A large boulder is ejected vertically upward from a volcano with an initial speed of 40.0 m/s. Ignore air resistance. (b) At what time is it moving at 20.0 m/s downward?

Video transcript

Welcome back everybody. We have a basketball that is being thrown straight up by the referee at the beginning of a game. Now we are told that it has a initial vertical velocity of 19. m/s. And at some point during its travel it will have a velocity of 9.81 m/s. Now, since this ball is in free fall as we are neglecting air resistance, its acceleration is equal to the acceleration due to gravity which is negative 9.81 m/s square. And we are asked to find the time when we reach that velocity of 9.81. Let's go ahead and use a cinematic formula here that states that our final vertical velocity equal to our initial vertical velocity plus our vertical acceleration times our time. Now we know some of these values, so let's go ahead and plug them in. Our final velocity is our 9.81 equal to our initial velocity of 19. plus our acceleration due to gravity times time. Now subtracting 19.62 on both sides. This yields that negative 9.81 is equal to negative 9. T Dividing both sides by -9.81. We get that T. Is equal to one second corresponding to our answer of a thank you guys so much for watching. Hope this video helped and we will see you all in the next one