1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A Plethora of Questions Relating to Basic Newtonian Physics

  1. Mar 9, 2015 #1
    Hi everyone.

    Thanks for attempting the audacious challenge of deciphering the undistilled chaos that is this thread. I'd appreciate any efforts to clarify my very much flawed understanding of basic physics.

    1. This is most likely a trivial question but why is the normal force perpendicular to the plane of contact? Is it a matter of definition as the perpendicular component of the contact force between two interacting bodies in contact, which also includes friction as its parallel component?

    If so, suppose that we take two idealized smooth surfaces devoid of any microscopic irregularities, why is the contact force then perpendicular? Is it because the normal force arises as the vector sum of all the separate repulsive forces between each pair of electrons across the two surfaces, which may occur in many directions but cancel each other into just a net force pointing perpendicularly?

    2. Relating to the first question, what determines the magnitude of the normal force? It is easy to calculate its magnitude in order to account for the observed motion of the affected body. But is it incorrect to take the approach of considering forces as the independent entities responsible for the motion rather than constructs which are devised to explain the observed motion? In other words, is there any theoretical way of predicting the magnitude of the normal force in a given situation without knowing the motion involved? Does it relate solely to the mechanical properties of the materials which constitute the two bodies?

    I think this is especially pertinent when it considering normal force between two colliding bodies, which, if I understand correctly, varies with time. So while I could calculate the average force of interaction from the impulse-momentum theorem, how could I deduce the normal force at any instant of time?

    3. On a related note, how do I calculate the tension of a pendulum string at any point in the bob's motion? Is there any way of deducing this tension value without considering the motion of the bob first? And does the formula for centripetal force apply for the normal component of an object in non-uniform circular motion such as a pendulum bob?

    4. Another confused question here. Suppose that we take a single frictionless and massless pulley with a massless string wrapped halfway around it. A tension force of magnitude T is applied at both ends. From considering the string and the pulley as a single system, I could see that a force of 2T is applied to the pulley? But when I consider the pulley as an independent component, my understanding is that the string applies a force equal to the sum of normal forces at all points of contact between the pulley and the string.. Besides considering the entire system, why does this force have a magnitude of 2T although the tension force is not directly applied to the pulley since tension acts along the length of the string?

    5. Last one here. With a car performing an unbanked turn, the centripetal force responsible for the motion is some kind of friction between the tires and the ground? But what motion is this friction force opposing such that it is directed towards the centre of the circular path?

    Again, thanks for reading this nonsense. Hope you can help out an uninitiated to physics.
  2. jcsd
  3. Mar 9, 2015 #2


    User Avatar
    Science Advisor

    1. Yes, per definition

    2. It comes from repulsion between atoms, which depends on material stiffness and compression.

    3. Yes, if you know the material properties and the elongation.

    4. Because of geometry.

    5. Friction opposes or prevents relative motion of the contact patches.
  4. Mar 9, 2015 #3
    Thanks for the quick reply.

    That's good to know. So do actual equations exist for these forces, albeit most likely being too complicated for me to understand?

    Could you please elaborate on this?

    But how are the contact patches attempting to move perpendicularly to the velocity of the car?

    And also
    Thanks for your time.
  5. Mar 9, 2015 #4


    User Avatar
    Science Advisor

    The deformation is also very small and thus hard to observe.

    Did you try to find a derivation online?

    They are attempting to move parallel to the axis of the turned steering wheels.
  6. Mar 9, 2015 #5
    Thanks again.

    So in practice, the only way to deduce a value for normal force or tension is to simply observe the motion of the affected object?

    I just found one and am in the process of reading through it. I'll see if I understand it.

    I can see that this would give rise to a force directed towards the centre of the circular motion, but what is causing the contact patches to move in this manner? Is it due to a force applied by the body of the car as it attempts to continue moving as per Newton's First Law?

    Thanks for your time.
  7. Mar 9, 2015 #6


    User Avatar
    Science Advisor

    You can also use a scale.
    The car wants to keep moving straight, but wheels resist motion parallel to their axis.
  8. Mar 10, 2015 #7
    So is the attempted motion of the contact patches parallel to the axis of the wheels due to the inertia of the car?

    I did find a diagram online but I'm not sure if it's correct or not. It may help with the explanation.

  9. Mar 10, 2015 #8


    User Avatar
    Science Advisor

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook