How is it possible for gravitational potential energy to be negative?
I Googled the answer, but all I could understand was that if the PE was zero at infinity as you move toward an object with gravity the KE must be positive because KE is the energy of motion so the the PE must be negative to equal a total of zero. Is this right? Or am I way off the mark?
PE is always to a reference. It is zero at infinity. But we commonly set some place (say ground level) and measure GPE in relation to that plane. Negative GPE means energy has to be given to get it back to zero. Example: refernce level is at 12 feet above ground. Below that, GPE is negative.
Okay, so using the ground level for your reference point if you went under the crust the GPE must be negative for the total energy to remain zero?
I think I get it! Thanks a lot!
That is it.
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You're actually on the right track! Gravitational potential energy can indeed be negative, and your explanation is partially correct. Let me explain it in more detail.
Gravitational potential energy is the energy possessed by an object due to its position in a gravitational field. The formula for gravitational potential energy is given by:
PE = mgh
Here, m represents the mass of the object, g is the acceleration due to gravity, and h is the height or distance from a reference point. Typically, the reference point is chosen to be at ground level, where the potential energy is considered to be zero.
Now, let's consider a scenario where an object is located at an elevated position. As you correctly mentioned, when we move the object from infinity towards the Earth, its potential energy decreases. This implies that the change in potential energy (ΔPE) will be negative.
The reason for this negativity is due to the choice of the reference point at ground level. By convention, potential energy is zero at ground level (or infinity). As the object moves closer to the Earth, its height (h) decreases, resulting in a negative change in potential energy. Therefore, the gravitational potential energy itself becomes negative at positions closer to the Earth's surface.
To understand it visually, consider throwing an object upwards. As it rises, its potential energy increases (becoming less negative), reaching zero at the peak of its trajectory. As it falls back to the ground, its potential energy becomes negative again, until it reaches its original height at ground level.
In summary, when we define the reference point for gravitational potential energy, it often results in negative values as objects move closer to the gravitational source. This negative value represents the decrease in potential energy as the object moves to a lower position.
If you have any more questions or need further clarification, please feel free to ask!