Please help clarify

[m.k.2015]

Hello everyone, 

I have been trying to study the gravitational potential energy concept from two sources. One of which described the change in gravitational potenial energy to be:

m*(V2 - V1)

= m*change in V

while the other described it as 

m*(V1 - V2)

= - m*change in V

so which one is it?

and can the change in gravitational potential energy be negative?

Thanks in advance

[Vioh]

Hello everyone, 

I have been trying to study the gravitational potential energy concept from two sources. One of which described the change in gravitational potenial energy to be:

m*(V2 - V1)

= m*change in V

while the other described it as 

m*(V1 - V2)

= - m*change in V

so which one is it?

and can the change in gravitational potential energy be negative?

Thanks in advance

 

Both sources are correct. They're just different representations of the same thing. However, the more important thing is that the equations that you quoted have nothing whatsoever to do with whether gravitational potential energy being positive or negative. Don't get confused by the negative sign in the equation.

 

In physics, we always define change in gravitation potential energy in this way:

 

So, ΔE_pcan happen to be negative if V₁ > V₂. When you deal with question relating to motion of a particle in a gravitation field, V₂ is always defined as the final position of the particle. It's just like the formula Q = mcΔT, in which ΔT can be a negative value as long as the final temperature is lower than the initial temperature. Hence, always use the formula that I gave above, especially if you don't want to get confused whenever you deal with motion of particles in a field.

[m.k.2015]

So I always work out the change and take the absolute value of it? That's what I used to do in Q=mc Î”T

[Vioh]

So I always work out the change and take the absolute value of it? That's what I used to do in Q=mc Î”T

 

*Change* is not defined as a positive value! So you don't need to take the absolute value of anything. Similarly, you don't take the absolute value of mcΔT because the sign can actually give you tonnes of information. If mcΔT is negative, then you know that the system loses heat. On the other hand, if mcΔT is positive, the system gains heat. So don't take the absolute value. Leave it as it is

[m.k.2015]

Alright I get it now .. The later minus the first no matter what the sign is .. Thank you