1. Newton’s 3rd Law: More Practice

2. Newton’s 3rd Law: More Practice

3. Newton’s 3rd Law: More Practice

4. Newton’s 3rd Law: More Practice

5. Newton’s 3rd Law: More Practice

6. Newton’s 3rd Law: More Practice

7. Newton’s 3rd Law: More Practice

8. Newton’s 3rd Law: More Practice

9. An Introduction to Friction: Student Learning Goal
Students will be able to perform an investigation to determine the factors affecting static and kinetic friction, and to determine the corresponding coefficient of friction between an everyday object and the surface with which it is in contact.
(C2.3)

10. An Introduction to Friction
SPH4C

11. Weight
Recall that the gravitational force acting on an object is also called the weight.

12. Weight
Recall that the gravitational force acting on an object is also called the weight.
Its magnitude is equal to:
where m is mass, measured in kg, and g = 9.8 m/s2.

13. Weight
Recall that the gravitational force acting on an object is also called the weight.
Its magnitude is equal to:
where m is mass, measured in kg, and g = 9.8 m/s2.
Weight is therefore measured in Newtons.

14. The Normal Force
Recall that the normal force is the force acting to keep objects apart.

15. The Normal Force
Recall that the normal force is the force acting to keep objects apart. For an object on a horizontal surface with no vertical applied force, the normal force will be equal to __________. FN Ff
e.g. a box pushed rightward across a table FA Fg

16. The Normal Force
Recall that the normal force is the force acting to keep objects apart. For an object on a horizontal surface with no vertical applied force, the normal force will be equal to the weight. FN Ff
e.g. a box pushed rightward across a table FA Fg

17. The Normal Force
Recall that the normal force is the force acting to keep objects apart. For an object on a horizontal surface with no vertical applied force, the normal force will be equal to the weight
(since the vertical forces must balance). FN Ff
e.g. a box pushed rightward across a table FA Fg

18. FN not equal to Fg
If there is a vertical applied force, the normal force will not equal the weight.
Example
A 5 kg book is resting on the table. If Ms. Rosebery is pushing down on the book with a force of 20 N, what is the normal force the table is exerting on the book?

19. FN not equal to Fg Example
A 5 kg book is resting on the table. If Ms. Rosebery is pushing down on the book with a force of 20 N, what is the normal force the table is exerting on the book?
FN= ?
FA= 20 N
Fg=(5 kg)(9.8 m/s2) = 50 N

20. FN not equal to Fg Example
A 5 kg book is resting on the table. If Ms. Rosebery is pushing down on the book with a force of 20 N, what is the normal force the table is exerting on the book?
The normal force must be 70 N [up].
FN= ?
FA= 20 N
Fg=(5 kg)(9.8 m/s2) = 50 N

21. Direction of FN
Note that the normal force is always perpendicular to the surface. If the surface is vertical (e.g. a magnetic board to which a magnet is attached), the normal force must be horizontal.

22. Friction
Recall that friction is the force acting to oppose any (attempted) motion and is therefore always opposite in direction to the attempted motion.
The magnitude of the force of friction will depend on the properties of the surfaces in contact

23. Friction
Recall that friction is the force acting to oppose any (attempted) motion and is therefore always opposite in direction to the attempted motion.
The magnitude of the force of friction will depend on the properties of the surfaces in contact and on the normal force acting on the object attempting to move .

24. Mu
The magnitude of the frictional force is given by:

25. Mu The magnitude of the frictional force is given by:
where m (mu) is the coefficient of friction, determined by the properties of the surfaces in contact.

26. Mu The magnitude of the frictional force is given by:
where m (mu) is the coefficient of friction, determined by the properties of the surfaces in contact.
m is dimensionless (no units) and is ≤ 1 (approx.).

27. Mu Examples E.g. For steel on wood, mk (kinetic) = 0.25
ms (static) = 0.45
Note that ms > mk : it’s harder to get things moving than keep things moving.

28. Mu Examples E.g. For steel on wood, mk (kinetic) = 0.25
ms (static) = 0.45
For steel on ice, mk (kinetic) =
ms (static) =

29. Mu Examples E.g. For steel on wood, mk (kinetic) = 0.25
ms (static) = 0.45
For steel on ice, mk (kinetic) = 0.010
ms (static) = 0.10
For rubber on dry concrete, mk (kinetic) =
ms (static) =

30. Mu Examples E.g. For steel on wood, mk (kinetic) = 0.25
ms (static) = 0.45
For steel on ice, mk (kinetic) = 0.010
ms (static) = 0.10
For rubber on dry concrete, mk (kinetic) = 1.0
ms (static) = 1.1

31. Example Problem
A 1200-kg car without ABS brakes is skidding on dry concrete. What is the magnitude of the force of friction acting on the car?

32. Example Problem
A 1200-kg car without ABS brakes is skidding on dry concrete. What is the magnitude of the force of friction acting on the car?

33. Example Problem
A 1200-kg car without ABS brakes is skidding on dry concrete. What is the magnitude of the force of friction acting on the car?

34. Example Problem
A 1200-kg car without ABS brakes is skidding on dry concrete. What is the magnitude of the force of friction acting on the car?

35. ABS Brakes
If the car does have ABS brakes, the car’s tires will continue to roll and grip the road while stopping and the relevant coefficient will be that of static friction:

36. ABS Brakes
If the car does have ABS brakes, the car’s tires will continue to roll and grip the road while stopping and the relevant coefficient will be that of static friction:

37. ABS Brakes
If the car does have ABS brakes, the car’s tires will continue to roll and grip the road while stopping and the relevant coefficient will be that of static friction:
i.e. there is more force stopping the car when the tires are rolling and not sliding

38. More Practice
Static and Kinetic Friction Lab