If we're lifting an object up, wouldn't the net work be equal to the change in kinetic energy PLUS the change in potential energy? (total change in mechanical energy?)

The work done by your force would be positive, and the total work done would be mgh, assuming the change in height to be h meters. So, according to the work-energy theorem, the final kinetic energy is mgh. Therefore, the square of final velocity should be 2gh. So, use the equations of motion to find out the final velocity. There is no need to find the initial velocity, assuming it started from rest. From the 3rd equation of motion, v^2 - u^2 = 2as, we find that v^2 = 2gh. Hence proved. Cheers!

how to find of an objects mass from force vs position graph?

First we need to find out the area under the force vs position graph which will give us the net work done and then we will set the net work = change in kinetic energy (Work-energy theorm) then we will be able to find the objects mass using the given data.

How do you find the initial kinetic energy?

initial kinetic energy is equal to 1/2*(mass of body)*(initial velocity of body)^2

since work is a vector, do we have work both on the y-axis and x-axis? for instance, if the force is made to a degree from the horizontal axis?

Work is not a vector. It is simply a quantity. If work is positive that means work is being done ON a system and if work is negative then that means that work is being done against the system. Hope this helps!

So the net force is all the work done on an object at the same instant?

the net force is different from the work done. first, they are different units. the net force is the vector sum of all the forces acting on an object while the total work is done is the sum of all the work done on the object. keep this in mind and don't confuse. Also, this may refer to the same instant or a period of time.

How do we know when to use cosine 180?

you can use the cos(x) when a certain degree of an angle is given, or if they want you to find the degree of an angle using a given/explained equation using the kinetic energy equation

Main content

Course: AP®︎/College Physics 1 > Unit 5

Lesson 3: Work-energy theorem

Work-energy theorem review

Google Classroom

Review the key concepts, equations, and skills for the work-energy theorem. Understand how the work-energy theorem only applies to the net work, not the work done by a single source.

Key terms

Term (symbol)	Meaning
Net work ( $W_{net}$ ‍)	Work done by the net force on an object. SI units of $Joules (J)$ ‍ or $N \cdot m$ ‍
Work-energy theorem	Net work done on an object equals the object’s change in kinetic energy. Also called the work-energy principle.

Equations

Equation	Symbols	Meaning in words
$\begin{aligned} W_{net} & = K - K_{0} \\ = Δ K \end{aligned}$ ‍	$W_{net}$ ‍ is net work, $K$ ‍ is final kinetic energy, $K_{0}$ ‍ is initial kinetic energy, and $Δ K$ ‍ is change in kinetic energy	The net work on an object is equal to the object’s final kinetic energy minus the initial kinetic energy.

Common mistakes and misconceptions

Sometimes people forget that the work-energy theorem only applies to the net work, not the work done by a single force. The work-energy theorem states that the net work done by the forces on an object equals the change in its kinetic energy.

Learn more

For deeper explanations of the work-energy theorem, see our video work and the work-energy principle.

To check your understanding and work toward mastering these concepts, check out calculating change in kinetic energy from a force and velocity and mass from force vs. position graphs.

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jabepun2018
Posted 3 years ago. Direct link to jabepun2018's post “If we're lifting an objec...”
If we're lifting an object up, wouldn't the net work be equal to the change in kinetic energy PLUS the change in potential energy? (total change in mechanical energy?)
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(11 votes)
Answer
- Athreya Ganesh
  Posted 3 years ago. Direct link to Athreya Ganesh's post “The work done by your for...”
  The work done by your force would be positive, and the total work done would be mgh, assuming the change in height to be h meters. So, according to the work-energy theorem, the final kinetic energy is mgh. Therefore, the square of final velocity should be 2gh. So, use the equations of motion to find out the final velocity. There is no need to find the initial velocity, assuming it started from rest.
  
  From the 3rd equation of motion, v^2 - u^2 = 2as, we find that v^2 = 2gh.
  
  Hence proved.
  Cheers!
  Comment on Athreya Ganesh's post “The work done by your for...”
  (6 votes)
arazaman
Posted 4 years ago. Direct link to arazaman's post “how to find of an objects...”
how to find of an objects mass from force vs position graph?
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(4 votes)
Answer
- 𝓜𝓪𝓱𝓮𝓼𝓱 𝓜𝓪𝓱𝓮𝓷𝓭𝓻𝓪𝓴𝓪𝓻
  Posted 4 years ago. Direct link to 𝓜𝓪𝓱𝓮𝓼𝓱 𝓜𝓪𝓱𝓮𝓷𝓭𝓻𝓪𝓴𝓪𝓻's post “First we need to find out...”
  First we need to find out the area under the force vs position graph which will give us the net work done and then we will set the
  net work = change in kinetic energy
  (Work-energy theorm)
  then we will be able to find the objects mass using the given data.
  Comment on 𝓜𝓪𝓱𝓮𝓼𝓱 𝓜𝓪𝓱𝓮𝓷𝓭𝓻𝓪𝓴𝓪𝓻's post “First we need to find out...”
  (6 votes)
Shenya Patrice
Posted 5 years ago. Direct link to Shenya Patrice's post “How do you find the initi...”
How do you find the initial kinetic energy?
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(3 votes)
Answer
- Abhimanyu Bhadauria
  Posted 5 years ago. Direct link to Abhimanyu Bhadauria's post “initial kinetic energy is...”
  initial kinetic energy is equal to 1/2*(mass of body)*(initial velocity of body)^2
  Comment on Abhimanyu Bhadauria's post “initial kinetic energy is...”
  (6 votes)
Michelle
Posted 3 years ago. Direct link to Michelle's post “since work is a vector, d...”
since work is a vector, do we have work both on the y-axis and x-axis? for instance, if the force is made to a degree from the horizontal axis?
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(1 vote)
Answer
- obiwan kenobi
  Posted 3 years ago. Direct link to obiwan kenobi's post “Work is not a vector. It ...”
  Work is not a vector. It is simply a quantity. If work is positive that means work is being done ON a system and if work is negative then that means that work is being done against the system. Hope this helps!
  Button navigates to signup page
  (2 votes)
Ashwin
Posted 6 years ago. Direct link to Ashwin's post “So the net force is all t...”
So the net force is all the work done on an object at the same instant?
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(1 vote)
Answer
- Ammu
  Posted 6 years ago. Direct link to Ammu's post “the net force is differen...”
  the net force is different from the work done. first, they are different units. the net force is the vector sum of all the forces acting on an object while the total work is done is the sum of all the work done on the object. keep this in mind and don't confuse. Also, this may refer to the same instant or a period of time.
  Button navigates to signup page
  (2 votes)
camacho.vivian759
Posted a year ago. Direct link to camacho.vivian759's post “How do we know when to us...”
How do we know when to use cosine 180?
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(1 vote)
Answer
- Gum Seng Nhkum
  Posted a year ago. Direct link to Gum Seng Nhkum's post “you can use the cos(x) wh...”
  you can use the cos(x) when a certain degree of an angle is given, or if they want you to find the degree of an angle using a given/explained equation using the kinetic energy equation
  Button navigates to signup page
  (2 votes)
ssroythegreat
Posted 5 years ago. Direct link to ssroythegreat's post “Is it for external forces...”
Is it for external forces or internal forces also be counted?
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(1 vote)
Answer
- NEKIBUR ZAMAN
  Posted 5 years ago. Direct link to NEKIBUR ZAMAN's post “if you are dealing with w...”
  if you are dealing with work energy theorem then internal energy may not be counted in some case.....[like if the internal energy doesnot do any work]
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  (1 vote)
Tom
Posted 3 years ago. Direct link to Tom's post “very perfect explaination”
very perfect explaination
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(1 vote)
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sofia.babovic
Posted a year ago. Direct link to sofia.babovic's post “How Can We Use the Work-E...”
How Can We Use the Work-Energy Theorem to Explain and Predict Behavior of a System That Consists of a Ball, a Ramp, and a Cup?
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(1 vote)
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pavel.buravtsov
Posted 2 years ago. Direct link to pavel.buravtsov's post “Why work equals change in...”
Why work equals change in kinetic energy though? Wouldn't it be right to say the change in energy since kinetic is equal to potential??
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(1 vote)
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