Fizzy_Cyst
Well-Known Member
I always get heaps of questions about GPE, both in class and on the forums. I got aNOTHER question yesterday, so I thought I would share my response with you guys, hopefully it will help a couple of you! The asker was concerned with 3 things, which I have addressed -- please excuse spelling mistakes, it is prolly my little girl smashing my keyboard.
why it takes a negative value and stuff.
With regards to the negative sign -- basically GPE is present whenever an object can 'fall' due to a gravitational field. There are two instances in which an object cannot fall -- when it has nowhere to fall and when it is not present within a gravitational field -- If the object cannot fall, then it does not possess and GPE -- so essentially, we define infinity as Zero GPE seeing as it is outside of the gravitational field of an object and hence cannot fall. As the object moves closer towards the Earth, then it's GPE is decreasing, therefore, seeing as infinity is zero GPE and GPE is decreasing, as it has less distance it is able to fall as you get closer to Earth, then the GPE must be negative.
Explain that a change in gravitational potential energy is related to work done
About the work done, basically work done is the same as 'energy input' into an object. Example, if you are driving a car at 60km/h and you want to make it go 100km/h -- you need to input energy into it (chemical energy in the petrol -- which is then converted in KE of the object), the extra kinetic energy cannot simply be plucked out of nowhere, it must be the result of a transfer or transformation of energy as stated by the law of conservation of energy.
So, if you have an object at a certain orbit (say altitude 250km) and you want to put it into an orbit of 1000km altitude, then you need to put more energy into the object to make it move away from the Earth (against gravity), obviously the object has two different values of GPE when it is in the two different positions, as it can 'fall' further when in 1000km orbit -- hence has higher (less negative) GPE at this position. Now, the difference between the two values of GPE is the minimum energy needed to be input into the object to move it between the two points, hence the 'work' needed to be 'done on the object'
Define gravitational potential energy as the work done to move an object from a very large distance away to a point in a gravitational field
That being said, change in GPE is final GPE - initial GPE. If our initial GPE is zero (GPE at infinity or very large distance away), you can see that Change in GPE = final GPE - 0 = final GPE.
We have already said that change in GPE = Work done, therefore final GPE = work done to move object from infinity to a point in grav field.
Sorry if it is a bit brief, typing with a 7 mionth old on ur lap is bnot easy to do!
why it takes a negative value and stuff.
With regards to the negative sign -- basically GPE is present whenever an object can 'fall' due to a gravitational field. There are two instances in which an object cannot fall -- when it has nowhere to fall and when it is not present within a gravitational field -- If the object cannot fall, then it does not possess and GPE -- so essentially, we define infinity as Zero GPE seeing as it is outside of the gravitational field of an object and hence cannot fall. As the object moves closer towards the Earth, then it's GPE is decreasing, therefore, seeing as infinity is zero GPE and GPE is decreasing, as it has less distance it is able to fall as you get closer to Earth, then the GPE must be negative.
Explain that a change in gravitational potential energy is related to work done
About the work done, basically work done is the same as 'energy input' into an object. Example, if you are driving a car at 60km/h and you want to make it go 100km/h -- you need to input energy into it (chemical energy in the petrol -- which is then converted in KE of the object), the extra kinetic energy cannot simply be plucked out of nowhere, it must be the result of a transfer or transformation of energy as stated by the law of conservation of energy.
So, if you have an object at a certain orbit (say altitude 250km) and you want to put it into an orbit of 1000km altitude, then you need to put more energy into the object to make it move away from the Earth (against gravity), obviously the object has two different values of GPE when it is in the two different positions, as it can 'fall' further when in 1000km orbit -- hence has higher (less negative) GPE at this position. Now, the difference between the two values of GPE is the minimum energy needed to be input into the object to move it between the two points, hence the 'work' needed to be 'done on the object'
Define gravitational potential energy as the work done to move an object from a very large distance away to a point in a gravitational field
That being said, change in GPE is final GPE - initial GPE. If our initial GPE is zero (GPE at infinity or very large distance away), you can see that Change in GPE = final GPE - 0 = final GPE.
We have already said that change in GPE = Work done, therefore final GPE = work done to move object from infinity to a point in grav field.
Sorry if it is a bit brief, typing with a 7 mionth old on ur lap is bnot easy to do!
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