# What would the *Energy equation be for this problem?

#### Np14

1. A 0.1 kilogram solid rubber ball is attached to the end of an 0.80 meter length of light thread. The ball is swung in a vertical circle, as shown in the diagram above. Point P, the lowest point of the circle, is 0.20 meter above the floor. The speed of the ball at the top of the circle is 6.0 meters per second, and the total energy of the ball is kept constant.

2. **ex. (Ug + Wf = K)

3. Would it just be Ug = k

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#### kuruman

Homework Helper
Gold Member
1. A 0.1 kilogram solid rubber ball is attached to the end of an 0.80 meter length of light thread. The ball is swung in a vertical circle, as shown in the diagram above. Point P, the lowest point of the circle, is 0.20 meter above the floor. The speed of the ball at the top of the circle is 6.0 meters per second, and the total energy of the ball is kept constant.

2. **ex. (Ug + Wf = K)

3. Would it just be Ug = Ug ?
Please post the diagram related to the question and refrain from using boldface for everything. Also, Ug is always equal to itself. Please make a better effort to show your reasoning.

Mentor

#### Np14

Please post the diagram related to the question and refrain from using boldface for everything. Also, Ug is always equal to itself. Please make a better effort to show your reasoning.
I don't think you understand how an energy equation works.

#### Doc Al

Mentor
I don't think you understand how an energy equation works.
You wrote Ug = Ug. What did you mean by that?

#### Np14

You wrote Ug = Ug. What did you mean by that?
I meant that energy is conserved. Since the equation for PE is m*g*h, the height is changing in both instances. So the force of gravity on the object must change as well.

#### Doc Al

Mentor
I meant that energy is conserved.
Well, that's not what you wrote.

Since the equation for PE is m*g*h, the height is changing in both instances. So the force of gravity on the object must change as well.
The force of gravity won't change, but the gravitational PE will.

Just write an expression for the total energy and set it equal to a constant.

#### Np14

I meant that energy is conserved. Since the equation for PE is m*g*h, the height is changing in both instances. So the force of gravity on the object must change as well.
I honestly have no clue what the equation would be for this problem. No work is being done since the displacement is zero, but I don't know if the PE becomes zero as well once the object goes to the bottom of the circle.

#### Doc Al

Mentor
Only two forms of energy are involved here. One is potential. What's the other?

#### Np14

Only two forms of energy are involved here. One is potential. What's the other?
Kinetic I guess.

#### Doc Al

Mentor
Kinetic I guess.
Exactly. So the total energy is PE + KE. Since energy is conserved, PE + KE = const.

You can write an expression for each energy term and evaluate their total.

#### Np14

By const, you mean a constant, nonzero number?

What would that represent?

Mentor

#### Np14

That is what I figured, thanks

#### kuruman

Homework Helper
Gold Member
That is what I figured, thanks
Can you find a numerical value for that constant? Please specify where you take the gravitational potential energy to be zero.

#### SammyS

Staff Emeritus
Homework Helper
Gold Member
Please post the diagram related to the question and refrain from using boldface for everything. Also, Ug is always equal to itself. Please make a better effort to show your reasoning.
I don't think you understand how an energy equation works.

You might be interested to know that @kuruman is a retired physics professor with a PhD degree. He certainly understands the principle of Conservation of Energy.
( I suppose that his avatar doesn't look all that professorial , )

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"What would the *Energy equation be for this problem?"

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