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Homework Help: Newtons law of motion questions

  1. Jul 29, 2008 #1
    1. The problem statement, all variables and given/known data
    Tell me what is wrong with this question-

    A donkey which has heard of Newtons third law but not understood it very well refuses to pull a cart attached to it because it argues that: ' If i pull on the cart with a certain force, Newton's third law would imply an equal but opposite force, which would then mean that the total force on the cart is zero, and so it will not move. so why waste my effort?i wont pull'. What's wrong with the donkey's argument?

    3. The attempt at a solution
    Is it because there is no reaction to the cart. for example the space shuttle example is a gud one. the downward thrust produces an equal upward trust. there is only the reaction with the ground the cart has which is friction so the donkey will be able to pull on it since the friction force is very small and there is no reaction with the cart.
    Last edited: Jul 29, 2008
  2. jcsd
  3. Jul 29, 2008 #2
    Try to do a free-body diagram of the donkey when it is moving - think about where forces are being applied.
  4. Jul 30, 2008 #3
    this is a very common question which comes in different forms... since it is against the forum rules, i wouldn't give you a direct answer... but think seriously about who is exerting a force and where it is being felt... and then go ahead to draw individual free body diagrams for the donkey and the cart (as badphysicist tells you to do) and figure out why the donkey moves despite the action-reaction pair...
  5. Jul 30, 2008 #4


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    There is an important conditon that must be met (which we are asking you to recognize).

    If the cart were all that the donkey were applying force against, it would be absolutely correct. (See, it's not that dumb...).

    But further, if the frictional force between the donkey's hooves and the ground matched the frictional force between the cart's wheels and the ground, it would again get nowhere and be correct. (Think of a comparison between your towing something across dry sand or wet ice and doing the same across dry, firm ground or pavement.)

    So what must be true if pulling a cart is to happen at all? As badphysicist says, you want to think about the forces on the donkey, but also look at the forces on the cart.
  6. Jul 30, 2008 #5
    well the forces i know of r the frictional force between the ground and the donkey/cart. there is also the gravitational pull exerted in the cart.

    the donkey is exerting a force on the cart as it is pulling it.

    plz correct me where ever it is incorrect
  7. Aug 1, 2008 #6
    hi! i think you are making it a bit too complicated... to begin with, gravity doesn't play much of a role out here... it is obviously there, but it doesn't help the cart to move, provided the road is perfectly horizontal... so don't take into account forces which are simply going to clutter things up... simply think about the force the donkey exerts upon the cart... who is applying this force? where is the force being felt? now, as Newton's third law suggests, every force will have a reaction force... now, think about who/what is applying this reaction force and where is it being felt...
  8. Aug 2, 2008 #7


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    Yes, the donkey-cart system has net zero force. The donkey is also exerting a force on the ground. Donkey+cart-ground is also zero. Who suffers? What's holding the earth in place?
    Last edited: Aug 2, 2008
  9. Aug 2, 2008 #8
    thx for making things a bit clearer gc2004.

    well come to think about it, the donkey exerts a force on the cart since it is pulling it and thus the cart would exert a force on the wheels which would then in turn move forward. The force is being felt by the wheels and since the donkey force is more than the cart force, the cart moves forward


    god i suck at this:p
  10. Aug 2, 2008 #9
    Oh yea and this is for Dick's question

    the earth is held together by gravity since all matter attracts all other matter through a force called gravity.
  11. Aug 3, 2008 #10


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    Yes, but that isn't quite what I meant. Picture just the donkey and cart with a stretched spring between them. Nothing else around. The forces are balanced. When you let them go they accelerate towards each other. Having balanced forces doesn't mean nothing can move, just that the center of mass of the whole system can't accelerate. In this case the system has three main parts, donkey, cart and earth. Action may equal reaction but the donkey/cart can still accelerate forward as long as the earth accelerates backwards.
  12. Aug 4, 2008 #11
    I quite dont get it what u mean by the earth moving backwards?

    wat does that hav to do with the donkey?
  13. Aug 4, 2008 #12


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    Action/reaction is also know as conservation of momentum. If donkey/cart acquire forward momentum something else (the earth) gains momentum in the opposite direction. But rereading the question, I think they may be after a simpler answer. When the donkey exerts a force on the cart, the reaction force acts on the donkey, not the cart. So they don't 'add to zero'. There is only one force on the cart.
  14. Aug 4, 2008 #13
    thx for clearing things up for me but how does the reaction force act in the donkey? wait force is either pull or push rite? so wen the donkey pulls the cart it is exerting a force? tat is the action rite? and so the reaction force (pull) moves the cart forward. ut doesnt this cancel out since the action/ reaction is equal.
    how come they dont add to zero?

    sorry but i hav to clear my doubts or i will never get this :)
  15. Aug 4, 2008 #14


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    Think of four forces, A the force the donkey exerts on the cart, B the force the cart exerts on the donkey, C the force the donkey exerts on the ground and D the force the ground exerts on the donkey. A and B are opposite and equal, as are C and D. Only force A acts on the cart. Forces B and D act on the donkey. So as long as D>B the cart/donkey can accelerate forward. A and B don't cancel because they act on different objects.
  16. Aug 4, 2008 #15
    What happens if the forces are balanced?
  17. Aug 4, 2008 #16
    thx dick. u have been of great help to me. thx for clearing this mess.
  18. Aug 20, 2008 #17
    Delete thread please
  19. Aug 21, 2008 #18
    delete thread plz
  20. Aug 21, 2008 #19


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    You'll need to contact a moderator if you really want to get rid of this. It's not the usual thing to delete a thread when they are finished. They contents could help someone else.
  21. Jul 31, 2010 #20
    the force applied by the donkey is a muscular force using his feet to push himself forward while the reaction force is not. the cart is not alone bieng pulled but along with it the wheel too, which reduces the friction force against the cart and it moves easily.
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