used p total before = p total after
p total after can be found using p = mv (since only one ball is in motion we can ignore the second ball)
p= 10 40
=400 kg m/s
p total after must also be 400
p total before = p ball1 + p ball2
m1v1 + m2v2
isolate for the mass of ball 2
m2= (p total - m1v1) / v2
= (400 - 10(20))/ 20
= 200/20
= 10 kg
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