1. Forces

2. What is a Force?
A force is a push or pull acting on an object that changes the motion of the object.
Force is a vector, so it requires a magnitude and direction

3. S.I. Unit of Force

4. How do Forces Combine? More than one force often acts on an object.
When all the forces acting on an object are added together, you determine the net force, FNET (or Resultant Force, FR) on the object.
An object with a net force more than 0 N on it will change its state of motion.

5. Forces in the Same Direction
When forces are applied in the same direction, they are added to determine the size of the net force.

6. Forces in Different Directions
When two forces act in opposite directions, you subtract the smaller force from the larger force to determine the net force.
The net force will be in the same direction as the larger force.

7. Balanced and Unbalanced Forces
When the forces on an object produce a net force of 0 N, the forces are balanced.
There is no change in the motion of the object.

8. Balanced and Unbalanced Forces
When the net force on an object is not 0N, the forces on the object are unbalanced.
Unbalanced forces produce a change in motion of an object.

9. Balanced and Unbalanced Forces

10. Example 1
UNC and NC State have a tug-of-war match with three people on each side. UNC’s players pull with a force of 12N, 30N, and 28N. NC State’s players pull with a force of 8N, 36N, and 40N. What is the Net Force?

11. Example 2
A dog is on a leash and the owner holds the dog back with a Force of 60N at a 40° angle. Determine the horizontal and vertical forces.

13. Inertia
Inertia measures the tendency of an object to resist changes in motion.
Galileo came up with the idea of inertia
Objects do not want their motion to change
Mass measures how much inertia an object has (More mass = More inertia)

14. Newton’s First Law of Motion (Law of Inertia)
If no unbalanced forces act on a moving object, then the object will continue to move with a constant velocity (constant speed in a straight line). If an object is at rest it will stay at rest.
Newton took his concept of forces and combined it with Galileo’s idea of inertia

15. Warm-up
What is the difference between mass vs. weight?

16. Types of Forces
Contact Force – Forces that act through direct contact between two objects
Applied Forces, Friction
Long Range Forces – Forces that can act over distances
Gravity, Electromagnetic Force (EMF)

17. A force that is applied to an object by a person or another object.

18. Force of Thrust
Thrust is a mechanical force generated by the engines to move an aircraft or rocket through the air
Drag: a force exerted on an object moving through a fluid (like air)

19. A force that attracts any objects with mass.

20. Newton’s Second Law of Motion
Newton discovered the idea of a Force
He found the Force is proportional to the Acceleration of an object (more Force = more Acceleration)
He found the Force is proportional to the mass of the object (more mass = more force needed).

21. Newton’s Second Law of Motion

22. Example 1
What is the net force required to accelerate a car at a rate of 2m/s² if the car has a mass of 3000 kg?

23. Example 2
A 10,000 g bowling ball would require what force to accelerate down an alleyway at a rate of 3m/s²?

24. Example 3
What is the mass of a falling rock if it produces a force of 147 N?

25. Example 4
How fast will a 1500 kg car accelerate if it produces 5000 N of force?

26. Mass and Weight
Mass is amount of matter that an object possess. Mass does not change with location.
Weight is the gravitational force that a large body (such as a planet) exerts on another object.

27. Weight Weight is a Force! It is measured in Newtons.
Weight = Mass x Acceleration due to Gravity (Newton’s 2nd law)
W = mg
Weight does change with location! (Acceleration due to gravity, “g”, will change with location)

28. Newton’s Law of Universal Gravitation

29. Example 1
Two spherical objects have masses of 200 kg and 500 kg. Their centers are separated by a distance of 25 m. Find the gravitational attraction between them.

30. Example 1
Two spherical objects have masses of 200 kg and 500 kg. Their centers are separated by a distance of 25 m. Find the gravitational attraction between them.

31. Example 2

32. Example 2

33. Example 3
Two spherical objects have equal masses and experience a gravitational force of 25 N towards one another. Their centers are 36 cm apart. Determine each of their masses.

34. Example 3
Two spherical objects have equal masses and experience a gravitational force of 25 N towards one another. Their centers are 36 cm apart. Determine each of their masses.

35. Warm-up
*Get out your HW (Newton’s 2nd Law Practice) I am coming around to check it is complete!
An object in free fall has a mass of 2000g. What is the net force the object experiences?
Reminders
Projectile Motion Quiz Tomorrow!
Unit 2 Test Corrections Due (11/19)

36. Newton’s Second Law of Motion HW Answers
F=132 N
F=9,800 N
a=10 m/s^2
F=3000 N
F=250 N
m=50 kg
m=5 kg; m=3.75 kg
a=4 m/s^2

37. Newton’s Second Law of Motion HW Answers
8. a=4 m/s^2 9. a=4 m/s^2 10. F=1200 N m/s^ m/s^2 (free fall…falling due to the force of gravity) 13. F=294 N; Net Force=244 N due to air resistance

38. Air Resistance
Free Fall – The situation where gravity is the only force acting on an object (Assume No Air Resistance)
Air Resistance – The force the air applies on a moving object. It attempts to slow down falling objects (similar to friction)
Objects in a vacuum chamber do not experience Air Resistance because there is no air!

39. Bowling Ball & Feather Drop

40. Projectile Motion Quiz Review…

41. The forces that surfaces exert to prevent solid objects from passing through each other.
Always perpendicular to the surface the object is on.

42. The force exerted by a rope, cable, chain, etc.
The force exerted by a rope, cable, chain, etc.
Tension always acts away from the object along the rope

43. What Causes Friction?
Friction is the force that opposes the motion between two surfaces that touch.
The surface of any object is rough.
Even an object that feels smooth is covered with tiny hills and valleys.
The contact between the hills of valleys of two surfaces causes them to stick, resulting in friction.

44. What Causes Friction? The amount of friction depends on:
Roughness of the surfaces
Force pushing the surfaces together

45. The force generated by two surfaces that contact and slide against each other.
Friction causes an acceleration (slowing down the motion of an object)
Friction always acts parallel to the surface in the opposite direction of motion
Friction resists motion.
Friction produces heat.

46. Kinetic Friction A force that acts between moving surfaces. Examples
Sliding Friction: pushing an object across a surface
Rolling Friction: between wheels and a surface
Fluid Friction: opposes the motion of objects traveling through a fluid (air or water)

47. Static Friction A force that keeps an object at rest.
It must be overcome to start moving the object. Once the object is in motion, it experiences kinetic friction.

48. Reducing Friction Use wheels or rollers instead of pushing an object
Lubricants - produce a smooth layer between the surfaces (Ex. Motor oil, wax, grease)
Sanding the Surfaces – smooth the surfaces

49. Extra Below that we haven’t discussed in class..

50. Solving Net Force Problems
Make a Free Body Diagram (Label all forces and the acceleration (for direction) of the object)
Use the following Equations to solve for the unknown
FNET (x-direction) = max
FNET (y-direction) = may
f = m Fn

51. Calculating the Frictional Force
Friction depends on:
Forces acting between the surfaces (Normal Force)
Nature of the surfaces (Coefficient of Friction)

52. Calculating the Frictional Force
Normal Force (FN) – Force that acts perpendicular to the surface and away from the surface
The Normal Force (FN) is usually found by summing the forces in the y-direction

53. Calculating the Frictional Force
Coefficient of Friction (m) – describes how rough/smooth the surfaces are
Rough Surfaces – High value of m m > 0.5
Smooth Surface – Low value of m m < 0.5

54. Calculating the Frictional Force
The frictional force is the product of the coefficient of friction and the normal force
Frictional force = (Coefficient of Friction) x (Normal Force)
f = m FN

55. Springs
Hooke’s Law – the Force of a Spring is proportional to the distance from the equilibrium position
FS = kd
FS = Restoring Force of the Spring
d = displacement from the equilibrium position
k = spring constant for that particular spring

56. Newton’s Laws-Recap
1st Law: (inertia: objects tend to do what they are doing)
cannon ball will rest until a force is put on it
ball will roll straight until ramp puts a force on it
2nd Law: (f = m x a)
greater force put on ball accelerates it more
greater mass of ball but greater force on water
3rd Law: (every action has an equal but opposite reaction)
ball moves right, cannon recoils left
ball move down, water splashes up
Newton's Laws - YouTube