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