# I Potential energy and gravity

1. Jun 22, 2017

### rishabh685

hey guys! i am very confused about the concept of potential energy and gravity. we know that as an object is continuously lifted above ground, its potential energy increases. but i was wondering if that potential energy is physically affecting the body, like if the potential energy increases, does that mean the body is being pulled towards the ground by a stronger force as the height increases ? please help !

2. Jun 22, 2017

### sophiecentaur

3. Jun 22, 2017

### Staff: Mentor

No, as the relevant equations tell you, the force is approximately constant over short distances and decreases over long distances.

4. Jun 22, 2017

### rishabh685

i don't think that potential energy will affect the body physically in any way, because i find the concept of potential energy hypothetical.

5. Jun 22, 2017

### rishabh685

does that mean that i require the same force to lift a body to a height of 100m and another to 50m ? i am not talking about power, i mean a constant supply of force

6. Jun 22, 2017

### Staff: Mentor

Yes, that is correct.

7. Jun 22, 2017

### rishabh685

then what does potential energy do in real world ? does it only come in action when the body is left to fall ?

Last edited: Jun 22, 2017
8. Jun 22, 2017

### Staff: Mentor

You probably could have answered your question yourself. If you step on a bathroom scale on the 2nd floor, does it show the same as on the 1st floor?
The scale is measuring the force needed to hold your body up.

9. Jun 22, 2017

### Staff: Mentor

Well, it also comes into play when you raise an object. Any time an object rises or falls.

10. Jun 22, 2017

### Staff: Mentor

Gravity is not the only kind of potential energy. Think of winding a spring for example.

Last edited: Jun 24, 2017
11. Jun 24, 2017

Well then i would like to introduce the case for a vehicle (4-wheels, drive on rear wheel only) to climb a slope.

Will the torque required at the wheels at (i) base of slope
(ii) a certain height on the slope, will be the same to sustain the vehicle at that exact location or it will be different ?
I believe it will be different due to rise in potential energy which will in turn try to minimize its value by pulling the vehicle more and more down.

12. Jun 24, 2017

### Staff: Mentor

No, it won't be. That's not what potential energy is. You really should be looking at the relevant equations; w=fd, f=mg, PE=mgh and Newton's law of gravity, which I don't feel like typing. What do they tell you about how gravitational force changes with elevation.

13. Jun 24, 2017

### sophiecentaur

It (Gravitational Potential Energy) doesn't "do" anything. It represents the work that has been (or could have been done) in getting a body to a certain height, relative to a chosen starting point. That energy can be 'got out' in the form of work as it goes back to ithe original level. Did you find that out in your research? A reservoir full of water 'has' potential energy, which can be used to generate Electrical Power. Potential Energy is Stored and can be 'used' at some time later and converted / transferred to another form.
Wiki tells you of the formulae which will tell you the actual amount of Potential Energy under different conditions. As with most of Physics, without using the formulae, you can't determine how much energy is involved.

14. Jun 24, 2017

### sophiecentaur

It isn't a matter of 'belief'. Potential Energy is defined in a strict way and the only way to get the right answer is to use the formulae and put in the particular values. It isn't a subjective thing.

15. Jun 24, 2017

i need to know then why the same potential energy which is defined in a strict way will try to minimize its value by pulling the vehicle dowwards.
i am really stuck.

16. Jun 24, 2017

### sophiecentaur

If the value of PE at one position (A) is greater (more positive) than the value of PE at another position (B) then Energy is available to move an object from B to A. The Force acting on the object is given by the negative of the gradient (rate of change with distance) of the Potential Energy. This is what is always found by experiment so we can call such a relationship a Scientific Law. Also, the Maths leads us to the same conclusion.
It works for all forms of PE: if you do work by stretching a spring, you increase the (Elastic) potential energy and the force (we all know) will be in the direction that will shorten (negative direction) the spring.

Edit: PS, you use the words " try to minimize". Nothing in Physics actually 'tries' to do anything. 'Try' implies some purpose and thought. It is always a good idea to try to avoid anthropomorphism in Science.

17. Jun 24, 2017

### Staff: Mentor

This question makes more sense. Potential energy doesn't try to do anything. Why don't you outline a specific scenario, draw a diagram, attach some numbers to it, and calculate what is happening? It is actually very simple!