Ball a, with a mass of 10kg, is moving to the right at 20 m/s. ball b is moving to the left at 20 m/s. upon collision, ball b comes to a standstill, and ball a moves to the left at twice its original speed. what is the mass of ball b? show your work

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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