# Homework Help: I would love some help on this problem

1. Nov 1, 2007

### brendonnn

Okay, so! My physics is lacking, but my life sciences are excellent, but that won't save me.

How do I determine object's acceleration and velocity when it strikes the ground given:

An object is thrown up into the air and then falls back to earth, remains in the air for 5 seconds before striking the ground; neglect air resistance.

Last edited: Nov 1, 2007
2. Nov 1, 2007

### Dick

Look up some equations about kinematics of falling objects. It's really a very popular topic. Let us know what you think and then we can get back to you. It's really not hard. But you have to start. Even a well-reasoned guess would help.

3. Nov 1, 2007

### brendonnn

You know how to do it, but you just want me to try?

Well this is what I thought but I am unsure if it is right.

its original and final velocity will be 0 and that it takes 2.5 seconds to reach its maximum height and 2.5 more seconds to hit the ground. I think
Does this make sense at all?

displacement = initial velocity(change in time) + 1/2(acceleration)(change in time)^2
so

x = 0(10) + 1/2(9.81)(10)^2 ?

4. Nov 1, 2007

### Dick

I know how to do it, and, yes, I want you to try. Good try. The total displacement is zero, since it came back to the ground. This means the initial velocity can't be zero. I would change 9.81 to -9.81 since the acceleration is downwards, and I would also ask why you are putting total time to be 10 since it was only in the air for 5s?

5. Nov 1, 2007

### brendonnn

I thought about that as soon as I posted, well about the displacement being zero, and the 10 was a typo.

6. Nov 1, 2007

### Dick

Then you have it solved, right?

7. Nov 1, 2007

### brendonnn

is this right?

displacement = (initial velocity)(change in time) + 1/2(acceleration)(change in time)^2

0 = x(5) + 1/2(-9.81)(5)^2

approximately 24.5 m/s

?

8. Nov 1, 2007

### Dick

Of course it's right. I told you you just had to try.

9. Nov 1, 2007

### brendonnn

it traveled approximately 30.66 meters into the air and its final velocity before striking the ground was 17.34

10. Nov 1, 2007

### Feldoh

There's symmetry in this example. The magnitude of the initial and final velocities should be the same.

11. Nov 1, 2007

### Dick

Uh. No. What makes you think that. The initial velocity was 24.5m/sec. Shouldn't it strike the ground at the same velocity? The height is ok, though.

12. Nov 1, 2007

### brendonnn

Okay, and thanks for the help. Physics is not my forte.

13. Nov 1, 2007

### Dick

I could be. You aren't that bad at it. Life sciences isn't easy either.

14. Nov 1, 2007

### brendonnn

I have a question that could make me seem potentially stupid.

Is it feasible to utilize electric currents to split ocean water into hydrogen and oxygen for the desalinization of ocean water and providing oxygen for submarines

15. Nov 1, 2007

### Dick

Depends on what you mean by "feasible". You could do either one. Is it economical is another question. Depends on where the current comes from and what it costs and how much you want the product, etc.

16. Nov 1, 2007

### brendonnn

Thanks for the answer and the push for me to do well on the problem.