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1) David has a mass of 86.4-kg and is skating with in-line skates. He sees his 24.8-kg younger brother up ahead standing on the sidewalk, with his back turned. Coming up from behind, he grabs his brother and rolls off at a speed of 2.99 m/s. Ignoring friction, find david's speed just before he grabbed his brother.

I do not know how to start this.

The law of conservation of linear momentum
m1•v = (m1+m2) •u
v= (m1+m2) •u/m1 =(86.4+24.8) •2.99/86.4 =3.85 m/s

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