Ernie's Ball Fall: Potential & Kinetic Energy

In summary: N/AIn summary, Ernie is holding a 2 kg ball at rest at the opening (top) of a 10 m deep well. He releases the ball so that it falls into the well. Immediately before the ball hits the bottom, the sum of it's kinetic energy and potential energy is:* 0* 196 J* -196 J* 392 J* -392 J
  • #1
Dark Visitor
215
0
Ernie is holding a 2 kg ball at rest at the opening (top) of a 10 m deep well. He releases the ball so that it falls into the well. Assume the bottom of the well represents zero potential energy position. Immediately before the ball hits the bottom, the sum of it's kinetic energy and potential energy is:

* 0
* 196 J
* -196 J
* 392 J
* -392 J


The only 2 equations I see usable for this are:

Umg = mgy
K = .5mv2


What I was thinking of doing was getting the kinetic energy, but I realized there is no velocity given, so I am not sure if I should find that, or if I don't need to bother with it. We know the initial velocity is zero.

I also thought that using the Potential energy equation, we would get:
Umg = (2 kg)(9.8 m/s2)(10 m) = 196 J

but I am unsure because I don't know if the 10 m is negative because the well is under ground, or if it stays positive.


I could use some help starting this problem. I need it by tomorrow night, but I would like to get it done a.s.a.p. Thanks for any help.
 
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  • #2
Simply use energy conservation. The problem defines the zero potential E point as the bottom of the well. So all of the energy must be of what type when the ball hits bottom? OK, when the ball is at rest, all of the energy must be of what type? Calculate it. Finally, can kinetic energy be negative?
 
  • #3
When it hits bottom, I guess it would be kinetic since potential is zero?

When the ball is at rest, it should be potential because there is no motion involved yet.

And I don't think kinetic energy can be negative.
 
  • #4
Dark Visitor said:
When it hits bottom, I guess it would be kinetic since potential is zero?

When the ball is at rest, it should be potential because there is no motion involved yet.

And I don't think kinetic energy can be negative.
Good answer. So use one of the formulas above to calculate potential energy.

PS: This is a great reminder of why words are so important and at times misleading/confusing.
 
  • #5
Thanks, but I am still confused. I don't understand how I can calculate anything yet.
 
  • #6
The kinetic energy is zero when the ball is at rest. The total energy is then given by mgy.
 
  • #7
So my answer is just mgh, which is:

mgh = (2 kg)(9.8 m/s2)(10 m) = 196 J

That's it?
 
  • #8
Thats it. :cool:
 
  • #9
Okay. Thanks a lot. It seems so easy after you helped me, but I was really stumped on that one.
 
  • #10
You're welcome. Physics and math is often like that--the first time you do a problem of a certain type, it is hard. Hundredth time, much, much easier.
 
  • #11
True. And since I am a fan of neither, it makes it that much harder.
 

Related to Ernie's Ball Fall: Potential & Kinetic Energy

1. What is the concept of potential and kinetic energy in "Ernie's Ball Fall"?

In "Ernie's Ball Fall", potential energy refers to the stored energy that the ball possesses due to its position above the ground. As the ball falls, this potential energy is converted into kinetic energy, which is the energy of motion.

2. How does the height of the platform affect the potential and kinetic energy of the ball in "Ernie's Ball Fall"?

The higher the platform, the greater the potential energy the ball has. This means that as it falls, it will have a higher speed and therefore more kinetic energy. The opposite is also true; a lower platform will result in less potential energy and a slower fall with less kinetic energy.

3. What factors affect the potential and kinetic energy of the ball in "Ernie's Ball Fall"?

The main factor that affects the potential and kinetic energy of the ball is the height of the platform. Other factors can include the mass of the ball, air resistance, and any external forces acting on the ball, such as friction or wind.

4. How does the energy of the ball change as it falls in "Ernie's Ball Fall"?

As the ball falls, its potential energy decreases while its kinetic energy increases. This is because the potential energy is being converted into kinetic energy due to the force of gravity pulling the ball towards the ground.

5. What real-life applications can be seen in "Ernie's Ball Fall" with potential and kinetic energy?

The concept of potential and kinetic energy can be seen in many real-life scenarios, such as a rollercoaster, a swinging pendulum, or a falling object. Understanding these energy forms can also help in fields such as engineering and physics, where energy conservation is an essential concept.

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