What is the magnitude of the total momentum of the two-puck system after the collision?

Question

Two pucks moving on a friction less air table are about to collide, as shown. The 1.5 kg puck is moving directly east at 2.0 m/s. The 4.0 kg puck is moving directly north at 1.0 m/s.

What is the magnitude of the total momentum of the two-puck system after the collision? 

(A) 1.0kg∙m/s 

(B) 3.5kg∙m/s 

(C) 5.0kg∙m/s 

(D) 7.0kg∙m/s 

(E) 5.5√5kg∙m/s 

Answer 1

Correct option (C) 5.0kg∙m/s

Total momentum before must equal total momentum after. Before, there is an x momentum of (2)(1.5) = 3 and a y momentum of (4)(1) = 4 giving a total resultant momentum before using the Pythagorean theorem of 5. The total after must also be 5.