1. Newton Card Sort
Find your cards from last class.

2. 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.

3. 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.

4. 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?

5. 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?

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

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

8. FORCES THAT OPPOSE MOTION
ITS NOT JUST
SCIENCE FRICTION

9. 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?

10. 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.

11. 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.

12. 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

13. 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

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

15. 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:

16. 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.

17. 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

18. 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.
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

19. 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

20. 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

21. 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

22. 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

24. What does this mean for sports?
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