Conceptual Physics

http://mrlafazia.com/PHYS111/lessons/201011Fall/PHYS111_13.ppt

TUESDAY October 5th, 2010

LESSON GOALS:• Introduce Newton’s Laws of Motion• Demonstrate inertia concept• Simulate Newton’s Laws concept• Demonstrate equilibrium of forces• Complete lab experiment on Discovering Hooke’s Law• Assign HMWK #5• Complete (time permitting) lab experiment on Pressure on your Feet

1/18

Newton’s Laws of Motion- an introduction -

2/18

First off, we must ask…“What ARE Forces”??

• Put simply, a Force is “a push or a pull”.

• Looking at the BIG PICTURE, a Force is defined as “a way to transfer Energy”.

3/18

Vector or Scalar??

• Strictly speaking, Forces are VECTORS

• For this reason, we represent Forces with arrows. The longer the arrow, the greater the magnitude (number-value), and the angle of the arrow tells us the direction of the Force!

4/18

F.B.D.-ing

• This is best seen by drawing a Free-Body Diagram (as important as Dimensional Analysis!)

• A Free-Body Diagram (or FBD) is a tool that takes an object all by itself and shows ONLY the Forces acting on THAT OBJECT!

5/18

An Example of FBD-ing

fk

Fg

N

Fpush

Box being pushed along rough surface:

6/18

Why do we need to know about Newton’s Laws of Motion???

• Newton’s 3 Laws of Motion help us to understand WHY an object moves (or why one doesn’t move).

7/18

Law # 1

• The Law of Inertia– What is inertia??

• Bill Nye says…?• Definition (the tendency of….)

• So…what IS Newton’s 1st Law of Motion??– “An object in motion will tend to stay in motion,

and an object at rest will tend to stay at rest, unless acted upon by an outside, unbalanced Force.”

8/18

Law #2

• “The acceleration an object experiences is directly proportional to the overall Force acting on it and inversely proportional to the object’s mass.”

• This is hard to picture, so let’s just remember it as an equation:

F = m a∙9/18

Law #3

• …often known as the “action/reaction” Law

• “For every action there is an equal and opposite reaction.”

– Examples: “Normal” force from floor, pushing against a wall, letting air out of a balloon, &tc.

• Note: “Normal” means “Perpendicular” or “Orthogonal”. Never think of it as the “regular” force.

10/18

DEMO: Inertia on a Stick

• Here is a very striking (and memorable) demonstration on INERTIA….

11/18

Simulation

Interactive Physics 6.1 • Discussion: “Why does the heavier object

NOT fall faster”??

12/18

DEMO: Groovy Gravity

• Here is a demonstration on how forces can be acting while in equilibrium (and the resulting motion).

• The demonstration can also show what happens when forces are NOT in equilibrium…

• …this demo is affectionately known as “The Physics Peace Pipe”

13/18

LAB: Hooke’s Law

• We have already used this LAW! Now we will “discover” it.

• Hooke’s Law (eXcel spreadsheet)– complete these in groups at your lab stations– http://mrlafazia.com/labs/HookesLaw.xls– reflection: Hooke’s Law practicum analysis

questions (#’s 1-6) • as projected onto board

– THESE will be your graded lab write-up portion.

14/18

Lab: Pressure on your Feet

• Pressure on Your Feet– Formula for Pressure calculation

• “How does Pressure differ from Force”?

– No lab write-up, here. Instead your grade is determined by my observation of your participation.

15/18

HMWK #5p. 47 #’s 7-10; 13-15.p. 48 #’s 17; 18.

16/18

Recommended Reading:Sections 3.2-3.7

Sections 4.1-4.4; 4.6

These sections of the text prepare you for the Force concepts from Unit 2. We will look more at the motion concepts in later

lessons within Unit 2.

17/18

Looking Ahead:

18/18

• Next class we continue our look at Force & Motion concepts.