Explain Physics Please.

Question

Ball A of mass 5 kilograms moving at 20 meters per second collides with ball B of unknown mass moving at 10 meters per second in the same direction.

 

After the collision, ball A moves at 10 meters per second and ball B at 15 meters per second, both still in the same direction. What is the mass of the ball?

 

The answer would be 10 kg.

 

But why?? Explain…

Answer 1

Conservation of momentum states :

 

MA*VAi + MB*VBi  = MA*VAf + MB*VBf      Note the sums are the same but velocities before and after are different

 

MA = 5  kg          MASS A

VAi = 20 m/s      VEL of MASS A  INITIAL

MB = ?                MASS B

VBi = 10 m/s      VEL of MASS B  INITIAL

 

VAf = 10             VEL of MASS A FINAL

VBf = 15             VEL of MASS B FINAL

 

substitute in our Momentum conservation equation and solve for MB

 

5*20 + MB*10 = 5*10 + MB*15

 

solve for MB

 

100 +  MB*10 = 50 + MB*15

100 - 50 = MB*(15-10) 

 50 = MB*5

 

 MB = 10