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Homework Help: Conceptual Newton's Law's Questions - HS Freshman Physics

  1. Dec 20, 2014 #1
    NOTE: The following questions are not part of any homework assigned. This is part of finding extra concept-related questions to help me study for my physics unit test.

    The problem statement, all variables and given/known data

    I have conceptual questions, not problems. I'll provide sufficient evidence that I've attempted to think about them.

    1. A boy seems to fall backward in an accelerating bus. What property does this illustrate?

    • I'm honestly swaying between the law of inertia and the third law.

    • I'm thinking law of inertia because the boy is resisting, then "non resisting" a change in motion...

    • But I'm thinking third law because the bus is moving forward and the boy is falling backward. (Sorry if this is an obvious question.)

    2. What is the difference between inertial and noninertial reference frames?

    • Just a question about vocabulary terms. I know that inertia is the tendency for objects to resist a change in motion. Would non-inertial mean that the object doesn't experience as much inertia, or no inertia at all?

    3. A fisherman stands in a boat that is moving forward toward a beach. What happens to him when the boat hits the beach?

    • Thinking that it may have to do with Newton's third law (i.e. when boat hits dock, fisherman falls back)?

    • Is this a trick question or more obvious than it seems :D ?
    4. A passenger sits in a stationary train. There are some objects on a table: an apple, a box of candy, and a can of soda. What happens to all these objects with respect to the passenger when the train accelerates forward?

    • Of course the objects fall backward, but why?

    • Is it the third law acting again? As the train accelerates forward, the passenger and his objects move back....

    • BUT: "what happens to all these objects with respect to the passenger"; is this a trick question, leading me to believe that the passenger is facing the back, instead of the front of the bus?

    5. A rock is thrown vertically upward and stops for an instant at its highest point. Is the rock in equilibrium at this point? Are there forces acting on it?

    • This one threw me off - the object isn't moving, but there is a force of gravity/air resistance?

    • This led me to the next question....

    6. Is it possible for an object to have a zero acceleration and zero velocity when only one force acts on it?

    • Not sure if it's completely relevant, but I remember a question that asked if an object could have a zero velocity and non-zero acceleration, and found that at an object's peak, the acceleration would be non-zero and the velocity would be 0.

    • Now on to this question, I'm thinking as hard as I can to think of an example in which there is zero acceleration/velocity with only ONE force acting on it.

    • All I can think of is if the force is equal to 0, but this obviously wouldn't be possible

    7. Two boys are pulling a spring scale in opposite directions. What is the reading of the spring scale if each boy applies a force of 50N?

    • Our class hasn't really covered spring forces/Hooke's laws other than in free-body diagrams, but I'm still curious about this.

    • 50+50=100N?
  2. jcsd
  3. Dec 20, 2014 #2

    Simon Bridge

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    Science Advisor
    Homework Helper

    1 - 4. I think you need to formally write out Newton's Laws, in English.

    1st Law: An object remains at rest (or in a state of uniform velocity) until acted on by an unbalanced force.

    i.e. objects in the train (Q4):
    If you imagine the table is frictionless, then the objects on the table attempt to maintain their state of rest ... the train accelerates, so it will leave the objects behind.

    The passenger travels with the train because they are constrained by the seat.

    Since the table is not frictionless, the objects fall over instead of just sliding backwards.
    That should help you with the others.

    5. you are correct so far, but you did not take that thought to it's logical conclusion: even though the rock is stationary at that point, it is acted on by an unbalanced force. Therefore: what can you say about whether the rock is in equilibrium or not?

    6. Take a look at Newton's second law.
    How are acceleration and force related to each other?
    If the acceleration were 0, what is the force?

    7. You don't need Hook's law for this one.
    Imagine both boys were pulling on a length of non-stretchy string - and you are asked to work out the tension in the string.
    Since nothing is moving, then, to each boy, the other boy may as well be a wall.
    So imagine one boy pulling with 50N on a non-stretchy string attached to some immovable object like a wall.
    What is the tension in the string?
  4. Dec 21, 2014 #3
    Thank you! I just have a few questions...

    For 1-4, I understand the laws well (wording and application), just not enough to put into these situations.

    • For number 1, I still can't see whether this is the 1st or 3rd law, because the boy is falling back - is this a reaction or is this a resistance to change in motion?

    • Number 2, I did a bit more research. Inertial is "following" the law of inertia (of course all objects do) by resisting a change in motion (i.e. constant velocity) while non-inertia would be an accelerating object? There were pretty vague descriptions, but that's what I've pieced together so far.

    • Number 3, still not sure. I saw that this example was used on another website to show the third law of motion, but I'm still not sure how the two relate. In other words, what is the action-reaction force pair here?

    For number 6, I'm assuming this scenario is not possible (if there is a force >0, then the acceleration must be greater than 0).

    And for number 7, would the force be 50N then?

    Thanks again!

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