# Physics- Momentum question

• Dec 16th 2009, 03:03 PM
Caturdayz
Physics- Momentum question
A A 235 KG ball traveling 25 m/s strikes a second ball with a mass of 245 Kg moving at a speed of 35 m/s. They become stuck together and travel in a straight line. How fast are they moving after they become entangled?
_

Don't think I am doing this right, but I tried...

$(235 / 480)*(60)= 29.38 m/s$

As

$(Ma+Mb)* V'= Pfinal$
• Dec 16th 2009, 04:57 PM
skeeter
Quote:

Originally Posted by Caturdayz
A A 235 KG ball traveling 25 m/s strikes a second ball with a mass of 245 Kg moving at a speed of 35 m/s. They become stuck together and travel in a straight line. How fast are they moving after they become entangled?
_

Don't think I am doing this right, but I tried...

$(235 / 480)*(60)= 29.38 m/s$

As

$(Ma+Mb)* V'= Pfinal$

since momentum is a vector ... initial directions of motion for each ball?
• Dec 16th 2009, 06:07 PM
Krahl
conservation of momentum?
ma+m'b=Mc
i)assuming they are travelling towards each other;
m=235
a=25 travelling left

m'=245
b=-35 travelling right

235(25)+245(-35)=480c
c=-5.625 so travelling right

ii)assuming they are travelling in the same direction;

m=235
a=25 travelling left

m'=245
b=35 travelling left

but m' starts behind m

235(25)+245(35)=480c
c=30.1
• Dec 22nd 2009, 04:31 PM
Vitruvian
Quote:

Originally Posted by Krahl
conservation of momentum?
ma+m'b=Mc
i)assuming they are travelling towards each other;
m=235
a=25 travelling left

m'=245
b=-35 travelling right

235(25)+245(-35)=480c
c=-5.625 so travelling right

ii)assuming they are travelling in the same direction;

m=235
a=25 travelling left

m'=245
b=35 travelling left

but m' starts behind m

235(25)+245(35)=480c
c=30.1

The problem with your solution is that you are assuming the first ball is always moving towards the left. If this is true, then your solution is impossible because in the first case it asserts that the 245 g mass is moving to the right... that means in the 2nd case, the 245 g mass is moving ahead of the 235 g mass making the collision impossible seeing as the 245 g mass is moving faster. Therefore, your first answer is wrong: the masses will actually move in the left direction. (c = + 5.625)
• Dec 23rd 2009, 08:08 AM
Krahl
@ vitruvian
check again.
for the first one you can always choose another frame of reference so that right is left its the same. the thing is i defined which ball is travelling in which direction. so my answer is actually saying 5.625 in the direction of the 245kg ball where as in your answer we dont know.

in the second one
so your saying if i go in a straight line at 35mph and you are in front at 25mph i wouldnt reach you?

since we know they travel in a straight line and strike , its gotta be one of these two cases
• Dec 23rd 2009, 10:51 AM
Vitruvian
Quote:

Originally Posted by Krahl
@ vitruvian
check again.
for the first one you can always choose another frame of reference so that right is left its the same. the thing is i defined which ball is travelling in which direction. so my answer is actually saying 5.625 in the direction of the 245kg ball where as in your answer we dont know.

I misread your sign convention on that one, I apologise for the error.

Quote:

in the second one
so your saying if i go in a straight line at 35mph and you are in front at 25mph i wouldnt reach you?

since we know they travel in a straight line and strike , its gotta be one of these two cases
I was infering quite the opposite. Once again, I misread your sign convention and incorrectly assumed you were working with the opposite scenario.
• Dec 23rd 2009, 10:57 AM
Krahl
no harm done :)