Angular Momentum of Ball of Mass Question

[pradeepk]

Homework Statement

A ball of mass 0.3 kg is fired horizontally off the side of a building of height 10m at a velocity of 5m/s. Take the initial firing position as the origin. Find the angular momentum of the ball about the origin just before it hits the ground.

Homework Equations

L=r x p

The Attempt at a Solution

So I know you need to find the r position vector, and I know how to do this. So, you can use the equation yf=yo + voy(t) -0.5gt² and you solve for the time to get t=$$\sqrt{}2yo/g$$. So then you can use xf=xo + vox(t), and you get that the x position is x=Vo$$\sqrt{}2yo/g$$$$\hat{}x$$ My question is about the y position vector. My professor gave that y position is -yo$$\hat{}y$$. Intuitively this does not make sense to me, how is the position of y only changing by its initial position?

Homework Statement

Homework Equations

The Attempt at a Solution

 

[olivermsun]

My professor gave that y position is -yo$$\hat{}y$$. Intuitively this does not make sense to me, how is the position of y only changing by its initial position?

From the problem description.

"Take the initial firing position as the origin. Find the angular momentum of the ball about the origin just before it hits the ground."

(emphasis added)

 

[pradeepk]

From the problem description.

"Take the initial firing position as the origin. Find the angular momentum of the ball about the origin just before it hits the ground."

(emphasis added)

ohh of course..when it hits the ground its y position will be the negative of where it started. Thanks a lot. I don't know why I couldn't think of that

 

[olivermsun]

No prob. ;)