Ok so I'm in 7th grade and I think I'm pretty good in science class. The other day we had some math questions involving some equations. One question was something like a spaceship hit the ground with a velocity of 100 m/s and then it asked like what was its mass. This question also included the gravity of the planet (like 7.8 m/s squared). The next question was what was the force of the impact. The answer was something low because the force of gravity was 7.8. I was wondering how F=ma would work here because the velocity makes it seem like the impact would have much more force than what it was. Then i started thinking about if you threw a ball straight up. As soon as it left your hand the acceleration would be downward at 9.8 because of gravity. So the acceleration upward is -9.8 even though its still going up. I know that if it hits something while going up its going to have a force so i don't see how F=ma works if acceleration is negative. Just today i started wondering if this was because of momentum and if you would somehow get p=mv into there. Thanks for all of the responses. I know this should be extremely easy for you guys.