Kinetic and Static Friction - AP Physics C Mechanics

1. Identify the unit of friction coefficients.

   Unitless. Coefficients are dimensionless ratios of forces.

2. Define the coefficient of static friction.

   The ratio of maximum static friction to the normal force. It quantifies the material's resistance to starting motion.

3. What is the effect of surface area on friction?

   No direct effect. Friction depends only on normal force and surface properties.

4. Determine the limiting static friction if $\mu_s = 0.5$ and $F_n = 100 \text{N}$.

   $f_s = 50 \text{N}$. Maximum static friction is $\mu_s F_n = 0.5 \times 100 = 50$ N.

5. Which is generally larger: static or kinetic friction coefficient?

   Static friction coefficient. It's harder to start motion than to maintain it.

6. Can static friction do work on an object?

   No, static friction does not do work. Static friction prevents motion, so displacement is zero.

7. Define the coefficient of kinetic friction.

   The ratio of kinetic friction to the normal force. It quantifies the resistance during sliding motion.

8. What happens to friction when an object transitions from static to kinetic?

   Friction decreases. Kinetic friction is typically less than maximum static friction.

9. What is the maximum static friction force called?

   Limiting static friction. This is the maximum value static friction can reach.

10. If $f_k = 50 \text{N}$ and $F_n = 250 \text{N}$, find $\mu_k$.

    $\mu_k = 0.2$. Using $\mu_k = \frac{f_k}{F_n} = \frac{50}{250} = 0.2$.

11. What does the static frictional force depend on?

    The coefficient of static friction and normal force. These two factors determine the maximum static friction force.

12. What does kinetic friction depend on?

    The coefficient of kinetic friction and normal force. These are the only factors that determine kinetic friction magnitude.

13. Is friction a contact force?

    Yes, friction is a contact force. Friction requires physical contact between surfaces.

14. Find the static friction force if $\mu_s = 0.4$ and $F_n = 200 \text{N}$.

    $f_s \leq 80 \text{N}$. Maximum static friction is $\mu_s F_n = 0.4 \times 200 = 80$ N.

15. Calculate $\mu_k$ if $f_k = 15 \text{N}$ and $F_n = 75 \text{N}$.

    $\mu_k = 0.2$. Using $\mu_k = \frac{f_k}{F_n} = \frac{15}{75} = 0.2$.

16. State the range of static frictional force.

    $0 \leq f_s \leq \mu_s F_n$. Static friction adjusts from zero up to its maximum value.

17. Calculate friction if $\mu_k = 0.1$ and $F_n = 200 \text{N}$.

    $f_k = 20 \text{N}$. Using $f_k = \mu_k F_n = 0.1 \times 200 = 20$ N.

18. Determine if friction is conservative or non-conservative.

    Non-conservative. Friction converts mechanical energy to heat, making it non-conservative.

19. Identify the direction of frictional force relative to motion.

    Opposite to motion or impending motion. Friction always opposes relative motion between surfaces.

20. Find the coefficient of static friction if $f_s = 40 \text{N}$ and $F_n = 200 \text{N}$.

    $\mu_s = 0.2$. Using $\mu_s = \frac{f_s}{F_n} = \frac{40}{200} = 0.2$.

21. What is the formula for static frictional force?

    $f_s \, \leq \, \mu_s F_n$. This inequality shows static friction can range from zero up to its maximum value.

22. What effect does increasing normal force have on friction?

    Increases frictional force. Friction is directly proportional to normal force.

23. State the primary factors affecting frictional force.

    Normal force and the coefficient of friction. These determine the magnitude of frictional resistance.

24. Identify the force that initiates motion overcoming static friction.

    Applied force greater than static friction. Applied force must exceed maximum static friction to start motion.

25. Identify whether friction opposes or assists motion.

    Opposes motion. Friction always acts to resist relative motion.

26. What is the formula for kinetic frictional force?

    $f_k = \mu_k F_n$. Kinetic friction has a constant magnitude proportional to normal force.

27. Find $f_s$ if $\mu_s = 0.6$ and $F_n = 150 \text{N}$.

    $f_s \leq 90 \text{N}$. Maximum static friction is $\mu_s F_n = 0.6 \times 150 = 90$ N.

28. Calculate the normal force on a $5 \text{kg}$ object on a horizontal surface.

    $F_n = 49 \text{N}$. Using $F_n = mg = 5 \times 9.8 = 49$ N.

29. Does static friction vary with the applied force?

    Yes, up to its maximum value. Static friction adjusts to match applied force until its maximum.

30. Find $f_k$ if $\mu_k = 0.15$ and $F_n = 300 \text{N}$.

    $f_k = 45 \text{N}$. Using $f_k = \mu_k F_n = 0.15 \times 300 = 45$ N.

31. For an object on a flat surface, what is the normal force equal to?

    Equal to the gravitational force on the object. On level ground, normal force equals weight $mg$.

32. State the relationship between $f_s$ and $\mu_s$.

    $f_s \leq \mu_s F_n$. This inequality defines the range of static friction values.

33. Find $f_k$ if $\mu_k = 0.25$ and $F_n = 400 \text{N}$.

    $f_k = 100 \text{N}$. Using $f_k = \mu_k F_n = 0.25 \times 400 = 100$ N.

34. If $\mu_s = 0.3$ and $\mu_k = 0.25$, which is larger?

    $\mu_s$. Static coefficient (0.3) exceeds kinetic coefficient (0.25).

35. Find the kinetic friction force if $\mu_k = 0.3$ and $F_n = 100 \text{N}$.

    $f_k = 30 \text{N}$. Using $f_k = \mu_k F_n = 0.3 \times 100 = 30$ N.

36. Identify if kinetic friction can do work.

    Yes, kinetic friction can do work. Kinetic friction acts through displacement, doing negative work.

37. What is the role of friction in walking?

    Prevents slipping by providing traction. Static friction between feet and ground enables forward motion.

38. Determine the force needed to maintain constant velocity if $f_k = 20 \text{N}$.

    20 N. Applied force must balance kinetic friction for constant velocity.

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