[SOLVED] momentum questions I've started reviewing topics for my semester final in my physics class and I realized that there were some fundamental questions about momentum in my textbook, which I couldn't really do... Here they are: -You are standing at the center of a stationary boat (1.8 m from shore) and you start walking towards the shore. Neglect frictional force on the boat from the water. Once you reach the end of the boat, you jump towards the shore. Does the motion of the boat change when you reach it? Common sense tells me no; but I'm not really sure. If there's no external force on the system, the common center of mass of the system should remain stationary and when you suddenly stop moving, the boat should stop moving as well. But doesn't the ground provide an external force in this system? Does this explain why the boat's motion is unaffected when you hit the ground (and is it)? -If all the cars in the world started driving eastward, what would happen to the length of the day? I assumed that the earth-cars system had no external forces on it. The earth normally rotates eastward; so if suddenly the cars of the earth-cars system started moving eastward, should the earth's rotation slow down, and days get longer? Is this correct? -The momentum of the center of mass of a system is the sum of the momenta of the individual particles, but the same is not true for kinetic energy. Why not? Is this because (if there is no external forces on the system) mechanical energy is constant, and since internal potential energy in a system can change, so can kinetic? Or am I going about this in a totally wrong way? -An asteroid 100 km in diameter is discovered to be on a collision course with Earth. In a show of international cooperation and mutual disarmament, nations lanch all their nuclear missisles at the asteroid and succeed in blowing it to smithereens. Will the center of mass of the asteroid still hit Earth? I think that the answer to this question is no, because if you think of the asteroid by itself, there is an external force on it (from the nuclear missles) and this can change the momentum of the center of mass. Is this incorrect? -An hourglass is inverted and placed on a scale. Compare scale readings a. before sand begins to hit the bottom; b. while sand is hitting the bottom; c. when all the sand is on the bottom. My answers were A. Simply the mass of the hourglass * 9.8 m/s^2; B. A larger value; C. the same answer as A. When the particles of sand hit the scale, don't they undergo a change in momentum (impulse) that has the same result as an actual force being applied to the scale over a small duration of time? When the sand particles are not hitting the scale, and there are no collisions taking place, I assumed weight was just mg and there was no impulse. Any help is appreciated in advance.