# Newton’s Laws.

Published on: Mon Feb 22 2010

We continued reviewing Newton’s three laws. 1) Objects in motion stay in motion. Objects at rest say at rest. 2) F = m/a 3) I think this is the law about equal but opposite forces. We analyzed two masses connected by a frictionless massless pulley and rope. Using just Force equals mass times acceleration we were able to determine the acceleration of the masses. Then once we knew the acceleration we were able to calculate the Tension Force on the rope. The professor checked his answers by thinking logically about his results. First he checked on the dimensions, then he thought about what would happen near the zero values and then near the equilibrium states. Friction We started on Friction today, because friction is really quite an important thing. If you look very closely at the borders between objects, you will see their small rough surfaces, trying to catch on to each other. The friction force always points in opposition to external forces. As long as a body does not move, the External Force and Friction force are equal. Once the body moves the external Force has overpowered the Friction force. If you place an object on a plane, then tilt the plane until the object slides off, you can find the value of the friction force. Friction is expressed using a greek letter “μ” read this “mu”. There are two types of friction we are considering. Static Friction, which is the friction required to get an object moving. This is represented by the subscript s, and look like μs. It is a dimensionless quantity and represents a fraction of the normal force required for horizontal movement. The second type of friction is Kinetic Friction. It is represented by the subscript k, and looks like μk. Static Friction is neat. It is the tangent of the angle created by the tilting of the plane.