1. Today is Wednesday, April 24, 2013
HOMEWORK:
- Read Chapter 13 (finish reading by Saturday)
Complete Chapter combined Homework Packet (due May 10)
ABSOLUTE LAST CALL FOR midterms!
Warm UP:
Discuss with your SHOULDER PARTNER: “What are two types of forces we talked about yesterday?”

2. Goal for Today
SWBAT demonstrate an understanding of different types of forces by identifying and describing electric force and friction.
SWBAT demonstrate an understanding of Newton’s three laws by identifying what laws are at work in different examples.

3. Electric Forces REVIEW: the structure of an atom –
proton, neutron, electron
protons and neutrons in nucleus
electrons around the outside
When molecules are formed, atoms can share or steal electrons
Let’s take a look at an example of static electricity with a balloon. Draw a diagram illustrating what happens to the charges when the balloon is rubbed and what happens when the balloon sticks to the wall.

5. Sometimes the charges will jump; this occurs naturally with lightening
Draw a diagram showing the negative charges collecting on the bottom of a cloud and the positive charges on the ground
Does this look similar to the balloon example?
This attraction will cause the electrons
to jump to the closest location

6. Friction - force that results when two materials rub against each other
Rub your hands together. What happens? Why?
There are times when you want more friction and times when you want less friction. Let’s brainstorm some examples of each. Take 2 minutes in table groups to do this.
Drop 2 pieces of paper on the ground. One piece is crumpled and the other is still a sheet
What happens? Why?
Air and water also have friction (resistance/drag)

7. Work – done when a force moves an object. It is measured in joules (J)
FORMULA: Work = Force x Distance (W = F x D)
Power – rate at which work is done. It is measured in watts (W)
FORMULA: Power = Work / Time (P = W/t)
Net force – combination of all the forces on an object  object moves? Stop moving? Change direction?
Let’s look at some diagrams and figure out the net forces involved…..as we are doing this, remember:
State of equilibrium – when all forces acting on an object balances each other
Unbalanced forces causes motion or change of motion

9. Step One: Assess the forces =
- Car is moving forward with 600N of force already
- Driver accelerates with 200N of force
- Air resistance is 300N coming from the other direction
Step Two: 600N + 200N = 800N
Step Three: 800N – 300N = 500N
Step Four: Final answer = the net force is 500N to the right

10. Newton’s first law (Law of Inertia) – an object will remain at rest or in constant motion unless a net force acts on it
Inertia – tendency of an object to resist any change in motion
The book on the table (or anything sitting still)
Marble rolling
Bike rolling on
Seat belt in a car
Bus

11. Newton’s second law – acceleration, mass, and net force are related
Newton’s second law – acceleration, mass, and net force are related. F = m x a
Acceleration – rate at which the velocity of an object changes over time
Explains that an object will change its velocity if pushed or pulled upon
If you do put a force on it, it will change velocity (acceleration) and it will change it with the direction of the force
Acceleration directly proportional to the force. If you push it 3 times harder, it will accelerate 3 times greater
Acceleration inversely proportional to mass. If you push on 2 objects with the same force but one has a mass 5 times greater, the acceleration will be 1/5 of the other object

12. Newton’s third law – for every action there is an equal and opposite reaction.
This is how a rocket works

13. Let’s close up with some practice questions…
Suppose you apply 300N to push your bike 90m. How much work is done?
If it takes 20s to push the bike 90 m, how much power was needed?
Riding the bike requires only 20N to cross 90 m, and it takes just 6 sec. How much work is done? How much power is needed?