Kinetic and Static Friction - AP Physics 1

1. If an object is moving at constant speed, what can be said about friction?

   Kinetic friction equals the applied force. No net force means friction balances applied force.

2. State whether static friction depends on the surface area of contact.

   No, it does not depend on surface area. Friction depends only on materials and normal force.

3. What does the symbol $N$ represent in friction formulas?

   The normal force. Perpendicular contact force between surfaces.

4. Define the coefficient of kinetic friction.

   The ratio of kinetic friction force to the normal force. Shows how friction scales with normal force for moving objects.

5. State whether kinetic friction depends on the surface area of contact.

   No, it does not depend on surface area. Friction depends only on materials and normal force.

6. What is the primary force opposing motion on a flat surface?

   Friction. Friction opposes relative motion between surfaces.

7. Identify the friction type when pushing an object that does not move.

   Static friction. Object remains at rest, so static friction acts.

8. If $f_k = 10 \text{ N}$ and $N = 50 \text{ N}$, find $\text{μ}_k$.

   $\text{μ}_k = 0.2$. Using $\text{μ}_k = f_k/N = 10/50 = 0.2$.

9. Find the normal force if $m = 10 \text{ kg}$ and $g = 9.8 \text{ m/s}^2$ on a flat surface.

   $N = 98 \text{ N}$. Using $N = mg = 10 \times 9.8 = 98$ N.

10. Identify the type of friction that occurs when an object is in motion.

    Kinetic friction. Acts on objects that are sliding.

11. Find the maximum static friction if $\text{μ}_s = 0.4$ and $N = 50 \text{ N}$.

    $f_s \text{ (max)} = 20 \text{ N}$. Using $f_s = \text{μ}_s N = 0.4 \times 50 = 20$ N.

12. What happens to static friction as the angle of an inclined plane increases?

    Static friction increases until the object starts to move. Adjusts to prevent sliding up to its maximum.

13. What effect does lubrication have on kinetic friction?

    It reduces kinetic friction. Lubrication decreases friction between surfaces.

14. How does surface roughness affect the coefficients of friction?

    Increases both coefficients. Rougher surfaces create more friction.

15. Determine the normal force on a $5 \text{ kg}$ object on a flat surface.

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

16. What does the symbol $f_s$ represent?

    The force of static friction. Standard notation for static friction force.

17. What effect does lubrication have on static friction?

    It reduces static friction. Lubrication decreases friction between surfaces.

18. Find the kinetic friction if $\text{μ}_k = 0.2$ and $N = 100 \text{ N}$.

    $f_k = 20 \text{ N}$. Using $f_k = \text{μ}_k N = 0.2 \times 100 = 20$ N.

19. Identify the type of friction that occurs when an object is at rest.

    Static friction. Acts on objects that aren't sliding.

20. What condition must be met for static friction to be at its maximum value?

    The object is on the verge of motion. When applied force equals maximum static friction.

21. What is the formula for the force of kinetic friction?

    $f_k = \mu_k N$. Kinetic friction equals coefficient times normal force.

22. Which is usually larger, $\text{μ}_s$ or $\text{μ}_k$?

    $\text{μ}_s$ is usually larger. It takes more force to start motion than maintain it.

23. Which option best describes friction between tires and a road while driving?

    Static friction. Tires grip the road without sliding.

24. What is the unit of measurement for the coefficients of friction?

    Dimensionless (no units). Coefficients are ratios of forces.

25. If $f_s = 15 \text{ N}$ and $N = 50 \text{ N}$, find $\text{μ}_s$.

    $\text{μ}_s = 0.3$. Using $\text{μ}_s = f_s/N = 15/50 = 0.3$.

26. What is the effect of increasing the mass on normal force?

    Normal force increases. More mass creates larger normal force.

27. What does $\text{μ}$ depend on?

    Material properties of surfaces. Coefficient depends on surface materials and conditions.

28. Define the coefficient of static friction.

    The ratio of maximum static friction force to the normal force. Shows how friction scales with normal force for stationary objects.

29. What happens to kinetic friction as the angle of an inclined plane increases?

    Kinetic friction remains constant regardless of angle. Kinetic friction magnitude doesn't change with angle.

30. What is the formula for the force of static friction?

    $f_s \text{ (max)} = \text{μ}_s N$. Maximum static friction equals coefficient times normal force.

31. Identify the friction type when an object slides down a slope.

    Kinetic friction. Object is moving, so kinetic friction acts.

32. Find kinetic friction if $\text{μ}_k = 0.1$ and $N = 250 \text{ N}$.

    $f_k = 25 \text{ N}$. Using $f_k = \text{μ}_k N = 0.1 \times 250 = 25$ N.

33. What does a higher coefficient of friction indicate?

    Greater resistance to motion. Higher coefficients mean more frictional force.

34. Find the static friction if $\text{μ}_s = 0.5$ and $N = 40 \text{ N}$.

    $f_s \text{ (max)} = 20 \text{ N}$. Using $f_s = \text{μ}_s N = 0.5 \times 40 = 20$ N.

35. Which option best describes friction between a sled and snow while sliding?

    Kinetic friction. Sled slides continuously across the surface.

36. Find static friction if $\text{μ}_s = 0.6$ and $N = 200 \text{ N}$.

    $f_s \text{ (max)} = 120 \text{ N}$. Using $f_s = \text{μ}_s N = 0.6 \times 200 = 120$ N.

37. Find the kinetic friction if $\text{μ}_k = 0.3$ and $N = 70 \text{ N}$.

    $f_k = 21 \text{ N}$. Using $f_k = \text{μ}_k N = 0.3 \times 70 = 21$ N.

38. If $\text{μ}_k = \text{μ}_s$, what can be inferred about motion?

    Frictional resistance is constant during motion. Same friction force whether moving or stationary.

39. What does the symbol $f_k$ represent?

    The force of kinetic friction. Standard notation for kinetic friction force.

Study these flashcards