Akid at the junior high cafeteria wants to propel an empty milk carton along a lunch table by hitting it with a 3.1 g spit ball. part a if he wants the speed of the 20 g carton just after the spit ball hits it to be 0.28 m/s , at what speed should his 3.1 g spit ball hit the carton?

Does the spit ball stick to the carton once it hits? assuming this, we solve this problem using conservation of momentum 
 the momentum of the spitball/carton system before collision is the same as the momentum after collision 
 momentum before collision = 0.0031kgxV where V is the initial speed of the spitball 
 momentum after collision = 0.0231kgx0.28m/s= 0.006468kgm/s
 (note that the mass after collision is the sum of the carton and spitball) 
 solving for V gives: 

 V= 0.006468kgm/s / 0.0031kg= 2.09 m/s
 answer
 his 3.1 g spit ball should hit the carton at 2.09 m/s

We answer this problem using conservation of momentum 
the momentum of the spitball/carton system before collision is the alike as the momentum after the collision 
momentum before collision = 0.0031 kg x V, the v here is the initial speed
momentum after collision = 0.0221kg x 0.28m/s = 0.0062  kg m/s 
(note that the mass after collision is the sum of the carton and spitball) 
solving for V gives: 
V = 0.0062  kg m/s  / 0.0221 kg = 0.280542986 m / s

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