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

Video transcript

Welcome back everybody. We have a bullet that is shot straight upward in order to scare off nearby thieves. We are told that the bullet has an initial velocity of 30m/s and at some point in its journey it will have a velocity of 15m/s. Now neglecting air resistance here, we know that this bullet is in free fall, so its acceleration is just going to be the acceleration due to gravity, which is negative 9.81 m per second squared. And we are asked to find at what time does the bullet reach 15 m per second. So let's go ahead and use a kid a Matic equation here that states that our final vertical velocity is equal to our initial vertical velocity plus our acceleration times time. We have a lot of these values. So let's go ahead and plug it in here. Final velocity. We're trying to find the time at which the velocity is 15. Is it equal to our initial 30 Plus acceleration of negative 9.81 time. Subtract 30 from both sides And we get that negative 15 is equal to negative 9.81 times. T Let's divide both sides by negative 9.81 And we get that E. is equal to negative 15, divided by negative 9.81 which when you plug into your calculator is 1.53 seconds. Or responding to our final answer choice of a thank you guys so much for watching. Hope this video helped. We will see you all in the next one