3.1 Force of Gravity
Gravitational Force
Gravitational Force
Gravitational Force - IMPORTANT
Gravitational Force - IMPORTANT
Gravitational Force
Force (gravitational) field is
Force (magnetic) field
GRAVITATIONAL field
The heavier the object, the bigger the
gravitational field.
GRAVITATIONAL field
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
Gravitational Field
GRAVITATIONAL FIELD STRENGTH
Fg
g=
m
Fg = gravitational force
m = mass
g = gravitational field strength
At Earth surface, g = 9.81 N/kg
Gravitational Force = Fg = WEIGHT
Near the surface of Earth – objects fall with the
same rate
Gravitational Force = Fg = WEIGHT
Rearranging the equation…
Fg
g=
m
Gravitational Force = Fg = WEIGHT
Rearranging the equation…
g·m = Fg
WEIGHT vs. MASS
Mass is the amount of matter in an object
WEIGHT vs. MASS
The mass of an object is the same
anywhere in the universe
= never changes for an object
WEIGHT vs. MASS
WEIGHT of an object will change
depending on:
WEIGHT of an object will change depending on:
the amount of the gravitational force that
is exerted on the object
WEIGHT = Fg on Earth vs Moon
WEIGHT vs. MASS
Mass units are: g or kg
Weight units are: Newtons
QUICK Check page 77
882N
64.9 kg
1.7 N/kg
HOMEWORK
Problems: worksheet #1
Page: 1
36
Measuring the Force of Gravity
The usual way is to balance the force of gravity with
another force acting upward (with springs or coils)
G = Universal Gravitational Constant
m1 and m2 = masses of bodies attracting each other
r = the distance between the centres of the two bodies
G = Universal Gravitational Constant
m1 and m2 = masses of bodies attracting each other
r = the distance between the centres of the two bodies
G = Universal Gravitational Constant =
6.67 x 10-11 Nm2/kg2
•determined by Henry Cavendish
HOMEWORK
Problems:
Read pages 79 - 80
Quick Check on pg 80
45
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
QUICK Check page 80
¼ Fg
1/9 Fg
4Fg
9 Fg
QUICK Check page 80
9.80 x
1.62 x
2
10
2
10
N
N
1.4 x 10-11 N
9.80 x
1.62 x
2
10
2
10
N
N
1.4 x 10-11 N
12
Force of Gravity vs. Distance from Centre
Force of Gravity (N)
10
8
6
4
2
0
0.157
0.0785
0.0523
0.0392
0.0314
Distance from Center of Earth (Mm) = r
Force of Gravity (N)
12
Force of Gravity vs. Distance
from Centre
10
8
6
4
2
0
6.37
12.74
19.11
25.48
31.85
Distance from Center of Earth (Mm) = r
Force of Gravity (N)
12
Force of Gravity vs. Distance
from Centre
10
8
6
4
r-1
r-2
(Mm-1)
(Mm-1)
0.1570
0.0785
0.0523
0.0246
0.0392
0.0314
2
0
6.37
12.74
19.11
25.48
31.85
Distance from Center of Earth (Mm) = r
6.16 x 10-3
2.74 x 10-3
1.54 x 10-3
9.86 x 10-4
Force of Gravity (N)
12
Force of Gravity vs. Distance
from Centre
10
8
6
4
r-2
(Mm-1)
(Mm-1)
0.1570
0.0785
0.0523
0.0246
0.0392
0.0314
2
0
6.37
12.74
19.11
25.48
31.85
12
Force of Gravity (N)
r-1
Force of Gravity vs. INVERSE
Distance from Centre
10
8
6
4
2
0
0
0.05
0.1
0.15
0.2
Distance from Center of Earth-1 (Mm) = r-1
6.16 x 10-3
2.74 x 10-3
1.54 x 10-3
9.86 x 10-4
Force of Gravity (N)
12
Force of Gravity vs. Distance
from Centre
10
8
6
4
r-2
(Mm-1)
(Mm-1)
0.1570
0.0785
0.0523
0.0246
0.0392
0.0314
2
0
6.37
12.74
19.11
25.48
31.85
12
Force of Gravity (N)
r-1
Force of Gravity vs. INVERSE
Distance from Centre
10
8
6
4
2
0
0
0.005
0.01
0.015
0.02
0.025
0.03
Distance from Center of Earth-1 (Mm) = r-1
6.16 x 10-3
2.74 x 10-3
1.54 x 10-3
9.86 x 10-4
12
Force of Gravity vs.
Distance from Centre
Force of Gravity (N)
Force of Gravity (N)
10
12
8
6
4
8
6
4
2
2
0
0
0
6.37
12.74
19.11
25.48
31.85
12
10
Force of Gravity (N)
10
8
6
4
2
0
0
0.05
0.1
0.15
Distance from Center of Earth-1 (Mm) = r-1
0.2
0.005
0.01
0.015
0.02
0.025
0.03
Distance from Center of Earth-1 (Mm) = r-2
Force of Gravity (N)
12
10
8
6
4
2
0
0
0.005
0.01
1 a)
Fg vs 1/r2 OR Fg vs r-2
0.015
0.02
0.025
0.03
Distance from Center of Earth-1 (Mm) = r-2
1 b)
y = Fg
x = 1/r2
m = 400 N·m2
b=0N
Force of Gravity (N)
12
10
8
6
4
2
0
0
0.005
0.01
0.015
0.02
0.025
0.03
Distance from Center of Earth-1 (Mm) = r-2
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
G =6.67 x 10-11 Nm2/kg2
m1(Earth) = 5.98 x 10 24 kg
m2(rock) = 1.00 kg
r (radius) = 6.38 x 10 6 m
Fg = 9.80 N
Force of Gravity (N)
12
10
8
6
4
2
0
0
0.005
0.01
0.015
0.02
0.025
0.03
Distance from Center of Earth-1 (Mm) = r-2
G =6.67 x 10-11 Nm2/kg2
m1(Earth) = 5.98 x 10 24 kg
m2(rock) = 1.00 kg
r (radius) = 6.38 x 10 7 m
HOMEWORK
Problems: worksheet #1
Page: 2 - 3
59
HOMEWORK
Page:
83 (textbook)
3.1 Review Questions
Problems:
all even
60