Wednesday, October 24, 2012 Warm-Up! Write the following question and a thoughtful answer in your notebook: Look in Chapter 5... Why does a heavy skydiver have a terminal speed greater than a light skydiver? What can be done so that the terminal speeds are equal? Be sure to provide sketches with your explanations. Wednesday, October 24, 2012

Answers: Q: Why does a heavy skydiver have a terminal speed greater than a light skydiver? A: The air resistance to weight ratio is less for the heavier diver. Q: What can be done so that the terminal speeds are equal? A: Vary the surface areas: to slow down the heavier diver, provide more surface area to create more air resistance. Be sure to provide sketches with your explanations.

Finish discussing Chapter 5 concepts: Today's agenda: Finish discussing Chapter 5 concepts: Friction, pressure, air resistance Remember, upcoming events (next week; will fall on either Wednesday or Thursday): - notebook & portfolio check - test over Chapters 3, 4, 5 Please NOTE: because your notebooks and portfolios are more substantial at this point, so will be the points possible.

Friction Chapter 5 (continued...) A force between all materials with surfaces in contact with each other. - Friction always depends on the kinds of materials that are in contact with each other and by how much force the surfaces are pressed together.

Friction always acts in the opposite direction of relative motion. Sketch this in your notes: List some common examples that you've experienced.

What is a FBD? - Free body diagram (FBD) - shows all forces acting on an object. See pg 64 Sketch this in your notes: These are very useful when identifying forces such as friction.

Friction (continued) Fluid friction occurs when objects move through a fluid. What is a Fluid - anything that flows (liquids, gases). - Air resistance is the friction acting on something moving through air.

Friction (continued) Static Friction: friction between two stationary objects Sliding Friction: friction between two objects that are sliding against one another Rolling Friction: friction between two surfaces when one is rolling Static Friction > Sliding Friction > Rolling Friction

Friction (continued) To overcome static friction, one must push harder until the surfaces in contact “break” apart. - Once the object begins moving, you push it so that it moves at a constant speed. - This force is often less than the force needed to overcome static friction. Fpush= Force of Sliding Friction

Friction (continued)... Air Friction/Resistance Remember, a fluid is anything that flows (including gases and liquids) AIR FRICTION (aka Air Resistance) loose molecules sliding past each other Mainly depends on: 1. Surface Area 2. Mass 3. Speed

Air Resistance - Air noticeably slows the motion of things with large surface areas like falling feathers or pieces of paper. - But, air resistance less noticeably affects the motion of more compact objects like stones and baseballs.

- Air resistance can affect acceleration of objects that are not inside a vacuum (for example, terminal speed). - In many cases, though, the effect of air resistance is small enough to be neglected (especially with objects that have small surface area to weight ratios). - With negligible air resistance, falling objects can be considered to be falling freely (ie, they don't slow down due to friction).

Write these in your notebooks... (use chapter 5) 1. Define the concept of terminal speed. 2. How does it affect falling objects that have different size, mass, etc.? 3. How does the air resistance and the weight of a falling object compare when terminal speed is reached? 4. How does this concept change how we've been discussing free fall on earth so far?

Answers to notebook questions: 1. Define the concept of terminal speed. The point at which a freely falling object stops accelerating downward and maintains a constant speed (acceleration stops; zero) because air resistance/friction balances the object's weight. 2. How does it affect falling objects that have different size, mass, etc.? These factors can affect the amount of friction and resistance, as well as the ratio between weight and air resistance due to area. (give examples) Coin vs. paper, etc. 3. How does the air resistance and the weight of a falling object compare when terminal speed is reached? Air resistance = weight; heavier objects have much higher terminal speeds. 4. How does this concept change how we've been discussing free fall on earth so far? Instead of the idea that a freely falling object will keep speeding up every second the entire time it is falling, we now know that the object will eventually reach a top speed and exceed that.

Pressure - How force is applied For a constant force, an increase in the area of contact will result in a decrease in the pressure. Pressure - the amount of force per unit of area. pressure = force/area of application or P= F/A

Pressure, continued... Pressure is measured in Newtons per square meter, or pascals (Pa). The smaller the area supporting a given force, the greater the pressure on that surface. P= F/A