Monday, March 17, 2008 - St Patrick's Day

Went around room asking students what they told their parents about last Wednesday's Inertia Labs. Some students made the potato head for parents and took pictures for extra credit.

Students explained the two cases for the pulling string mini-station. Discussed which was due to weight and which was due to inertia.

Collected write-ups.

A force is simply a push or a pull.

Explained what is meant by a net force - vector sum of all forces acting on an object.

Introduced Newton's First Law of Motion: If there is no net external force acting on an object, an object at rest remains at rest and an object in motion keeps moving in a straight line with constant speed.

Constant velocity means NO NET FORCE.

Mass, measured in kg, is a measure of inertia. If you toss an object from hand to hand you are measuring its inertia.

Volume and weight are NOT the same as mass.

Weight, measured in Newtons, is the force of gravity on an object. If you hold an object in your hand, you are measuring its weight.

A mass of 1 kg weighs 10 N at the surface of the Earth. It also weighs about 2.2 lbs. One Newton is about the same as 0.22 lbs ~ 0.25 lbs. Therefore, instead of ordering a quarter pounder, you could order a Newtonburger.

Problem of who has more gold - 100 lbs of gold on Earth or 100 lbs of gold on Moon.

Quiz on inertia.

Normal is a mathematical term meaning perpendicular to a surface. A normal force, which you only have with a surface (NOT with strings) is the force perpendicular to that surface.

Introduced the idea of free body diagrams. Isolate an object and show only the external forces that act on that object. You do NOT show velocity vectors, etc. or any forces that that object exerts on something else. Did a few examples of drawing free body diagrams.

Did some examples in statics.

Handed out RA 4.1 due tomorrow.