Newton Card Sort Find your cards from last class.

CLAIM-SUPPORT-QUESTION Make a claim about the statements on the next slide. A claim is an explanation or interpretation of some aspect of what is being examined.

Statements: Accelerating a shot put is harder than accelerating a baseball. Running on the beach is harder than running on a track. The harder you throw a basketball down, the higher it bounces back up. Playing basketball is more fatiguing than jogging.

CLAIM-SUPPORT-QUESTION Identify support for your claim. What things do you see, feel, or know that lend evidence to your claim? Specifically, what things have we talked about in this class that would support your claim?

CLAIM-SUPPORT-QUESTION Raise a question related to your claim. What may make you doubt the claim? What seems left hanging? What isn’t fully explained? What further ideas or issues does your claim raise?

SHARE YOUR THOUGHTS Find a partner and share your claim, support, and question for each of the 4 statements previously made.

EXTENSION OF KNOWLEDGE Which of Newton’s 3 Laws do you think best explains friction?

FORCES THAT OPPOSE MOTION ITS NOT JUST SCIENCE FRICTION

Friction Self-Assessment You will have 5 minutes to write down what you know about friction in your notebook. Questions to help guide your response: When, where, and how is friction useful? When, where and why do the effects of friction need to be reduced? How and where does friction act on the human body? Why is the static friction greater than the kinetic friction? (static friction – effects of friction before an object is moving; kinetic friction – effects of friction after the object has started moving). In ice sports, why are blades thin and sharp?

Graffiti Around the room there are posters for each of the questions you just answered. You will be broken into groups of 4-5. Each group will be assigned a poster. Your group will have 2 minutes to answer the question. Each of you should have a pen/pencil to write with. When the timer rings, rotate clockwise to the next poster. You will continue this until you return to your original poster.

The Force of Friction Opposes motion – occurs when surfaces slide over each other Depends on the kinds of material and how much they are pressed together. Is due to tiny surface bumps and to “stickiness” of the atoms on a material’s surface. Example: Friction between a crate on a smooth wooden floor is less than that on a rough floor.

FRICTION ISN’T ALL BAD It is essential – without it, the world would be very different walking, gripping objects running and even standing in place would be impossible

MORE FRICTION Friction does not exist if an object at rest with no external force acting on it If a force is applied, friction resists the movement If the friction resisting motion is equal to the applied force, then no motion occurs Friction that holds an object in place is called static friction - represented by fs

Gravity acts on the box: static friction holds the box in place No force acts on the box so no friction exists

Surface Area and Friction Take a moment to answer this question on a sheet of paper: How does surface area affect friction? Experiment: With a partner, you are going to analyze how surface area affects friction. Copy this table on your paper:

Step One Use the fishing line to attach your book to the spring scale as shown. Determine the weight of the book by hanging it from the spring scale. Check with your teacher for an alternative method if the book weighs too much for the spring scale. Step Two Lay the book flat on a table so that the largest surface of the book is in contact with the table. With the fishing line still around the book from step one, drag the book steadily across the table with the spring scale. Determine the amount of pulling force necessary to keep the book moving slowly at a constant velocity across the table. Read the force measurement from the spring scale. Record this pulling force in the data table. Repeat these procedures three times. Compute the average pulling force of the three trials and record the average in the data table. Step Three Reposition the book so that a smaller surface (the edge opposite the binding) is in contact with the table. Repeat the dragging procedures describe in step 2 three more times. Record the pulling force measurement for all three trials in the data table. Compute the average force of the three trials and record the average in the data table. Be sure that the force reading on the spring scale is only the pulling force.

SURFACE AREA AND FRICTION Friction is independent of an object’s surface area - it varies with the mass of the object - the greater an object’s mass, the more force is applied at the point of contact between the objects

What does this mean for sports? Why are ice skates sharp and thin? Take a moment to write your answer in your notebook. Share your answer with a partner. http://www.nbclearn.com/science-and-engineering-of-the- 2014-olympic-winter-games As we glide across the ice, we exert pressure on the thin blade, and are therefore creating a small stream of water in our path by melting that ice. The water between the blade and the ice is what we really glide across. Right when we stop exerting pressure, that water freezes almost instantly due to the cold temperature surrounding it

SLIDING FRICTION AKA - KINETIC FRICTION Once the force overcomes static friction, the object begins to move - the motion is still opposed by friction – sliding friction or kinetic friction - represented by fk - Sliding friction resisting an object’s motion is not affected by the speed at which the object moves

STATIC VS KINETIC FRICTION In general, static friction is greater than kinetic friction -meaning: it takes more force to start an object moving than it does to keep it moving

COMPARING STATIC AND SLIDING FRICTION Coefficient of Coefficient of Material Static Friction Sliding Friction Hard Steel on Hard Steel 0.78 0.42 Glass on Glass 0.94 0.04 Oak on Oak 0.54 0.32 Teflon on Steel 0.04 0.04 Aluminum on Steel 0.61 0.41 Rubber Tire on Concrete (low speed) 0.9 0.7 Rubber Tire on Wet Concrete (low speed) 0.7 0.5 Steel on Ice (skates) 0.02 0.01 Waxed Skis on Wet Snow 0.14 0.10

FORCE OF FRICTION The force of friction is equal to the normal force multiplied by the coefficient of friction between two surfaces fs = FN μs and fk = FN μk FN w Ff

What does this mean for sports? http://www.glogster.com/chels328/physics-of-ice-skating/g- 6n845bn3r41tq7murk80oa0