A Day of Days

Between the storm that left everyone sodden and full day of forces, I'm ready for a nap...

B and C Blocks took time to discuss Newton's 2nd and 3rd Laws of Motion. Many people think of Newton's 2nd Law of Motion purely as the formula F_net = ma; however, that's a little shallow. From an equation standpoint, you should really think of it as a = F_net/m. An object's acceleration is directly proportional to the net applied force and inversely proportional to the object's inertia (measured by the mass). For a single object - increase F_net and the acceleration increases proportionally. For a given magnitude of force, the larger the mass of the object, the smaller the resultant acceleration. The equation is fine for calculations, but keep the general concept in mind, too. It can make general predictions and comparing objects in similar circumstances simple to do.

Newton's 3rd Law of Motion is one that seems so simple, but people really just don't get it. The phrase "for every action, there is an equal and opposite reaction" is so misused, I have to take a lie down sometimes. Newton-3 only deals with the size and direction of forces that arise when objects contact each other. Period. End of it. Finished. No farther you shall go... If it hit a desk with 20N of force downward, the desk applies a 20N upward force on me. If I want to take things a step further and predict what will happen to the motion of the desk and/or my hand after the contact, I have to haul out Newton-2. I know the magnitudes and directions of the forces, now, I have to work with each object's mass to determine their acceleration. Newton-3 does not, in any way, speak to the responses of the objects to the forces. Also, remember that the forces are simultaneously applied - there is no lag time, even though the term "action-reaction forces" is frequently used. Hopefully, today's demonstrations helped solidify a little of this in your minds and you bring the ideas with you to class tomorrow, when we take up looking at specific forces - weight and friction.

E Block conducted their Atwood's Machine lab and, for the conclusion section of your write-up, make sure to consider our discussion yesterday of forces, equilibrium, inertia and Newton's 1st Law of Motion. The wise student might peek ahead to Newton-2 and Newton-3 for additional information to include in your synopsis.

F Block discussed their Atwood's Machine lab and used that to highlight ideas about forces. The concepts of net force, equilibrium, inertia and Newton's 1st Law of Motion were nicely demonstrated by your lab, as was the bones of Newton's 2nd Law of Motion, which we'll discuss in class tomorrow. For the homework tonight, pull out those vector operations skills... you're gonna need them...