Fiddling with Forces

Physics B discussed yesterday's friction lab before jumping into our new unit on forces. Friction is a specific force, but it does what all forces do - act to accelerate objects. We discussed the idea of net force (F_net) and what it meant when F_net = 0 N and ≠ 0 N. You should be able to calculate the net force acting on any object and be prepared to draw a free-body diagram to document the forces that were included in the net force calculation. Remember that forces can be contact forces, like a physical push or pull, or field forces, like gravitational or magnetic forces, but they do the same job and contribute to net force calculations. Tomorrow, we'll start to specifically line forces to motion and changes in motion as we dive into Newton's laws of motion.

Physics A started a lab investigation on static and kinetic friction. Static friction acts on objects at rest on surfaces and kinetic friction acts on objects in motion across or through surfaces. Both frictional forces arise from irregularities on the surfaces of the materials in contact, but static friction is boosted by the formation of adhesive, electrostatic bonds between the surfaces. Because of this, for the same objects in contact, the peak static friction (F_s,max) will always be greater than the kinetic friction (F_k). The data you are collecting is showing this relationship nicely and slotting in the additional concept of coefficient of friction. We'll talk about that tomorrow as more groups get to that calculation and discuss what it means specifically for our surfaces in the experiment.

Introductory Physics reviewed their MCAS practice for motion and certain force ideas. Folks seemed to do a nice job with that, but the open responses could have used more clarity, organization, detail and vocabulary for some questions. We'll practice this throughout the year, but start looking at your short answer questions and homework questions with an eye towards how this question would score if someone was grading it for your MCAS. Tomorrow is test day and on Thursday, we'll get deeper into the ideas of forces.

Honors Physics had more practice with working with forces and force diagrams. A good sketch, with all forces identified and labeled is a great help in solving these problems. Sometimes it is easiest to work backwards with these situations. Writing down what you are asked to determine (acceleration, net force, coefficient of friction, etc.) can start you thinking about what you directly need to solve for that variable (and there may be more than one possible pathway that you'll pare down to one as you evaluate the information), then what do you need to solve for the things to solve for that, etc. Tomorrow's lab will give you a better idea about the conceptual nature of friction and the coefficient of friction and we'll take Thursday to review the chapter ideas and tonight's homework. Test of Friday, so be prepared. I'll get a couple of videos up tomorrow for working with forces that might help you with the problems due on Thursday.