# Homework Help: H=rt-4.9t^2 questions

1. Jan 6, 2008

### Sophialiu609

t=number of seconds
h=height
r=initial speed
How do you solve these two problems?
1. A projectile is fired upward with an initial speed of 2940 m/s. After how many minutes does it hit the ground?
2. A ball is thrown upward from the top of a 98m tower with an initial speed of 39.2 m/s. When does it hit the ground?
I don't understand how you can find the answers to these types of problems

2. Jan 6, 2008

### Sophialiu609

I got this far on the second one...
h=rt- 4.9t²
-98=39.2t-4.9t²
-20=8t-t²
0=-(t²-8t -20)
0=-(t-10)(t+2)
(t-10)=0 (t+2)=0
t=10 t=-2

Last edited: Jan 6, 2008
3. Jan 6, 2008

### apoptosa

For the first:
If you don't need the height then why are you calculating it ?
Try Vf = Vi + at
Where t would be time of flight & Vf,Vi final and initial velocity respectively

Last edited: Jan 6, 2008
4. Jan 6, 2008

### Sophialiu609

Well, I'm supposed to put the problem into that formula, that's what I'm confused about.

5. Jan 6, 2008

### rocomath

Don't forget to convert time to minutes b/c the answer you will get is in seconds.

6. Jan 6, 2008

### t!m

For the first, if it's fired from the ground and lands on the ground, what is h (often times, more appropriately written as $$\Delta$$h)?

7. Jan 6, 2008

### hage567

For the first question: you can use the same equation as you did for problem #2. If the projectile returns to the ground, what is its displacement? What does that make h in your equation?

8. Jan 6, 2008

### Sophialiu609

Ok, so what I have so far is
0=2940t/60 -4.9t^2/60
0=-4.9t(t/60 -1)
0=-4.9t(t-1/60)
0=-4.9t(t-1)
t={0,1}
Is that right?

9. Jan 6, 2008

### Sophialiu609

0=2940t-4.9t²
60 60
0=2940t-4.9t²
0=600t-t²
0=-t(t-600)
-t=0 (t-600)=0
t=0 t=600

or is this right?

10. Jan 6, 2008

### apoptosa

Just a little tip, you might wanna work through things in algebra before you number plug :)

11. Jan 6, 2008

### rocomath

Would time 0 make sense as an answer?

12. Jan 6, 2008

### Sophialiu609

0 wouldn't make sense as an answer, my answer is in minutes~I think... but now i'm more confused...

13. Jan 6, 2008

### Sophialiu609

Is it 10 minutes...?

14. Jan 6, 2008

### rocomath

Stop guessing. What equation are you using?

And before plugging values into your equation, let's SOLVE for the variable we want.

15. Jan 6, 2008

### Sophialiu609

The one I posted as the title!
h=rt-4.9t^2

16. Jan 6, 2008

### rocomath

Well looks right to me. You have it in terms of minutes, so there you go.

17. Jan 6, 2008

### Sophialiu609

So was my answer correct? 10 minutes? Because 600/60=10

18. Jan 6, 2008

### Sophialiu609

oh ohkay, thank you i didn't see the second page