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The Fundamental Forces of Nature
3U Physics
The 4 Forces
The 4 fundamental forces of nature are how the fundamental particles of the universe interact.
The 4 Forces
The 4 fundamental forces of nature are how the fundamental particles of the universe interact.
All other forces (e.g. friction) are macroscopic effects of these 4 forces.
The 4 Forces
These forces are all action-at-a-distance (or non-contact forces) and are:
The 4 Forces
These forces are all action-at-a-distance (or non-contact forces) and are:
• Gravity
The 4 Forces
These forces are all action-at-a-distance (or non-contact forces) and are:
• Gravity
• Electromagnetism
(Magnetism is a relativistic effect of electrical force.)
The 4 Forces
These forces are all action-at-a-distance (or non-contact forces) and are:
• Gravity
• Electromagnetism
• The Weak Nuclear Force
The 4 Forces
These forces are all action-at-a-distance (or non-contact forces) and are:
• Gravity
• Electromagnetism
• The Weak Nuclear Force
• The Strong Nuclear Force
The Strong Nuclear Force
Also called the strong interaction, this is the force that binds all those electrically-repelled positively-charged particles of atomic nuclei together.
It is the strongest of the 4 forces but only on the scale of atomic nuclei. At larger distances, it’s unobservable.
The Weak Nuclear Force
Also called the weak interaction, this is the force responsible for some nuclear phenomena like beta decay.
Electromagnetism
The force that exists between all charged particles, this force is responsible for most of the observed phenomena of everyday life (the normal force, friction, etc.).
Unlike the nuclear forces, it is infinite in range.
Gravity
The weakest of the 4 forces, it is nevertheless extremely important because it is also infinite in range and exists between all particles with mass.
Gravity
The weakest of the 4 forces, it is nevertheless extremely important because it is also infinite in range and exists between all particles with mass.
And because it is always attractive, it cannot be shielded against.
The proportionality
Fm m
rG 1 22
The proportionality
Note that this r is measured between the centres of mass of the objects
Fm m
rG 1 22
The proportionality
The proportionality
The proportionality
The proportionality
The proportionality
The proportionality
The proportionality
The proportionality
The equation
G was not determined until almost a century after Newton . . .
F Gm m
r
w here G N m kg
G
1 22
11 2 26 6 7 1 0. /
The torsion balance
To determine G, Cavendish used a torsion balance, a light, rigid rod suspended by a wire with two spheres attached to the ends of the rod. When the long rod twisted, the torsion of the wire exerted a torsional force proportional to the angle of rotation of the rod.
Direction
Note that gravity is always an attractive force. The direction of the force is on the line between the centres of mass of the masses.
Example
Estimate the magnitude of the gravitational attraction between you and the person sitting next to you.
Example
Estimate the magnitude of the gravitational attraction between you and the person sitting next to you.
m kg
m kg
r m
1
2
6 0
6 0
1 0
.
Example
Estimate the magnitude of the gravitational attraction between you and the person sitting next to you.
F G
m m
r
F N m kgkg kg
m
F N
G
G
G
1 22
11 2 226 6 7 1 0
6 0 6 0
1 0
0 0 0 0 0 0 0 2
. /( . )
.
m kg
m kg
r m
1
2
6 0
6 0
1 0
.
Example
Compare this 0.0000002 N to a 60-kg person’s weight:
F m g kg Ngms 6 0 9 8 6 0 02.
g
Note that
where mE is the mass of the Earth
mE = 5.98 x 1024 kg
and rE is the radius of the Earth
rE= 6.38 x 106 m
F Gm m
rG
m
rm m g
so g Gm
r
GE
E
E
E
E
E
2 2
2
g
This g can be determined for any mass.
For example,
where mMoon is the mass of the Moon
and rMoon is the radius of the Moon
2Moon
MoonMoon
r
mGg
