To find the impulse you multiply the mass by the change in velocity (impulse=mass×Δvelocity). So in this case, 3 kg × 12 m/s ("12" because the object went from zero m/s to 12 m/s).
The answer is 36 kg m/s
If the collision is inelastic, there is every possibility that the large body will drag the small stationary body along with it in the direction of the collision. Some amount of heat, light and sound energy will also be produced due to the kinetic energy of the large body. I hope the answer helps you.
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It will be 49 Newtons of force in the down direction. To find the force in newtons, you multiply the mass (5 kg) by the gravity (which if 9.8).