Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

For the N's second law

  1. Jan 31, 2010 #1
    So if F=Ma

    then if there was no accleration but it was going at constant velocity

    there would still be force right? but what? and how
     
  2. jcsd
  3. Jan 31, 2010 #2
    If there is a constant velocity, yes, there is no acceleration, and yes, no Force.
    Things resist changes in motion, whether that is starting or stopping.
    Something moving at a constant velocity will go on forever without another force stopping it. In the real world that doesn't happen because of things like friction.
     
  4. Feb 1, 2010 #3
    a force is only present when an acceleration occurs. If an object is moving at uniform velocity, then there are no forces exerting themselves on that object, you can see from the equation F = Ma. A force might've exerted itself before to get an object to a certain velocity, but if it is moving at a uniform velocity, that force is no longer exerting itself on that object
     
  5. Feb 1, 2010 #4

    Claude Bile

    User Avatar
    Science Advisor

    Acceleration is zero only when there is no net force acting on an object.

    Claude.
     
  6. Feb 2, 2010 #5

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Excellent point. As I sit here at the computer, I do not accelerate and yet the Earth exerts a gravitational force on me.
     
  7. Feb 2, 2010 #6

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    I should hope that you DO accelerate (radially) or you will be following a straight line course out into space.
     
  8. Feb 2, 2010 #7

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Ha ha, true. But my acceleration is considerably less than g.
     
  9. Feb 2, 2010 #8

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Don'tcha just hate a smartypants?
     
  10. Feb 2, 2010 #9
    The force is directly proposnal to change in linear momentum so when the momentum is 0 the force is zero.
     
  11. Feb 3, 2010 #10

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    The Earth is exerting TWO forces on you. Gravity produces your Weight force and the Ground exerts an equal and opposite Reaction force. Net force is Zero so no acceleration. Remove the ground and you accelerate down the lift shaft at g acceleration.
     
  12. Feb 3, 2010 #11

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    If the OP is now learning about Newton's Laws, chances are they have not learned about momentum yet.

    Yes, exactly. The Earth is exerting forces on me, but it is zero net force. Thus zero acceleration.
     
  13. Feb 3, 2010 #12

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    I know you know that I know you know - it needed to be said to avoid confusion.
    And doesn't it just?
     
  14. Feb 3, 2010 #13
    Sorry, I'm feeling dense...

    So, F=Ma. Just because F1-F2 = 0 (net force, zero acceleration) how do you have any "F"s to add or subtract, because you don't have any acceleration?

    My couch must have force to balance out the force of gravity, but what is the acceleration of my couch? And if it's 0, then wouldn't the force be 0 also??
     
  15. Feb 3, 2010 #14

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Mais oui, mon ami.
    But, without a force F keeping you from falling, there would be an unbalanced weight force W pulling you down. F - W = 0 (Net)

    I suppose it depends your frame of reference. If you're in your chair, the only force you can really feel is the chair pushing up on your bum. Take your chair away and the only clue you would have about any force acting on you would be as you observed the sides of the lift shaft going upwards faster and faster.
    Our lives are lived with pairs of balanced forces in almost every experience we have.
     
  16. Feb 3, 2010 #15
    I'm still confused. If I try to lift a 1000kg weight, it's not going anywhere. I've done no work, and from my limited understanding I have not created/applied any acceleration, but I am still applying force to it, no?
     
  17. Feb 3, 2010 #16

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    When you are suspending the weight, (it's not accelerating) your force, just like the force which the ground was exerting on it, is balancing its weight force. Balanced forces = no acceleration. Irrespective of what you happen to be doing to the weight, if it is not accelerating, the sum of the forces on it must be zero.
    You could also say that you are accelerating it at -g, I suppose, and so the net / resultant acceleration would be g - g = 0 but we don't usually state it in that way.
     
  18. Feb 3, 2010 #17
    Okay, g-g=0 makes sense to me. BUT, if it's sitting on the ground, it's already at g-g, so my tugging at it does what? The net of the ground and my efforts are still g-g, so if I'm taking away some of a from the ground, then the ground is providing variable acceleration?

    Or let's take me out of it. Weight is sitting on the ground. Gravity is pulling down on it. It's not moving, so we have equal forces (gravity and the ground.) If somehow gravity increases to 1.5 g's, the ground will still be supporting the load and we still have equal forces. How does the force of the ground change? Shouldn't that be a constant?
     
  19. Feb 3, 2010 #18

    Doc Al

    User Avatar

    Staff: Mentor

    Don't think of each individual force on the object as providing its own acceleration. It's the net force that produces the acceleration. As long as the net force remains zero, there will be no acceleration. The more you lift, the less force the ground has to exert to support the object.

    No, the upward force that the ground exerts is not a constant. Think of it as a 'reactive' force--it's whatever it needs to be to prevent the object from crushing through it (up to the limit of its strength). Say you sat on the object. Now the ground has to push up on the object with a force equal to the combined weight of you and the object. Similarly, if you pull up on the object the ground force on it becomes less.
     
  20. Feb 3, 2010 #19
    But then we're back to the original issue: if F=ma and there's no a, then how do you have a non-zero F?

    Boy, my head is starting to hurt from what should be a simple concept. Sorry if I'm being dense.
     
    Last edited by a moderator: Feb 3, 2010
  21. Feb 3, 2010 #20

    Doc Al

    User Avatar

    Staff: Mentor

    The F in F=ma refers to the net force on the object. If the acceleration is zero, then the net force is zero.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook