# Homework Help: Kinematics and magnitude problem

1. Nov 29, 2004

### nightlaei71

What is the magnitude of velocity of an object after free falling from rest after 3 minutes?
(A) 9.8 m/s (D) 180 m/s
(B) 32 m/s (E) 1800 m/s
(C) 87 m/s

I think the answer is 1800 m/s but I'm not sure how to arrive at it.

2. Nov 29, 2004

### arildno

Well, what equation would you use?

3. Nov 29, 2004

### nightlaei71

I have no idea, the equations that we studied in this chapter don't seem to work with this problem.

4. Nov 29, 2004

### arildno

What does free fall mean?

5. Nov 29, 2004

### nightlaei71

it's falling without a drag-producing device like a parachute

6. Nov 29, 2004

### Dooga Blackrazor

Have you tried transfering your minutes into seconds before putting everything into your formula?

7. Nov 29, 2004

### nightlaei71

I don't even know what formula to use. I'm in an AP class that my teacher doesn't show up to. He just assigns us homework and shows us videos. The book isn't much help at all so I'm completely lost.

8. Nov 29, 2004

### Dooga Blackrazor

If those are the only questions you have I'd just ask someone before you have to return the assignment. The formula is probably fairly straight forward and easily explainable. Afterwards you can just do the other questions quickly.

If your book has the answer in the back I might be able to determine the formula.

9. Nov 29, 2004

### nightlaei71

10. Nov 29, 2004

### Staff: Mentor

Hint: What is the meaning of "acceleration due to gravity"?

11. Nov 29, 2004

### nightlaei71

the pull of gravity causes objects to accelerate towards the ground... that doesn't really help me

12. Nov 29, 2004

### Staff: Mentor

It would help you if you know what acceleration means. What's the definition of acceleration?

And what is the acceleration due to gravity?

13. Nov 29, 2004

### nightlaei71

You're right, it would help, but like I said, I really don't know. : /

14. Nov 29, 2004

### Staff: Mentor

Acceleration is the rate of change of velocity; the acceleration due to gravity is 9.8 $m/s^2$.

15. Nov 29, 2004

hi,
you could try using this equation
$$v=u+at$$ where v is the final velocity and u is the initial velocity
t is the time in seconds
a is acceleration
t=3*60 seconds = 180 seconds
now pop in the numbers and out will pop the answer

16. Nov 30, 2004

### aviv87

Actually, considering the options he was given, I'd say in his course the acceleration due to gravity is used as $$10\ m/s^2$$

17. Dec 2, 2004

### nightlaei71

"he"? I'd hope not...

18. Dec 4, 2004

### Raza

Question:
What is the magnitude of velocity of an object after free falling from rest after 3 minutes?

Solution:
Time:180s <----(3)(60) to convert to seconds
Acceleration:$$9.8m/s^2$$ <---- Gravitational Force on Earth
D=(A)(T)
D=(9.8)(180s)

1764m

I might be wrong

*Edit: When I read my post again, I got confused, are you trying to find out the distance?

19. Dec 4, 2004

### nelufar

Here since the object is falling freely from rest, initial velocity(u) is zero. So now you have to find the final velocity(v). Use v = u + at, where a= $$9.8m/s^2$$. So your answer is right. If you want to calculate distance, then use $$s = ut + {1 \over 2}a t^2$$.

20. Dec 4, 2004

### tony873004

A car goes from 0 to 60 in 8 seconds. If it could keep up this acceleration, how fast would it be going in 16 seconds?

Do your problem the same way.