## anonymous 4 years ago A Cannon shell is fired straight up from the ground at an initial speed of 225 m/s. after how much time is the shell at a height of 620 m above the ground and moving down.

1. anonymous

I attempted to use the equation $time=\frac{distance}{speed}$ but it didn't return an option that was a choice in the book, so i guess i need to do something else here.

2. anonymous

take into account that acceleration due to gravity is 9.81 and the question is asking when the ball is at a height of 620 m and ON ITS WAY DOWN. which means its a 2 part question. you have to find out how long it takes the ball to slow down to 0 speed from 225 at a negative acceleration of 9.81 then how much time it takes for the ball to get back to a height of 620 m with a positive acceleration of 9.81.

3. anonymous

alright let me try to work this out, and thanks for the help.

4. anonymous

you can use a kinematic equation here to find out these parts. first part is to find out where the ball stops and turns around. state your known values, which are acceleration= -9.81. Velocity initial = 225. Velocity Final= 0. so use the equation $V ^{2}=V _{0}^{2}+2a* \Delta X$ and solve for $\Delta X$ to find out how far the ball traveled up. then you can use another kinematic to find out how long that took. $\Delta X = .5*(V _{0} + V) \Delta t$ by solving for $\Delta t$ then you have to solve for the second part where its falling. this time your values are slightly different. we know our Velocity initial is 0. our delta x is our max height minus 620m and our acceleration is positive 9.81.

5. anonymous

6. anonymous

43 sec is the an option so it sounds right.

7. anonymous

is 19.99s an option too? because thats what i got

8. anonymous

They are 2.96, 17.3, 25.4, 33.6, 43.0

9. anonymous

its 43 because i forgot to add my time from part 1 to part 2

10. anonymous

The shell is moving down, so we should add the time the shell take to reach the highest position

11. anonymous

But I don't see why there are so many answers...

12. anonymous

they are just giving me some to pick from

13. anonymous

Ah, then surely is 43s

14. anonymous

multiple choice. yeah im almost 100% sure it should be 43 s because it took about 23 s to reach max height then it took about 19.99 or 20 s to get back down to 620m

15. anonymous

Right on the target ,dude.

16. anonymous

Thanks a ton for the help!

17. anonymous

no problem. you should be able to use the same method for your arrow question you posted

18. anonymous

Yea i'm going to look at it again in the morning and see what i can't figure out. This might just be a case of me being brain dead right now.