# Comet orbiting a Star - True and False

1. Mar 19, 2015

### Westin

1. The problem statement, all variables and given/known data

Consider the path of a comet orbiting a star, the system of the comet plus the star. Which of the following statements are correct?

Select all that are True.
1) As the comet slows down, the kinetic energy of the system decreases.
2) As the comet's kinetic energy increases, the gravitational potential energy of the system also increases.
3) External work must be done on the system to speed up the comet.
4) As the kinetic energy of the system increases, the gravitational potential energy of the system decreases.
5) As the comet slows down, energy is lost from the system.

2. Relevant equations
none

3. The attempt at a solution

1) True (If the speed is decreasing means the KE is decreasing)
2) False (If KE increases, then PE decreases)
3) False (Not sure on this one)
4) True (If KE increases, then PE decreases)
5) True ( If speed is decreasing mean it is losing energy)

This was incorrect. I am not sure why. Any input would be greatly appreciated. Thanks.

2. Mar 19, 2015

### Orodruin

Staff Emeritus
Think about this one for two more seconds.

3. Mar 19, 2015

### Westin

KE would decrease as PE would increase?

4. Mar 19, 2015

### Westin

So it's technically not losing "energy". So it would be false. Is my reasoning correct now?

5. Mar 20, 2015

### Orodruin

Staff Emeritus
Yes, unless there is an external force, the system energy is conserved. The comet will lose kinetic energy but gain potential.

6. Mar 20, 2015

### andrevdh

The effect of gravity is taken into consideration by the potential energy of the comet-star system.
That is the total mechanical energy can only be changed if another force, other than gravitational attraction, comes into play.

7. Mar 20, 2015

### Westin

thank you

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