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Ok so my teacher has recently covered this chapter, but one of the

  1. Jul 16, 2012 #1
    Ok so my teacher has recently covered this chapter, but one of the concepts that has been bugging me for ages. Fr an object to experience zero acceleration, the forces acting on it have to be balanced. That means tha applied force has to equal the frictional force so that the resultant force will be zero. But why, if the forces are balanced, won't they cancel out each other, and leave no force to actually cause the object to more at constant speed?yes I know that according to newtons second law, no net force equal zero acceleration, but where Does the object get its energy to do work? Ie remain at constant velo.i just can't seem to understand how, and my teacher doesnt want to answer my stupid question, so all help is greatly appreciated! Oh by the way, when there is a resultant force acting on a object, will the applied force still be acting on the same object? No right ?
  2. jcsd
  3. Jul 16, 2012 #2
    Think of an object in empty space, first. If I give it a push, it will continue to move in the direction I applied the force until someone or something else stops it. Until then, it will remain at that constant velocity. In order to stop the object, you will need make it's acceleration negative (slowing it down), which via Newton's second law, requires a net force that is pushing against it's motion. If the applied force is equal to the frictional force, then no acceleration is occurring, and the object will be moving with a constant velocity.

    EDIT: Oh, and to add. It gets it's ability to do work from the fact that you needed to accelerate it to get it to that constant velocity. This requires you to put a force on it, which will then be transferred by that object to whatever it does work on.
  4. Jul 16, 2012 #3
    Re: Dynamics

    When the object moves with constant speed there is no net work done by or on the object. Its kinetic energy is constant.

    In order to have some work involved the object must be either slowed down or accelerated.
    In both these cases the net force is not zero. To slow down, the applied force must be decreased, to accelerate it must be increased.

    I don't know what you mean in the the last two sentences.
  5. Jul 16, 2012 #4


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    Re: Dynamics

    If that is true, then why does my car engine have to keep burning fuel when I'm driving at constant speed on a level road?

    Answering the OP's question, you seem to be thinking that the only sort of energy involved here is mechanical energy. If you are pushing an object at constant speed against a friction force, the applied force (equal and opposite to the friction force) is doing work (= force x distance), and that work is mostly converted into heat. (Of course you can think of heat energy as the amount of kinetic energy stored in the vibratiion the atoms making up the material, but that is usually "too much information" for solving dynamics problems!)
  6. Jul 16, 2012 #5
    Re: Dynamics

    The others have already explained it rather well, but I'll add to that.

    - I hope that it's clear now that no work is needed to remain at constant speed if there is no friction. Think of falling on ice, how far you can glide thanks to little friction.

    - The subtle thing with friction is that work must be done to overcome the friction. And that work results in heat (think of the brakes of your car).

    So, to sum it all up (literally), and perhaps answering your last question:

    The force Fapp that you apply on the object remains on the object, and it is countered by the friction force and the force* due to inertia that counters acceleration Facc = -ma. The friction force Ffriction increases with increasing speed (for simplicity, let's neglect the force that is needed to get the object "unstick" from the floor). Thus the object accelerates until the friction force is equal (but opposite) to your applied force on the object. Schematically, from the moment of getting in motion on a smooth floor until reaching top speed:

    Fapp + Facc + Ffriction = 0

    1. Fapp ≈ -Facc = ma (if Ffriction is small at low speed)
    2. Fapp + Facc + Ffriction = 0 (the acceleration decreases because the friction force increases)
    3. Fapp + Ffriction = 0 (acceleration zero, your applied force is balanced by friction force)

    All forces are in dynamic equilibrium all the time (third law of Newton: "Whatever draws or presses another is as much drawn or pressed by that other").

    Does that help?

    *Note: confusingly, the term "inertial force" has been hijacked for something completely different; here we only discuss real forces.
    Last edited: Jul 16, 2012
  7. Jul 16, 2012 #6
    Friction. The friction between the road and your car resists it's motion. So, to overcome it, you must burn fuel to apply a force on your car to move it forward.
  8. Jul 17, 2012 #7
    I think u understand what I mean , but the object will not do work on anything else in it's path right ? It itself is doing work ?
  9. Jul 17, 2012 #8
    What do you mean by this ?
  10. Jul 17, 2012 #9


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    Re: Dynamics

    Oh well. If you ask a rhetorical question on a forum, there's always somebody ready to answer it :smile:
  11. Jul 17, 2012 #10
    Re: Dynamics

    I'm not sure what you mean with "resultant force", nor what you mean with "work together"...

    I did explain to you how the applied force minus the friction force results in acceleration of the object, and that this acceleration is controlled by the object's inertia. Perhaps it's useful to phrase it again differently: The object's inertia delivers the missing counter force by means of its acceleration.

    Thus in my earlier answer I tried to clarify the dynamic force balance that I thought you were after to understand by giving you an example of what happens, and why, as function of the time. Thus you can understand how dynamic force equilibrium results in a top speed - which is the point where your discussion with your teacher started off.

    If you can understand why and how that point was reached, then all following questions disappear. For example, "Fr an object to experience zero acceleration, the forces acting on it have to be balanced" is inaccurate: the sum of all forces is always balanced according to Newton's 3d law, as I explained. For the object of your example to experience zero acceleration, the friction force has to be equal and contrary to the applied force.
    What exactly is still not clear?
    That footnote was for some people who could misinterpret my referral to inertial effects as referral to a fictitious force, despite my careful phrasing. If you are not into such things, then you can safely ignore it. :smile:
    Last edited: Jul 17, 2012
  12. Jul 17, 2012 #11
    What do you mean ?

    Oh and mark m, the applied force on the car already balances the frictional force, since ye car moves at constant speed. So how can there be extra frictional force acting on the car ?
  13. Jul 17, 2012 #12
    In this case do you refer to inertial force as effects of mass ?
    I understand what you are trying to say , you are actually clear in yor explanations. It's just that I have a weird mindset I guess, and I havent found the actual answer I am looking for , but if I was learning this chapter for the first time and having difficulty grasping concepts, your post would be very helpful. Except , like I said , I don't like to accept things easily especially when I can't seem to visualise them. So yup. Thanks a lot though !!
  14. Jul 17, 2012 #13
    They don't work very well on the Internet. :smile

    What exactly do you mean?

    Let me give another explanation - constant velocity doesn't require any force, nor does it do any work. So, if the applied force and frictional force cancel out, you'll get zero acceleration. The velocity of the object is irrelevant, long as it's constant. Remember that Newton's second law is F = ma. If the friction and applied force are balanced then there isn't any acceleration. Notice, however, that it doesn't discriminate between different constant velocities (called inertial frames of reference). You could be moving over one million meters per hour, but as long as the friction is equal to the applied force, you won't change speed.
  15. Jul 17, 2012 #14
    Re: Dynamics

    Yes, certainly (except that I did not refer to "inertial force" but to "the force due to inertia", for the reason that I explained): http://dictionary.reference.com/browse/inertial?s=t
    You're welcome! :smile:
  16. Jul 18, 2012 #15
    Is this your explanation to why an object moving at constant velocity doesn't require a force ?
  17. Jul 18, 2012 #16
    I was referring to the fact that if this object, say, slammed into another, it would exert a force on it. I thought your original question regarded how the object got the energy to do this, which is what I explained.
  18. Jul 18, 2012 #17
    Re: Dynamics

    When a 200N force is applied to an object on a frictionless surface, object remains at constant velo. But when a force is applied to an object on a surface with friction, and the net force is 200N the object accelerates. So this means that the applied force is still acting on the object on the surface with friction right ?
  19. Jul 18, 2012 #18
    Well, let's break it down case by case.

    If we exert a force on an object on a frictionless surface, it will accelerate to a certain speed (given by F = ma), and then proceed at a constant velocity. And because of Newton's first law (an object in motion stays in motion until acted on by another force), it will continue at this constant velocity.

    Rather than applying the force once, let's say we continue to apply to force. Say, by strapping a rocket to the object. Now, the acceleration given by F = ma will stay constant (consent force means constant velocity), and the object will get faster and faster. As in the previous example, if we then stop applying the force (I.e., Turn off the rocket), the object will continue at whatever velocity it achieved and stop accelerating.

    Now, let's try a surface with friction. Friction is a constant force that pushes against your motion. Remember, constant force means constant acceleration. In the case of friction, this means constant deceleration. So, if we give it a one time force, such as a push (as in our first example), it will accelerate to some constant speed. But, in the first, example, the object continued at this speed because there was no outside force. But now there is, friction. Remember that I said that friction leads to a constant deceleration (until you hit 0 velocity, then friction stops, obviously). So, the object will decelerate down to 0 velocity.

    Next, let's say we apply a constant force again, but on a surface with friction. Firstly, the object accelerates to a certain speed (F = ma), but then the friction resists. In the last example, the force from friction decelerated the object. So, the force from friction is negative. If we respond with an equal but opposite constant force with our rocket, the two will cancel to zero. So, since F = ma, the object has no acceleration. So, whatever original constant velocity we got it to, it will stay at. Compare this to the case of constant force without friction, where, rather than having a constant velocity, the object had a constant acceleration.

    How's that?
  20. Jul 20, 2012 #19
    Re: Dynamics

    Yup I get you , but the constant force u mentioned in the last para is the applied force right ? But isnt the applied force only a one time force ?
  21. Jul 20, 2012 #20
    Re: Dynamics

    You can apply a force as long as you wish.
  22. Jul 24, 2012 #21
    Re: Dynamics

    Ok so normally when we apply force to an object , it is a constant force ? So this means that if there is friction present , and friction is a constant force, then the resultant force will be the only force acting on the car, the applied force will not be acting on the car anymore , right ?
  23. Jul 24, 2012 #22
    Re: Dynamics

    Applying force on the car means that that force acts on the car. You cannot say that aplied force doesnt act on a body, unless it is zero, in which case you can say no force was applied :)
  24. Jul 24, 2012 #23
    Re: Dynamics

    Maybe this helps some:

    How about some examples with a block of wood on a table in which the surfaces are rough between each other (ie friction exists)? We will only be interested in forces in the horizontal.

    Case I: We turn around and look at a block of wood moving across a table. The block of wood is slowing down while moving to the right some distance. We dont know how the block of wood got started, we dont know what or who pushed it if anything.

    Friction is the only horizontal force as well as the net force acting to the left causing the block to slow down. The net force causes any mass to accelerate in the same direction as that net force. Even though the block is moving to the right, it is accelerating (some say decelerating) to the left. Net work is also done to the left but it is considered negative work as the kinetic energy decreases

    Case II: While we apply a constant force to the right on the block by pushing with our finger, the block moves at a constant velocity to the right some distance. We are uninterested on how the block got started in the first place and as we push with a constant force on the block it does not change velocity.

    It must be that there is also a constant force (friction) to the left that is equal and opposite to the push with our finger. The block is not accelerating. There is no net force. + work is done to the right by our finger on the block. Negative work is done by friction to the left. There is no net work done. The block undergoes no change in kinetic energy, but it has kinetic energy.

    Case III: A block lies motionless on a table even though we apply a force with our finger to the right. We push harder and the block remains at rest. It moves no distance.

    There must be a force to the left that equals the force to the right. Even as we increase the force to the right, the force to the left must increase as the force to the right increases. There is no acceleration, no net force, zero work done. No kinetic energy to begin with or later.

    Case IV: A block lies motionless and we push it to the right with our finger the block starts to move to the right and continues to move faster as we keep our finger and a constant force on the block. (We ignore the diff between static and kinetic friction.)

    There is a force to the right larger than the force to the left (friction). Your finger does + work on the block, friction does negative work on the block. The block accelerates to the right. The force to the right must be larger than the force to the left. There is a net force to the right and net work. The block of wood gains kinetic energy.

    Case V: Take case IV but then stop pushing the block with your finger. You are now back to case I with the exception that we know what happened before to get the block moving.

    Sorry for the long winded possibly simplistic explanation. If it does not help the OP I hope it helps someone else perusing the thread.
  25. Jul 31, 2012 #24
    Re: Dynamics

    If we think in terms of energy, when a constant force is applied to the block, it has constant amount of kinetic energy, which is constantly tackled by the same amount of uniform friction , hence won't all the kinetic energy be converted into heat and sound energy ? But then the object is supposed to move at constant speed With the same amOunt of kinetic energy ?
  26. Jul 31, 2012 #25
    Re: Dynamics

    The block only has constant kinetic energy when its speed is constant.
    Yes. See again my post #5: this is the third phase, when the force of friction exactly counters the applied force.
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