Warm-Up: Why are ice skates thin and sharp? (Question from last class).

FORCES THAT OPPOSE MOTION ITS NOT JUST SCIENCE FRICTION

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.

QUESTIONS Was your hypothesis correct based on the data you collected? What affect does surface area have on frictions? Support your answer using your data.

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? Last class, we said that ice skates are sharp and narrow to reduce surface area, and therefore reduce friction. This contradicts what we just learned about surface area. What do you think then is the reason for thin, sharp skates? Chemistry of ice: http://www.nbclearn.com/science-and-engineering-of-the-2014-olympic-winter-games Friction produces heat, which melts the ice just enough to create a lubricant for the athlete.

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

FRICTION AND SPORTS

FRICTION AND RUNNING Although friction typically acts against motion, it is the very reason why forward movement exists during walking and running. Read the following handout on running and friction. How does friction help prevent your from slipping when you initially make contact with the ground? How does friction help propel you forward towards the end of your stride? What do you think a resultant force is? Using Newton’s Laws, how does the resultant force tell us what direction your leg will be moving?

CURLING http://www.nbclearn.com/portal/site/learn/science-of-the- olympic-winter-games

Summary: Friction and Surface Area If the area of contact doubles, you may think that you should get twice as much friction. But when you double the length of an object, you halve the force on each square centimeter, because less weight is above it to push down.

WINTER OLYMPICS PROJECT Homework: Watch at least 3 competitors compete in your sport. Hockey – watch 15 minutes of a game. There is an App that allows you to watch any sports that have already competed any time you want. Write down key terms commentators use to describe your sporting event. How are athletes’ scores computed? Is it based on time, techniques, certain jumps/movements, etc. Wednesday we will be in the computer lab. By the end of class Wednesday, your group needs to have your rough draft of the summary of your sport completed.

Globster http://www.glogster.com/chels328/physics-of-ice-skating/g- 6n845bn3r41tq7murk80oa0