Please help me with this SAT question from a released test.
A toy truck with a mass of $0.6$ kg initially coasts horizontally at a speed of $2$ meters per second. A child drops a beanbag with a mass of $0.2$ kg straight down onto the truck. What is the speed of the truck after it?
My teacher told me to use conservation of momentum in the horizontal direction. However, when the beanbag is dropped, isn’t the ground pushing the truck up so that an external force is acting on the system? If not what does an external force mean in momentum conservation as momentum is conserved if and only if no external forces act.
Also, I tried to use the fact that energy is conserved so that $(0.5) (0.6 )2^2 = 0.5 (0.6+0.2) v^2 $ but this doesn’t work. As the correct answer is $1.5$. Why is the kinetic energy not conserved during this collision?
Thank you in advance