Question

. .A 5kg object moving at 10m/s collides with a stationary 3kg object in an isolated system. After the collision, the two object stick together and move as one mass. What can be said about the total momentum and kinetic energy of the system before and after the collision?
A. The total momentum after the collision is greater than before, and the kinetic energy remains the same
B. The total momentum is conserved, but the kinetic energy after the collision is less than before.
C. The total momentum decreases, and the kinetic energy after the collision is greater than before
D. Both the total momentum and kinetic energy are conserved after the collision
s

Answer 1

Answer: B.

Explanation/calculation:
- Initial momentum: p_i = 5(10) + 3(0) = 50 kg·m/s.
- After they stick, total mass = 8 kg, so v_f = p_i / 8 = 50/8 = 6.25 m/s. Momentum after = 8(6.25) = 50 kg·m/s — momentum is conserved.
- Initial kinetic energy: KE_i = 1/2(5)(10^2) = 250 J.
- Final kinetic energy: KE_f = 1/2(8)(6.25^2) = 156.25 J.
KE decreases (250 J → 156.25 J); the lost energy is converted to internal energy, sound, deformation, etc.