# Newton's Laws

1. Oct 5, 2004

### Shay10825

Hi everyone! I need some help.

1.)A .2 kg rock is thrown upward with an initial speed 15 m/s. What is the force on the ball in N when it reaches its maximum height?

Would I use kinematics to find the max height then use F=ma? If so how?

2.) A woman whose mass as the surface of the earth is 52 kg dives off a 2 m board into a swimming pool. What is her mass in kg while she is in a freefall?

~Thanks

Last edited: Oct 5, 2004
2. Oct 5, 2004

### Sirus

$F_{net}=ma$, as you know. What is the net force acting on the rock while it is in the air? Hint: the net force on the rock is constant throughout its trajectory, nelgecting friction.

3. Oct 5, 2004

### Shay10825

But how would you do this for the max height?

4. Oct 5, 2004

### Gokul43201

Staff Emeritus
Shay, for a minute forget your problem and answer this question : "What are the different forces that are acting on an object that is in the air, and not in contact with any other object ?"

5. Oct 5, 2004

### Shay10825

If I just do F=ma then I need the acceleration. How do I fond the acceleration and max height with only the velocity and mass?

6. Oct 5, 2004

### Gokul43201

Staff Emeritus
What is the definition of mass, and how does an object's mass change ?

7. Oct 5, 2004

### Shay10825

gravity is the only force

8. Oct 5, 2004

### Gokul43201

Staff Emeritus

9. Oct 5, 2004

### Shay10825

Does it change when it is in freefall?

10. Oct 5, 2004

### Gokul43201

Staff Emeritus
Correct. So would that not be the answer to the question ?

How do you calculate the force due to gravity ? Does it have another name ?

11. Oct 5, 2004

### Gokul43201

Staff Emeritus
Okay, I'll give you a definition, and you tell me whether or not the mass changes.

Definition : The mass is the amount of matter contained in an object, (sort of like the number of atoms/molecules in the object).

12. Oct 5, 2004

### Shay10825

Is it 9.8 m/s^2 ?

13. Oct 5, 2004

### Shay10825

I think the mass does not change.

14. Oct 5, 2004

### Gokul43201

Staff Emeritus
That's the acceleration due to gravity, represented by the symbol 'g'.

The force due to gravity involves this g and the mass m. And there's a common name for it - begins with a "W". What is this name and formula for the force ?

15. Oct 5, 2004

### Gokul43201

Staff Emeritus
Correct, you can change the mass only by removing or adding stuff to the object.

16. Oct 5, 2004

### Shay10825

Weight
Fg = mg

17. Oct 5, 2004

### Shay10825

Ohhhhhhhhh!

Fg = ma
Fg= (.2)(9.8)
Fg=1.96

BUt why did they say "when it reaches its maximum height" and why does it talk about the initial speed? Is this just to confuse me?

18. Oct 5, 2004

### Shay10825

OK now I understand why 52 is the answer to #2. Is what I did for #1 correct?

19. Oct 5, 2004

### Gokul43201

Staff Emeritus
Correct. It's a trick question. The whole thing about the velocity and the maximum height is a distraction. They have no relevance to the problem.

20. Oct 5, 2004

Thanks