Question

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 Earth’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:

Answer 1

options??

Answer 2

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

Answer 3

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

Answer 4

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

Answer 5

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

Answer 6

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.

Answer 7

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

Answer 8

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

Answer 9

u took downwards negative?

Answer 10

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.

Answer 11

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

Answer 12

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

Answer 13

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

Answer 14

no, always took downwards negative and upwards positive

Answer 15

v=0+10*2
v=20m/s downwards

and

v=15-20
v=-5m/s upwards

Answer 16

no!!!!

Answer 17

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.

Answer 18

wait u messed up!!

Answer 19

no,, 15 m/s down.

Answer 20

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

Answer 21

right

Answer 22

0.25*35=8.75

Answer 23

-20+8.75

Answer 24

oh :o

Answer 25

-11.25/0.75=-15m/s

Answer 26

i see! :D

Answer 27

okk