What is the mistake in assuming that a finite force can start an object at rest?

  • Thread starter valjok
  • Start date
  • Tags
    Movement
In summary, The power is the amount of energy being transferred, the energy change P = Fv at specific moment of time. The kinetic energy is proportional to speed: E = mv2/2. I wonder how a finite force starts moving an object at rest. The zero velocity implies zero power P=0, which implies zero kinetic energy inflow dE = 0 * dt = 0. Ultimately, as the energy does not change, the speed persists 0. In other words, the speed does not change because it is zero and it remains zero because it does not change. We cannot start off. Where is the mistake?In summary, power is the rate of energy transfer and is equal to the product of force and
  • #1
valjok
71
0
The power is the amount of energy being transferred, the energy change P = Fv at specific moment of time. The kinetic energy is proportional to speed: E = mv2/2. I wonder how a finite force starts moving an object at rest. The zero velocity implies zero power P=0, which implies zero kinetic energy inflow dE = 0 * dt = 0. Ultimately, as the energy does not change, the speed persists 0. In other words, the speed does not change because it is zero and it remains zero because it does not change. We cannot start off. Where is the mistake?
 
Physics news on Phys.org
  • #2
Nowhere in any of that was a mention of acceleration and velocity is a dependent variable. Those are output calculations, not inputs. What makes an object accelerate is f=ma (a=f/m).
 
  • #3
To expand upon what Russ wrote:

At the moment the object is at rest,
Power P=0
Kinetic Energy E=0
But dE is not zero.
dE = d(mv^2)/2 = mdv
Just because v is zero does not mean that dv is zero. By definition dv/dt is acceleration. If you are about to start pushing on the object then acceleration = F/m. So after a finite period of pushing, v is not zero either and E and P now have finite magnitudes too.
 
  • #4
Russ, introducing a new term should not break the solid mechanics theory, which, as I have explained above, does not need you acceleration entity to prove that no acceleration exists :) Use Accam razor. Do you claim that Newton mechanics in inconsistent? :)
 
Last edited:
  • #5
MikeLizzi said:
dE = d(mv^2)/2 = mdv

I may completely mistake but dE/dv = m/2 d(v^2)/dv = mv rather than m, so dE = mv dv. That might me a clue to the answer. I mean that dv = dE / mv = 0/0 indefinite. In other words, the speed may change despite dE is zero. Right?


MikeLizzi said:
To expand upon what Russ wrote:

At the moment the object is at rest,
Power P=0
Kinetic Energy E=0
But dE is not zero.

dE is equal to power: dE = P dt = Fv dt. It cannot be that the power is 0 while energy is changing. The power is the 'speed' of energy, energy is not moving if its speed is 0. From the latter formula it is clearly seen that dE is zero when v = 0.
 
Last edited:
  • #6
valjok said:
I may completele mistake but dE/dv = md(v^2)/2/dv = mv rather than m, so dE = mv dv. That might me a clue to answer.


dE is equal to power: dE = P = Fvdt. It cannot be that the power is 0 while energy is changing. If power is 'speed' of energy, energy is not moving if its speed is 0. From the latter formula it is clearly seen that dE is zero when v = 0.

dE is not zero when v = zero. that's my whole point. This last equation for power is wrong. Check your units. Power can't be equal to Fv and Fvdt.
 
Last edited:
  • #7
MikeLizzi said:
dE is not zero when v = zero. that's my whole point. This last equation for power is wrong. Check your units. Power can't be equal to Fv and Fvdt.

Please check the formula now. Tell me if it is correct and how can dE be different from 0 if v = 0? Don't try to dodge anyone by suggesting that dE is not proportional to v. ;)
 
Last edited:
  • #8
valjok said:
Please check the formula now. Tell me if it is correct and how can dE be different from 0 if v = 0? Don't try to dodge anyone by suggesting that dE is not proportional to v. ;)

You also made a mistake in declaring dE = mv dv. Again, check your units. dE = mdv. dE is NOT proportional to v. dE is proportional to dv.

Really smart people think this stuff is easy. Myself, I had a hard time learning it. Looks like you are having a hard time too. Let me assure you there is no contradiction. I have used this stuff to design machinery for many years. Sometime I screw up, but the physics always works.
 
  • #9
valjok said:
Russ, introducing a new term should not break the solid mechanics theory, which, as I have explained above, does not need you acceleration entity to prove that no acceleration exists :) Use Accam razor. Do you claim that Newton mechanics in inconsistent? :)
Huh? Solid mechanics theory? Occam's razor? Newton mechanics inconsistent? That entire post is meaningless gibberish!
 
  • #10
MikeLizzi said:
You also made a mistake in declaring dE = mv dv. Again, check your units. dE = mdv. dE is NOT proportional to v. dE is proportional to dv.

Really smart people think this stuff is easy. Myself, I had a hard time learning it. Looks like you are having a hard time too. Let me assure you there is no contradiction. I have used this stuff to design machinery for many years. Sometime I screw up, but the physics always works.

I believe that both x and dx measure in the same units. Hence, both E and dE are measured in [joules] or [kg meters^2] rather than your [kg meters], which is a unit for momentum. In my galaxy, mdv = dp rather than dE. May be it is good that I do not design machinery.
 
Last edited:
  • #11
I give up, voljok. Maybe someone else can help you. I can't.
 
  • #12
Probably not, but one more try before we lock the thread:

Mike is correct: the units of x and dx are not the same. Taking the derivative (the "d") turns units of meters (for example) to meters per second. The derivative of distance is velocity. Further, you said above that the derivative of energy is power - but now you say the units are the same. So do you think power and energy are the same thing?

Valjok, your entire line of reasoning needs to be tossed in the trash. Your idea doesn't hold water.
 
  • #13
russ_watters said:
Probably not, but one more try before we lock the thread:

Mike is correct: the units of x and dx are not the same. Taking the derivative (the "d") turns units of meters (for example) to meters per second. The derivative of distance is velocity. Further, you said above that the derivative of energy is power - but now you say the units are the same. So do you think power and energy are the same thing?

Valjok, your entire line of reasoning needs to be tossed in the trash. Your idea doesn't hold water.

What? "d" does not stand for 'deriviative'! it stands for http://en.wikipedia.org/wiki/Differential_(infinitesimal)" ! I can explain to the children that deriviative is a ratio of differentials: dx/dt. This is how we get [meters/sec]. Neither dx alone nor dt are measured in meters per second. Where do I say that power and energy measure in the same units?

If you are a moderator on a phys-math forum, it is the forum which must be tossed into the trash along with the whole education system, which produces such awesomely illiterate "experts".
 
Last edited by a moderator:
  • #14
It's been a while since I took calc and I forgot the definition, so I'll give you "derivative" vs "differential", but in this case, that it was dx/dt was assumed. However, finding a nitpicky mistake of mine does not change anything about your entire point being wrong. Thread locked.
 

What is the purpose of starting off the movement?

The purpose of starting off the movement is to initiate and inspire change within a particular field or cause. It is often done to address a specific problem or issue and to motivate others to take action.

How can I start off a movement?

Starting off a movement requires identifying a cause or issue that you are passionate about and creating a plan of action. This may involve researching and gathering information, reaching out to like-minded individuals, and utilizing social media and other platforms to raise awareness and gain support.

What are some effective strategies for starting off a movement?

Some effective strategies for starting off a movement include clearly defining the purpose and goals of the movement, creating a strong message that resonates with others, utilizing social media and other forms of communication to spread the message, and organizing events and activities to engage and involve others.

How can I make sure my movement is sustainable?

To ensure that your movement is sustainable, it is important to have a solid plan in place and to continuously assess and adapt your strategies as needed. It is also crucial to involve and empower others to take on leadership roles within the movement and to maintain open communication with all members.

What are some potential challenges in starting off a movement?

Some potential challenges in starting off a movement may include gaining support and visibility, facing opposition or backlash, and maintaining momentum and sustainability. It is important to anticipate and address these challenges, and to stay resilient and determined in your efforts.

Similar threads

I Mechanical power generated by a force F
    • Classical Physics
2
Replies
41
Views
2K
I Pushing a rod in a magnetic field
    • Classical Physics
Replies
15
Views
498
I I need some support, please, for a gravity assist analysis
    • Classical Physics
3
Replies
86
Views
4K
I System, potential energy, and nonconservative forces
    • Classical Physics
Replies
6
Views
804
I Understanding the concept of energy
    • Classical Physics
Replies
7
Views
1K
I Work - Energy Principle Application to Fluid Flow
    • Classical Physics
2
Replies
48
Views
2K
I Conservative forces and internal and external forces
    • Classical Physics
Replies
9
Views
1K
I The far reaching ramifications of the work-energy theorem
    • Classical Physics
Replies
31
Views
2K
I De Broglie Wavelength at rest: λ = h/p = h/0 when v=0?
    • Other Physics Topics
Replies
5
Views
832
B How does liquid starts moving when there is Pascal's law
    • Classical Physics
Replies
15
Views
2K

Hot Threads

Recent Insights

Back
Top