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External and internal forces

In Grade 10, you saw that mechanical energy was conserved in the absence of external forces. It is important to know whether a force is an internal force or an external force in the system, because this is related to whether the force can change an object's total mechanical energy when it does work on an object.

When an external force (for example friction, air resistance, applied force) does work on an object, the total mechanical energy (KE + PE) of that object changes. If positive work is done, then the object will gain energy. If negative work is done, then the object will lose energy. The gain or loss in energy can be in the form of potential energy, kinetic energy, or both. However, the work which is done is equal to the change in mechanical energy of the object.

Investigations : external forces

We can investigate the effect of external forces on an object's total mechanical energy by rolling a ball along the floor from point A to point B.

Find a nice smooth surface (e.g. a highly polished floor), mark off two positions, A and B, and roll the ball between them.

The total mechanical energy of the ball, at each point, is the sum of its kinetic energy (KE) and gravitational potential energy (PE):

E total , A = KE A + PE A = 1 2 m v A 2 + m g h A = 1 2 m v A 2 + m g ( 0 ) = 1 2 m v A 2
E total , B = KE B + PE B = 1 2 m v B 2 + m g h B = 1 2 m v B 2 + m g ( 0 ) = 1 2 m v B 2

In the absence of friction and other external forces, the ball should slide along the floor and its speed should be the same at positions A and B. Since there are no external forces acting on the ball, its total mechanical energy at points A and B are equal.

v A = v B 1 2 m v A 2 = 1 2 m v B 2 E total , A = E total , B

Now, let's investigate what happens when there is friction (an external force ) acting on the ball.

Roll the ball along a rough surface or a carpeted floor. What happens to the speed of the ball at point A compared to point B?

If the surface you are rolling the ball along is very rough and provides a large external frictional force, then the ball should be moving much slower at point B than at point A.

Let's now compare the total mechanical energy of the ball at points A and B:

E total , A = KE A + PE A = 1 2 m v A 2 + m g h A = 1 2 m v A 2 + m g ( 0 ) = 1 2 m v A 2
E total , B = KE B + PE B = 1 2 m v B 2 + m g h B = 1 2 m v B 2 + m g ( 0 ) = 1 2 m v B 2

However, in this case, v A v B and therefore E total , A E total , B . Since

v A > v B E total , A > E total , B

Therefore, the ball has lost mechanical energy as it moves across the carpet. However, although the ball has lost mechanical energy, energy in the larger system has still been conserved. In this case, the missingenergy is the work done by the carpet through applying a frictional force on the ball. In this case the carpet is doing negative work on the ball.

When an internal force does work on an object by an (for example, gravitational and spring forces), the total mechanical energy (KE + PE) of that object remains constant but the object's energy can change form. For example, as an object falls in a gravitational field from a high elevation to a lower elevation, some of the object's potential energy is changed into kinetic energy. However, the sum of the kinetic and potential energies remain constant. When the only forces doing work are internal forces, energy changes forms - from kinetic to potential (or vice versa); yet the total amount of mechanical energy is conserved.

Questions & Answers

hi please help me how to balance redox reactions?
Brian Reply
Which equation do u wanna balance
i tried to write it but my phone just can't write it Cr2 O72-(aq) + H2S (aq) Cr3 (aq) + S(s)
hello. what is the difference between a primary, secondary and tertiary alcohol
Thokozani Reply
primary the C=bonded to 1 Carbon atom.... secondary=bonded to 2 carbon atoms tertiary=bonded to 3 carbon atoms
I hope you're answered
thanks Christina
My pleasure
Thank you Christina. This is very helpful 😀👍👍💯
Anytime Thokozani
How to calculate pH?
thank you guys i didn't know about the primary, secondary and tertiary alcohols
thank you Christina
can somebody help me with functional isomers
Baningi Reply
function isomer has the same molecular formula but different functional group
what's momentum
Luvelo Reply
the product of an objects mass times velocity. it is mainly prevalent in collisions.
Methane contains C and H. This compound is
Juan Reply
ketones's functional group
Moloi Reply
Why does the carboxyl group have acidc properties?
Evi Reply
carboxylic acids
Besides improving appearance what is another reason why medal is electro plated
Elizabeth Reply
it is made of metal
what is different between Dependent variable and independent variables
difference between A.c and D.c
AC has slip rings DC has split rings commutator
to avoid rust and to improve strength and conductivity
the variable whose value doesn't depend on other is known as independent variable ex time
are UV rays dangerous?
Khathutshelo Reply
what's this?
if there is extensive exposure, it can lead to skin cancer
yes depend upon their intensity
i hv a problem with this chapter because i also have problem with Newton's laws of motion
Kearabetswe Reply
I have a problem with calculation in Newton's law of motion
me also VuyokAzi
what's the question
Which area in particular?
just get the hang if free body diagrams,know the directions.
I know how to draw free diagram I have a problem with calculation in second law
what is the wave model of atom ?
Trazy Reply
guys what is momentum
@Luvelo... Momentum is the product of an object's mass and its velocity.
the model based on wave nature of electron
Can you state the Doppler effect in words
Nombuyiselo Reply
State Coulomb's law in words
the magnitude of the electrostatic force exerted point on each other is directly proportional to the product on magnitude of charges and inversely proportional to the square distance between them
the change in frequency of the sound detected by a listener because sound source and listener have different velocity
Newton's first law of motion
Nnuso Reply
When an object is at rest or traveling at a constant velocity it will remain at rest unless an unbalanced force acts upon it.
yes that's true
Always know Newton's second law of motion. It appears to be in every question paper
Is the normal force always 0
Mpilo Reply
no. newtons 3rd law states that if something exerts a force into something else it will experience that same magnitude of force but in the opposite direction. so the net force is equal to the force the object applies to the surface but in the opposite direction
define the term rate of reaction in word
David Reply
I think it a chemical process in which substance act mutually on each other.
how many bones are in the human body?
it's the rate at which the reactants are able to change to products
206 bones

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Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
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