# 2-d kinematics problem

1. Sep 27, 2010

### stressedgirl

1. The problem statement, all variables and given/known data
If you can hurl a ball so that its initial speed is 30 m/s, what is the widest river it can cross?

v1 = 30m/s

2. Relevant equations
I am not sure with only the one variable given.

3. The attempt at a solution

2. Sep 27, 2010

### rock.freak667

Yes split the inital velocity into vertical and horizontal components.

Now consider vertical motion, when the ball lands on the ground again, the displacement is zero. Use y=vyt_0.5gt2 to get the time for this motion.

Now horizontally the Range (how wide your river is) is given by R=vxt

that 't' in the equation for 'R' is the time for the motion above.

3. Sep 27, 2010

### stressedgirl

Umm.. I don't understand. How would the 't' answer the question? Wouldn't I need a 'd' since it's asking how wide the river could be?

edit: SOrry I don't really understand what I'm even supposed to draw in the diagrams of the components

4. Sep 27, 2010

### rock.freak667

Solving for the 't' would get the 'd'.

Draw the 30m/s at an angle θ and split it into vertical and horizontal components.

5. Sep 27, 2010

### stressedgirl

Can I get something step by step please? I drew the components but I don't get why I even have to have both an x and the y. I just have two sets of axes and a theta angle at 90degrees

6. Sep 27, 2010

### rock.freak667

You will see why you need it soon.

Did you get your components as

y: 30sinθ

x: 30 cosθ

These are your initial velocity components

Now consider vertical motion: y=vyt-0.5gt2

When the ball hits the other side of the river, y = 0. So solve for 't' and tell me what you get.