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Questions about basic physics concepts

  1. Dec 8, 2012 #1
    Some silly questions I've just never been able to figure out, and it really bothers me when I don't know if something is a concept like speed, or just a way of describing something else and not in itself tangible. If anyone could take the time to clear these (or some of them) up for me, it would be greatly appreciated.

    - What exactly is torque? Is it actually some special sort of force, or is it just a normal force that just happens to rotate an object because the object is rotatable or because the direction of the force just happens to be such?

    - What does it mean that inertia is an object's "resistance" to change? How is the object resisting? Is inertia just a way of saying it takes a force to speed up/slow down an object? What is the difference between mass and inertia?

    - Momentum: If an object is resting on top of another object, and we push it so it falls off to the right, does the other object move left even when there is no friction between them? Why? Please explain momentum conservation.

    - When we say an object has potential energy, for example gravitational potential e., is this just the potential, or does the object literally gain energy? Does an object on the table literally have more "stored" energy than the same object on the ground? How? Same with kinetic energy.
  2. jcsd
  3. Dec 8, 2012 #2


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    Torque is the result of a force which is not directed at the center of your coordinate system (which can be arbitrary). Usually, this is relevant only if objects are allowed to rotate in some way.

    More mass => less acceleration at the same force. You can easily throw a pingpong ball, but it is hard to throw an elephant in the same way.

    Inertia is just an effect of mass, so you can consider them as the same.

    It does (unless the lower object is fixed in some way). No friction means no force parallel to the contact between the objects, but a force perpendicular to that is possible: You can push objects without friction, too.
    One object accelerates to the left, the other to the right, total momentum is conserved.

    Define "literally gain energy".
    It just depends on your definition.
  4. Dec 8, 2012 #3
    But if the force which pushes the top object off is parallel, how does it affect the bottom object? It doesn't "push" off of it.
  5. Dec 8, 2012 #4
    If you have a block on top of another block, all on top of a table, and every surface is frictionless, I don't see why horizontally pushing the top block should do anything to the lower one.
  6. Dec 8, 2012 #5


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    Ah well, I thought the upper object is tipping over the edge of the other. If they are just sliding on each other, the lower object does not move if you push the upper object. Momentum is conserved as you are pushed back from the upper object.
  7. Dec 8, 2012 #6
    Oh, you make a good point. I sort of approached his question naively, but you're right that the top block's center of mass will eventually move past the edge of the lower block, resulting in tipping which then does introduce some lateral forces.
  8. Dec 8, 2012 #7
    To clarify: total momentun of a system, such as the two mentioned "objects", is not conserved if there are external forces acting on the system.
  9. Dec 8, 2012 #8
    Okay, so lets say there's a rocket travelling upwards, and suddenly a door at the bottom of the rocket opens, and mass starts falling out (not being thrown or pushed out), without touching the rocket (it was all stacked on top of the bottom doors). The rocket doesn't have to gain momentum in this case because the external force is gravity (causing the mass to fall out)?
  10. Dec 8, 2012 #9
    If the rocket is accelerating upwards due to propulsion, the system rocket + mass is gaining momentum at a rate proportional to the propelling force (F = dp/dt, Newtons second law). If the mass is let go, but the propelling force is the same, the rocket will be gaining more momentum per time than it did before - it will be gaining the same amount of momentum as the system rocket + mass did before. If, howerver, the rocket is moving without propulsion and there is no other force acting on it than gravity, the mass will stay inside it, since gravity is exerting the same force on the mass as it is on the rocket.
  11. Dec 8, 2012 #10
    I don't think Newton's laws say that.

    Further thought for both potatocar and zarquon.

    Rocket propulsion works by ejecting mass in the opposite direction from the motion. There is no need for opening doors for stuff to fall out.

    potatocar, note the progress of this thread. I think you have too many questions in one post. I recommend that fewer in future will achieve better responses.

    Noone has yet addressed your potential energy question.

    A body in a potential field requires the input of work to be moved form some point A to some other B point at higher potential.
    This body can supply work if it moves from B to A.

    This potential field may be gravitational, electric or due to some other effect such as elasticity.

    So yes your object (brick) has more enrgy on the table than on the ground. It took work to lift the brick from the ground to the table and the brick can do work if it falls back - it can squash your toe.
  12. Dec 9, 2012 #11
    What i meant to say, was that nothing will fall out of the rocket if it's in free fall.
    I was regarding the propelling force as an external force, neglecting the associated mass loss, to make things simpler. But i would agree that a rocket is a bad example for explaining conservation of momentum.
  13. Dec 9, 2012 #12
    But that is not how rockets work, except in some science fiction.

    The system originally contains the rocket shell plus fuel.
    This has a compact centre of mass.

    You have to consider what happens to this centre of mass as the rocket and propellant gasses separate. They both still contribute to the analysis.
  14. Dec 9, 2012 #13
    Not sure if we're talking about the same "analysis" here. I was just trying to explain what happens to the object when released from the rocket.
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