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

2. FORCES THAT OPPOSE MOTION
ITS NOT JUST
SCIENCE FRICTION

3. 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.

4. 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

5. 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

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

7. 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:

8. 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.

9. 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.

10. 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

11. 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:
Friction produces heat, which melts the ice just enough to create a lubricant for the athlete.

12. 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

13. 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

14. COMPARING STATIC AND SLIDING FRICTION
Coefficient of Coefficient of Material Static Friction Sliding Friction Hard Steel on Hard Steel Glass on Glass Oak on Oak Teflon on Steel Aluminum on Steel Rubber Tire on Concrete (low speed) Rubber Tire on Wet Concrete (low speed) Steel on Ice (skates) Waxed Skis on Wet Snow

15. 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

16. FRICTION AND SPORTS

17. 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?

18. CURLING
olympic-winter-games

19. 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.

20. 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.

21. Globster
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