At the same instant that a 0.50-kg ball is dropped from 25 m above Earth, a second ball, with a
mass of 0.25 kg, is thrown straight upward from Earthâ€™s surface with an initial speed of 15 m/s.
They move along nearby lines and pass without colliding. At the end of 2.0 s the velocity of
the center of mass of the two-ball system is:

- DLS

- katieb

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- mayankdevnani

options??

- DLS

A. 11 m/s, down
B. 11 m/s, up
C. 15 m/s, down
D. 15 m/s, up
E. 20 m/s, down

- DLS

V(a)=20m/s
V(b)=-5m/s

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## More answers

- mayankdevnani

Since this is multiple choice, lets use 10 m/s^2 as the acceleration due to gravity.

- DLS

so
0.50*20+0.25*(-5)/0.75

- mayankdevnani

The trick is to look at the momentum of the system after 2 seconds. So for the heavy ball, after two seconds, it has a velocity of -20 m/s.

- DLS

10+1.25/0.75 (wil we have a minus sign or are we considering just the mag?

- mayankdevnani

Thus it has a momentum of -10 kg*m/s.

- DLS

u took downwards negative?

- mayankdevnani

yaa, i took downwards negative.
The light ball will have a velocity of -5 m/s (15 m/s going up minus 20 m/s due to gravity gives 5 m/s downward). So it has a momentum of -1.25 kg*m/s.

- mayankdevnani

In both cases, the negative sign indicates the momentum is downward, and the sum is -11.25 kg*m/s.

- mayankdevnani

Find the velocity of the system by dividing by the total mass (0.75 kg)??
then solve it and you get your answer.

- DLS

i took downwards positive and upwards negative,im getting something different

- mayankdevnani

no, always took downwards negative and upwards positive

- DLS

v=0+10*2
v=20m/s downwards
and
v=15-20
v=-5m/s upwards

- mayankdevnani

no!!!!

- mayankdevnani

Find the velocity of the system by dividing by the total mass (0.75 kg)
and you get 15 m/s down.
so your answer is option C) 15 m/s up.

- DLS

wait u messed up!!

- mayankdevnani

no,, 15 m/s down.

- DLS

im taking downwards negative and upwards positive so
for the 0.25 kg ball,
v=15+20=35?
we are taking upwards positive right

- mayankdevnani

right

- DLS

0.25*35=8.75

- DLS

-20+8.75

- DLS

oh :o

- DLS

-11.25/0.75=-15m/s

- DLS

i see! :D

- mayankdevnani

okk

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