Homework Due Ch 9 packet (12 pts) Tonight’s HW  Read & Study , do #8-12 p. 133  Ch 9 open notes quest on Feb 21 & 22. Will NOT take up the entire class block – we will start Ch. 12 on this day, too.  Extra Credit Web Quest due on Feb 21 & 22 Do Now  Watch the marble and plate demo. Draw / write what you see. Then, SILENTLY answer this question (in your notes): Using Newton’s Laws as the basis for your argument, explain WHY there MUST be a force causing the object to move in a circle.

I am missing… 2 1 classroom textbook (1 was returned) 2 bottles of white out 1 calculator Until these items are returned, borrowing privileges are suspended… bring your own textbooks and your own calculators.

In Summary… Newton’s 1 st Law states that UNLESS there is a net force, – an object at rest will stay at rest – An object in motion will stay in motion without acceleration In other words, it will not accelerate  it will not change its velocity  it will not speed up, slow down, or change direction. Does the marble travel in a straight line? So…does it change direction? So… does it accelerate?

In Summary… Newton’s 2 nd Law states that IF there is a net force, – an object will accelerate In other words  it will change its velocity  it will speed up, slow down, change direction or any combination of the 3. Does the marble travel in a straight line? So…does it change direction? So… does it accelerate? So… is there a force that causes this acceleration? Your goal is to figure out what direction this force acts (and therefore, what direction the acceleration acts, since the acceleration is in the same direction as the net force)

Centripetal Force Any force that causes an object to follow a circular path. – Centripetal means “center seeking” or “toward the center.” – The force that holds the occupants in a rotating ride.

Centri f ugal Force The Physics curse word!!! The forbidden F word – fake, false, fictitious, Very commonly misused. Means “center-fleeing” or “away from the center.” It is NOT real force because it is NOT part of an action-reaction pair! Misconception that centrifugal force pulls you outward during circular motion. THERE IS NO FORCE PULLING YOU OUTWARD! – Actually, it is just the inertia of the body traveling tangent to the circle.

As a ball is whirled above the head, it is the inertia of the ball that wants to continue moving in a straight line. Instead, the ball is forced inward to change its direction (Centripetal Force-supplied by string).