Force on a rock at maximum height and mass during freefall

[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?
The answer is 2

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?

The answer is 52.

~Thanks

 

[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.

 

[Shay10825]

But how would you do this for the max height?

 

[Gokul43201]

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 ?"

 

[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?

 

[Gokul43201]

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

 

[Shay10825]

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 ?"

gravity is the only force

 

[Gokul43201]

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?

Forget about this for now. You'll learn how to do this only after you understand the answer to my question.

 

[Shay10825]

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

Does it change when it is in freefall?

 

[Gokul43201]

gravity is the only force

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 ?

 

[Gokul43201]

Does it change when it is in freefall?

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).

 

[Shay10825]

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 ?

Is it 9.8 m/s^2 ?

 

[Shay10825]

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).

I think the mass does not change.

 

[Gokul43201]

Is it 9.8 m/s^2 ?

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 ?

 

[Gokul43201]

I think the mass does not change.

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

 

[Shay10825]

Weight
Fg = mg

 

[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?

 

[Shay10825]

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

 

[Gokul43201]

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.

 

[Shay10825]

Thanks