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3.1 Force of Gravity.

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Presentation on theme: "3.1 Force of Gravity."— Presentation transcript:

1 3.1 Force of Gravity

2 Words to Know for Chapter 3.1
A force Force Due to Gravity Gravitational Field Strength Gravity Inverse Square Law Mass Newton’s Law of Universal Gravitation Universal Gravitational Constant Weight

3 What causes an object to accelerate?
What causes an object to change direction? What causes an object to accelerate? What causes an object to change its state?

4 FORCE A pull or a push exerted on an object which may change the object’s state of rest, motion, and its shape

5 What kind of force is the ball experiencing in each picture?
Change of direction Change of velocity (stop moving) Change of velocity (moving from rest)

6 What kind of force is the ball experiencing in each picture?
In each picture, how do you know the ball is experiencing at least one force? Change of direction Change of velocity (stop moving) Change of velocity (moving from rest)

7 The ball’s speed and directions change
What kind of force is the ball experiencing in each picture? Change of direction Change of velocity (stop moving) The ball’s speed and directions change Change of velocity (moving from rest)

8 The ball’s speed and directions change
In the each case: FORCE changes VELOCITY with respect to TIME Change of direction Change of velocity (stop moving) The ball’s speed and directions change Change of velocity (moving from rest)

9 Representation of FORCE
SI unit: N (newton) = = kg ∙ m/s2 The amount of force that is acting on a 1 kg mass and producing acceleration of 1 m/s2

10 FORCE can act through contact or at a distance
Contact Force Field Force Name 2 of examples of each! Physical contact between two objects No physical contact between two objects

11 FORCE can act through contact or at a distance
Contact Force Field Force Pushing an object Pulling an object Gravitational Force Magnetic Force Electrical Force

12 WHAT FORCES ACT ON A TOY CAR?
1. What happened to the car before the collision (changes in motion?) 2. What happened to the car during the collision? 3. What happened to the car after the collision (changes in motion?) How can you describe the forces acting on the car without using words?

13 REMEMBER Diagrams will be studied in detailed in the next section
FORCE DIAGRAMS Diagrams will be studied in detailed in the next section Effect of force depends on magnitude and direction = FORCE is a vector REMEMBER

14 FORCE can act through contact or at a distance
Field Force No physical contact between two objects

15 Gravitational Force Any two bodies in the universe exert a gravitational force on each other

16 Gravitational Force The amount of force depends on how massive the bodies are and how apart they are!

17 Gravitational Force - IMPORTANT
1.It can’t be “shut off”

18 Gravitational Force - IMPORTANT
2. It is always an ATTRACTIVE force, never REPULSIVE

19 It creates gravitational field around an object
Gravitational Force It creates gravitational field around an object

20 Force (gravitational) field is
An area where force is exerted

21 Force (magnetic) field
Example:

22 Depends on the mass of an object
GRAVITATIONAL field Depends on the mass of an object The heavier the object, the bigger the gravitational field.

23 GRAVITATIONAL FIELD STRENGTH
A smaller object is then attracted to the centre of the Earth because Earth has larger mass The force within the gravitational field is called GRAVITATIONAL FIELD STRENGTH

24 Gravitational Field How can you see it?

25 GRAVITATIONAL FIELD STRENGTH
g = Fg m Fg = gravitational force m = mass g = gravitational field strength At Earth surface, g = 9.81 N/kg

26 Gravitational Force = Fg = WEIGHT
Near the surface of Earth – objects fall with the same rate

27 Gravitational Force = Fg = WEIGHT
Rearranging the equation… g = Fg m

28 Gravitational Force = Fg = WEIGHT
Rearranging the equation… g·m = Fg

29 Mass is the amount of matter in an object
WEIGHT vs. MASS Mass is the amount of matter in an object

30 The mass of an object is the same anywhere in the universe
WEIGHT vs. MASS The mass of an object is the same anywhere in the universe = never changes for an object

31 WEIGHT of an object will change depending on:
WEIGHT vs. MASS WEIGHT of an object will change depending on:

32 WEIGHT of an object will change depending on:
the amount of the gravitational force that is exerted on the object

33 WEIGHT = Fg on Earth vs Moon

34 Weight units are: Newtons
WEIGHT vs. MASS Mass units are: g or kg Weight units are: Newtons

35 QUICK Check page 77 882N 64.9 kg 1.7 N/kg

36 HOMEWORK Problems: worksheet #1 Page: 1

37 Measuring the Force of Gravity
The usual way is to balance the force of gravity with another force acting upward (with springs or coils)

38 Newton’s Law of Universal Gravitation
Sir Isaac Newton realized this…

39 Newton’s Law of Universal Gravitation
The Force of Gravity is an universal force which is exerted by any two objects on each other

40 Newton’s Law of Universal Gravitation
It’s the same for planets or apples...

41 Newton’s Law of Universal Gravitation
It’s a MUTUAL force between two objects

42 Newton’s Law of Universal Gravitation
It varies as the inverse of the square of the distances between two objects G = Universal Gravitational Constant m1 and m2 = masses of bodies attracting each other r = the distance between the centres of the two bodies

43 Newton’s Law of Universal Gravitation
DIRECTLY Proportional? INVERSELY Proportional? G = Universal Gravitational Constant m1 and m2 = masses of bodies attracting each other r = the distance between the centres of the two bodies

44 G = Universal Gravitational Constant
6.67 x Nm2/kg2 determined by Henry Cavendish

45 HOMEWORK Problems: Read pages Quick Check on pg 80

46 Earth’s Gravitational Field Strength
This means... The Gravitational Field Strength of Earth, g, depends ONLY on the mass of Earth AND the distance, r, from the centre of Earth to the centre of mass of the object that has mass m

47 QUICK Check page 80 ¼ Fg 1/9 Fg 4Fg 9 Fg

48 QUICK Check page 80 9.80 x 102 N 1.62 x 102 N 1.4 x N

49 Use MS Excel to draw the graphs
9.80 x 102 N 1.62 x 102 N 1.4 x N

50 Which of the Graphs does your
graph most resemble? Force of Gravity (N) Distance from Center of Earth (Mm) = r

51 Distance from Center of Earth (Mm) = r
Force of Gravity (N) Distance from Center of Earth (Mm) = r

52 Distance from Center of Earth (Mm) = r
0.1570 0.0246 0.0785 6.16 x 10-3 0.0523 2.74 x 10-3 0.0392 1.54 x 10-3 0.0314 9.86 x 10-4 Force of Gravity (N) Distance from Center of Earth (Mm) = r

53 Distance from Center of Earth-1 (Mm) = r-1
0.1570 0.0246 0.0785 6.16 x 10-3 0.0523 2.74 x 10-3 0.0392 1.54 x 10-3 0.0314 9.86 x 10-4 Force of Gravity (N) Fg vs r-1 Force of Gravity (N) Distance from Center of Earth-1 (Mm) = r-1

54 Force of Gravity vs. INVERSE Distance from Centre
(Mm-1) r-2 0.1570 0.0246 0.0785 6.16 x 10-3 0.0523 2.74 x 10-3 0.0392 1.54 x 10-3 0.0314 9.86 x 10-4 Force of Gravity (N) Force of Gravity vs. INVERSE Distance from Centre Fg vs r-2 Force of Gravity (N) Distance from Center of Earth-1 (Mm) = r-1

55 Fg vs r-2 Fg vs r-1 Force of Gravity (N) Force of Gravity (N)
Distance from Center of Earth-1 (Mm) = r-2 Fg vs r-1 Force of Gravity (N) Distance from Center of Earth-1 (Mm) = r-1

56 Fg vs r-2 Fg = 400 x 1/r2 y = mx + b 1 a) Fg vs 1/r2 OR Fg vs r-2 1 b)
Force of Gravity (N) Fg = 400 x 1/r2 Distance from Center of Earth-1 (Mm) = r-2 1 a) Fg vs 1/r2 OR Fg vs r-2 1 b) y = Fg x = 1/r2 m = 400 N·m2 b = 0 N y = mx + b

57 Fg vs r-2 Force of Gravity (N) Fg = 400 x 1/r2 4.90 = 400/r2 r = 9.04 Mm Distance from Center of Earth-1 (Mm) = r-2 G =6.67 x Nm2/kg2 m1(Earth) = 5.98 x kg m2(rock) = 1.00 kg r (radius) = 6.38 x 10 6 m Fg = 9.80 N 1. First calculate Fg of the rock on the Earth’s surface 2. Then, use that Fg/2 and the graph above to look up the distance

58 Fg vs r-2 Force of Gravity (N) Fg = 400 x 1/r2 Fg = N Distance from Center of Earth-1 (Mm) = r-2 G =6.67 x Nm2/kg2 m1(Earth) = 5.98 x kg m2(rock) = 1.00 kg r (radius) = 6.38 x 10 7 m

59 HOMEWORK Problems: worksheet #1 Page: 2 - 3

60 HOMEWORK Page: 83 (textbook) 3.1 Review Questions Problems: all even

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