vx and vy and total velocityView attachment 106222

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.

 

[Walter White]

9.8 is superman's velocity. I'm multiplying it by his mass because I want the momentum of Superman. The line is titled x because this is 2d momentum and we need both x and y to get final velocity of x and final velocity of y

 

[haruspex]

9.8 is superman's velocity

No, Superman's initial velocity is what you have to find. I see no 9.8m/s in the question statement.