<< Chapter < Page Chapter >> Page >
  • Explain gravitational potential energy in terms of work done against gravity.
  • Show that the gravitational potential energy of an object of mass m size 12{m} {} at height h size 12{h} {} on Earth is given by PE g = mgh .
  • Show how knowledge of the potential energy as a function of position can be used to simplify calculations and explain physical phenomena.

Work done against gravity

Climbing stairs and lifting objects is work in both the scientific and everyday sense—it is work done against the gravitational force. When there is work, there is a transformation of energy. The work done against the gravitational force goes into an important form of stored energy that we will explore in this section.

Let us calculate the work done in lifting an object of mass m size 12{m} {} through a height h size 12{h} {} , such as in [link] . If the object is lifted straight up at constant speed, then the force needed to lift it is equal to its weight mg size 12{ ital "mg"} {} . The work done on the mass is then W = Fd = mgh size 12{ ital "W = Fd = mgh"} {} . We define this to be the gravitational potential energy     ( PE g ) put into (or gained by) the object-Earth system. This energy is associated with the state of separation between two objects that attract each other by the gravitational force. For convenience, we refer to this as the PE g size 12{"PE" rSub { size 8{g} } } {} gained by the object, recognizing that this is energy stored in the gravitational field of Earth. Why do we use the word “system”? Potential energy is a property of a system rather than of a single object—due to its physical position. An object’s gravitational potential is due to its position relative to the surroundings within the Earth-object system. The force applied to the object is an external force, from outside the system. When it does positive work it increases the gravitational potential energy of the system. Because gravitational potential energy depends on relative position, we need a reference level at which to set the potential energy equal to 0. We usually choose this point to be Earth’s surface, but this point is arbitrary; what is important is the difference in gravitational potential energy, because this difference is what relates to the work done. The difference in gravitational potential energy of an object (in the Earth-object system) between two rungs of a ladder will be the same for the first two rungs as for the last two rungs.

Converting between potential energy and kinetic energy

Gravitational potential energy may be converted to other forms of energy, such as kinetic energy. If we release the mass, gravitational force will do an amount of work equal to mgh size 12{ ital "mgh"} {} on it, thereby increasing its kinetic energy by that same amount (by the work-energy theorem). We will find it more useful to consider just the conversion of PE g size 12{"PE" rSub { size 8{g} } } {} to KE size 12{"KE"} {} without explicitly considering the intermediate step of work. (See [link] .) This shortcut makes it is easier to solve problems using energy (if possible) rather than explicitly using forces.

(a) The weight attached to the cuckoo clock is raised by a height h shown by a displacement vector d pointing upward. The weight is attached to a winding chain labeled with a force F vector pointing downward. Vector d is also shown in the same direction as force F. E in is equal to W and W is equal to m g h. (b) The weight attached to the cuckoo clock moves downward. E out is equal to m g h.
(a) The work done to lift the weight is stored in the mass-Earth system as gravitational potential energy. (b) As the weight moves downward, this gravitational potential energy is transferred to the cuckoo clock.

Questions & Answers

2 how heat loss is prevented in a vacuum flask
Abdullah Reply
what is science
logical reasoning for a particular phenomenon.
I don't know anything about it 😔. I'm sorry, please forgive 😔
due to non in contact mean no conduction and no convection bec of non conducting base and walls and also their is a grape between the layer like to take the example of thermo flask
dimensions v²=u²+2at
Lagben Reply
what if time is not given in finding the average velocity?
Alan Reply
the magnetic circuit of a certain of the flux paths in each of the long and short sides being 25cm and 20cm reprectielectrove. there is an air gap of 2mm long in one the long sides if a flux density of 0.8weber/m is to produce in the magnet of 1500 turns..
Daniel Reply
How do you calculate precision
Sacky Reply
what module is that?
Chemisty 1A?
No it has something to do with measurements bro... What we did today in class
Tah bra honestly I didn't understand a thing in that class..when re your Tutorials?
Friday bro... But the topics we did are in this app... Just try to master them quickly before the test dates... Are you done with the Maths sheet
I eat ass
I'll work on the maths sheet tomorrow bra @Sacky Malyenge but I'll try mastering them
I'll eat your mom's ass with a side of tendies
@Fillemon Nanwaapo
lol, hush
There are very large numbers of charged particles in most objects. Why, then, don’t most objects exhibit static electricity?
Bilkisu Reply
Because there's an equal number of negative and positive charges... objects are neutral in nature
when a ball rolls on a smooth level ground,the motion of its centre is?
Mary Reply
what is electro magnetic field?
electromagnetic field is a special type of field been produced by electric charges..!!! like the word electro from Electricity and the word magnetic from Magnetism.. so it is more of a join field..!!!
Electromagnetic field is caused by moving electric charge
when a ball rolls on a smooth level ground,the motion of its centre is?
what's the relationship btw displacement and position
Declan Reply
displacement is the change of position 8======✊=D 💦💦
what is the meaning of elasticity
Pele Reply
is the ability of a material to or any object to expand to a limit point
this is about kinematics you bonk
what does emf/R mean
Eze Reply
What is work
Wisdom Reply
work is the product of force and perpendicular distance
Pls explain simple harmonic motion
Olaiya Reply
Any to and from motion of a fluid or any elastic object
a current of 5.5mA is flowing through a 3.3k resistor.compute th p.d developed across the resistor
Clifford Reply
A p.d of 24 volts exist across a 15 OHM'S resistor.calculate the current flowing the resistor
a current of 5.5mA is flowing through a 3.3kOHM'S resistor.compute th p.d developed across the resistor
solve it please
the so unit power is the watt(w)/joul/second (w1)/s
Jibo Reply
what is time
Jibo Reply
a measure of the duration of an event
Practice Key Terms 1

Get the best College physics course in your pocket!

Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'College physics' conversation and receive update notifications?