# Gravitational potential question

Watch
Announcements

Page 1 of 1

Go to first unread

Skip to page:

A spacecraft of mass 1.0 × 106 kg is in orbit around the Sun at a radius of 1.1 × 10^11 m

The spacecraft moves into a new orbit of radius 2.5 × 10^11 m around the Sun.

What is the total change in gravitational potential energy of the spacecraft?

A -6.76 x10^14

B -3.38 x 10^14

C 3.38 x 10^14

D 6.76 x10^14

I understand you have to find the change in gravitational potential then times this number by the mass to get the change in gravitational potential energy, i get the value of -6.76x10^14. But the answer is D. Why is the energy positive?

Does it have something to do that the gravitational potential energy is the energy needed to move an object from infinity to that point?

If you move an object from infinity to a point, the force is in the same direction as the displacement so the work is positive. But we are moving the satellite towards infinity, so the force pulling it back to earth is in the opposite direction to the displacement, so the work must be negative, but its not. Im so baffled. I hope i make some sort of sense?

Please could someone help me understand?

The spacecraft moves into a new orbit of radius 2.5 × 10^11 m around the Sun.

What is the total change in gravitational potential energy of the spacecraft?

A -6.76 x10^14

B -3.38 x 10^14

C 3.38 x 10^14

D 6.76 x10^14

I understand you have to find the change in gravitational potential then times this number by the mass to get the change in gravitational potential energy, i get the value of -6.76x10^14. But the answer is D. Why is the energy positive?

Does it have something to do that the gravitational potential energy is the energy needed to move an object from infinity to that point?

If you move an object from infinity to a point, the force is in the same direction as the displacement so the work is positive. But we are moving the satellite towards infinity, so the force pulling it back to earth is in the opposite direction to the displacement, so the work must be negative, but its not. Im so baffled. I hope i make some sort of sense?

Please could someone help me understand?

0

reply

Report

#2

(Original post by

A spacecraft of mass 1.0 × 106 kg is in orbit around the Sun at a radius of 1.1 × 10^11 m

The spacecraft moves into a new orbit of radius 2.5 × 10^11 m around the Sun.

What is the total change in gravitational potential energy of the spacecraft?

A -6.76 x10^14

B -3.38 x 10^14

C 3.38 x 10^14

D 6.76 x10^14

I understand you have to find the change in gravitational potential then times this number by the mass to get the change in gravitational potential energy, i get the value of -6.76x10^14. But the answer is D. Why is the energy positive?

Does it have something to do that the gravitational potential energy is the energy needed to move an object from infinity to that point?

If you move an object from infinity to a point, the force is in the same direction as the displacement so the work is positive. But we are moving the satellite towards infinity, so the force pulling it back to earth is in the opposite direction to the displacement, so the work must be negative, but its not. Im so baffled. I hope i make some sort of sense?

Please could someone help me understand?

**user12394385835**)A spacecraft of mass 1.0 × 106 kg is in orbit around the Sun at a radius of 1.1 × 10^11 m

The spacecraft moves into a new orbit of radius 2.5 × 10^11 m around the Sun.

What is the total change in gravitational potential energy of the spacecraft?

A -6.76 x10^14

B -3.38 x 10^14

C 3.38 x 10^14

D 6.76 x10^14

I understand you have to find the change in gravitational potential then times this number by the mass to get the change in gravitational potential energy, i get the value of -6.76x10^14. But the answer is D. Why is the energy positive?

Does it have something to do that the gravitational potential energy is the energy needed to move an object from infinity to that point?

If you move an object from infinity to a point, the force is in the same direction as the displacement so the work is positive. But we are moving the satellite towards infinity, so the force pulling it back to earth is in the opposite direction to the displacement, so the work must be negative, but its not. Im so baffled. I hope i make some sort of sense?

Please could someone help me understand?

0

reply

Report

#3

For Gravitational Potential energy it is assumed that at infinity it is equal to zero, as a object approaches a gravitating object it will gain Kinetic energy (as it accelarates to to the gravitational field of the object), this gain in energy cannot be from nowhere- this would violate the laws of conservation of energy, hence it is assumed that the potential energy is most negative when the radius of rotation approaches zero and most positive/ equal to zero when approaching infinity. Hence as you go away from/increase your radius of rotation from a gravitating object, you will gain potential energy (it will become less negative) and you will lose equal and opposite kinetic energy (it will become less positive)- therefore energy is conserved and at any point, the kinetic energy of the satellite is equal and opposite to the potential energy of the satellite. There is the theory.

Summary: you need to do work to move away from a gravitating object, therefore the change in potential energy will be positive...

Hope this helps!!

Summary: you need to do work to move away from a gravitating object, therefore the change in potential energy will be positive...

Hope this helps!!

0

reply

(Original post by

For Gravitational Potential energy it is assumed that at infinity it is equal to zero, as a object approaches a gravitating object it will gain Kinetic energy (as it accelarates to to the gravitational field of the object), this gain in energy cannot be from nowhere- this would violate the laws of conservation of energy, hence it is assumed that the potential energy is most negative when the radius of rotation approaches zero and most positive/ equal to zero when approaching infinity. Hence as you go away from/increase your radius of rotation from a gravitating object, you will gain potential energy (it will become less negative) and you will lose equal and opposite kinetic energy (it will become less positive)- therefore energy is conserved and at any point, the kinetic energy of the satellite is equal and opposite to the potential energy of the satellite. There is the theory.

Summary: you need to do work to move away from a gravitating object, therefore the change in potential energy will be positive...

Hope this helps!!

**Lyroknight**)For Gravitational Potential energy it is assumed that at infinity it is equal to zero, as a object approaches a gravitating object it will gain Kinetic energy (as it accelarates to to the gravitational field of the object), this gain in energy cannot be from nowhere- this would violate the laws of conservation of energy, hence it is assumed that the potential energy is most negative when the radius of rotation approaches zero and most positive/ equal to zero when approaching infinity. Hence as you go away from/increase your radius of rotation from a gravitating object, you will gain potential energy (it will become less negative) and you will lose equal and opposite kinetic energy (it will become less positive)- therefore energy is conserved and at any point, the kinetic energy of the satellite is equal and opposite to the potential energy of the satellite. There is the theory.

Summary: you need to do work to move away from a gravitating object, therefore the change in potential energy will be positive...

Hope this helps!!

0

reply

Report

#5

(Original post by

Ok oh, so when would the change in potential energy be negative? When it moves towards the gravitating object? why?

**user12394385835**)Ok oh, so when would the change in potential energy be negative? When it moves towards the gravitating object? why?

**when moving**

__negative____the gravitating object and__

**towards**__when moving__

**positive**__from a object. Even if you use the equation mgΔh, if you go from a high height of 5 metres to a height of 1 metre above the ground, you're Δh value will be 1-5=-4 and therefore your change in potential energy will be negative. As I said above this will be because as you move towards the gravitating object you will accelerate and therefore you will gain kinetic energy, therefore to conserve energy you must lost energy from your potential energy store. Is better to thing of the negative sign as the 'change in potential energy' instead of the object's current potential energy if what I said previously is confusing...__

**away**Hope this helps

Last edited by Lyroknight; 8 months ago

0

reply

(Original post by

Yes the change in potential energy will be

Hope this helps

**Lyroknight**)Yes the change in potential energy will be

**when moving**__negative____the gravitating object and__**towards**__when moving__**positive**__from a object. Even if you use the equation mgΔh, if you go from a high height of 5 metres to a height of 1 metre above the ground, you're Δh value will be 1-5=-4 and therefore your change in potential energy will be negative. As I said above this will be because as you move towards the gravitating object you will accelerate and therefore you will gain kinetic energy, therefore to conserve energy you must lost energy from your potential energy store. Is better to thing of the negative sign as the 'change in potential energy' instead of the object's current potential energy if what I said previously is confusing...__**away**Hope this helps

0

reply

Report

#7

(Original post by

Therefore the change in potential energy would also be negative when moving from infinity to the object. As the object moves towards the gravitating object it accelerates so gains KE so the change in potential energy must be negative to conserve energy. Now i know gravitational potential is just the change in gravitational potential energy divided by mass at that point in space. So is that why the gravitational potentials are negative?

**user12394385835**)Therefore the change in potential energy would also be negative when moving from infinity to the object. As the object moves towards the gravitating object it accelerates so gains KE so the change in potential energy must be negative to conserve energy. Now i know gravitational potential is just the change in gravitational potential energy divided by mass at that point in space. So is that why the gravitational potentials are negative?

0

reply

X

Page 1 of 1

Go to first unread

Skip to page:

### Quick Reply

Back

to top

to top