1. Newton’s Laws of Motion Chapters 2,3,6,7
Forces
Newton’s Laws of Motion
Chapters 2,3,6,7

2. Forces A push or pull that could cause a change in motion
Measured in Newtons (N)
kg∙m/s2
Many Types
Tension
Weight
Friction
Normal

3. Concurrent Forces Two concurrent forces acting on an object…..
Two forces acting concurrently……
Means the forces are acting on the same object at the same time

4. Newton’s 1st Law (N1L)
An object at rest will stay at rest unless acted upon by an external force
An object in motion will remain in motion unless acted upon by an external force.

5. Law of Inertia (N1L) Inertia Inertia Mass
Property of objects to resist changes in motion
Mass is the measure of inertia
Inertia Mass

6. Law of Inertia (N1L)
More mass, more inertia, less likely to change motion
Less mass, less inertia, more likely to change motion

8. Law of Equilibrium (N1L)
The sum of all the forces acting on an object equals zero.
Object may be at rest or moving at constant velocity

9. Example
Box at rest on a table

10. Example Force of table pushing up equals weight of box pushing down
Force of table pushing up – weight of box = 0

11. Normal Force, FN
Force exerted by a surface whenever an object is in contact with a surface
Normal force is always perpendicular to the surface

12. Force Diagram Also called Free Body Diagram
Shows all forces acting on an object
Arrows originate on object and point in the direction that the force is acting
Weight
Table Pushing Up

13. Example
A 50N chandelier hangs from ceiling by a single cord. Draw a free body diagram and determine the tension in the cord.
ΣF = 0
Tension, T
T - w =0
T = w
Weight, w
T = 50N

14. Another Example ΣF = 0 T – w – F =0 T = w + F T = 50N + 20N T = 70N
A 50N object hangs from ceiling by a single cord. A student pulls down on the object by 20N. Draw a free body diagram and determine the tension in the cord.
ΣF = 0 T
T – w – F =0
T = w + F
w = 50N
F = 20N
T = 50N + 20N
T = 70N

16. Newton’s 2nd Law (N2L)
The acceleration of an object is directly proportional to the net force acting on it, and inversely proportional to the mass of the object

17. N2L

18. N2L Example
A 5kg box is being pulled by a 10N force. What is the acceleration of the box?
5kg
F = 10N

19. N2L Example
A 5kg box is being pulled by 2 forces as shown. What is the acceleration of the box?
F1 = 10N
F2 = 5N
5kg

20. Weight, w
Force due to gravity, Fg

22. Friction, Ff Force that opposes motion.
Always in the direction opposite to motion
Friction v

23. Types of Friction Static Friction Kinetic Friction
Friction when an object is stationary, not moving
Must be overcome for an object to start moving
Kinetic Friction
Friction when an object is moving
Always less than static friction

24. Static Friction
A student exerts a force of 100N against a crate resting on the floor. If the crate does not move, what is the force of friction? Ff
F=100N

25. Kinetic Friction
A student exerts a force of 90N to keep a crate sliding across a level floor at a constant speed. What is the force of friction? v Ff
F=90N

26. Another Example
A 2000kg car is accelerating at a rate of 3m/s2. If the engine is exerting a force of 8000N, what is the force of friction? What type of friction is this? Ff
F=8000N a

27. Another Example
Kinetic Friction Ff
F=8000N a

28. One More Example
A 75kg skydiver is falling at his terminal velocity. What is the force of the air resistance? What type of friction is this? Ff
w = mg

29. One More Example Ff
Kinetic Friction
w = mg

31. Friction
What factors affect friction?
Type of material
Weight Ff F

32. Friction
Why does surface area not matter? FN Ff F Fg

33. Normal Force, FN
Force exerted by a surface whenever an object is in contact with a surface
Normal force is always perpendicular to the surface FN Fg

34. Friction The frictional force is due to a surface acting on an object
The frictional force is dependent on the force exerted by the surface
The frictional force is proportional to the Normal Force

35. Friction Ff=μFN Coefficient of Friction, μ
Factors in the type of material/surface
Two Types
μs, coefficient of static friction
μk, coefficient of kinetic friction

36. Coefficient of Friction,μ

37. Example
If it takes a force of 50N to start a 100N crate moving across a level floor, what is the coefficient of friction? FN Ff
F=50N
Fg=100N

38. Example
If it takes a force of 25N to keep a 100N crate moving across a level floor at constant speed, what is the coefficient of friction? FN Ff
F=25N
Fg=100N

39. One More Example
A block of copper sliding on steel requires a force of 25N to keep it moving across a level floor at constant speed, what is the weight of the block? FN Ff
F=25N Fg

40. One More Example FN Ff
F=25N Fg

42. Newton’s 3rd Law (N3L)
For every action there is an equal and opposite reaction
Forces occur in Action/Reaction pairs

43. N3L
When an object exerts a force on a second object, the second object exerts a force on the first object that is equal in magnitude and opposite in direction
Must be two objects acting on each other

44. N3L Example A Chandelier hanging from a ceiling
Wire pulls up on chandelier
Chandelier pulls down on wire

45. N3L Example

46. N3L Example

47. N3L Example
Action: Man pushes stroller
Reaction: Stroller pushes man

49. Forces Review
A student pushes a crate across a floor at a constant speed. What forces are acting on the crate? FN v Ff F Fg