Section 4
Universal Forces
Key Concepts
• What force can attract and repel?
• What force holds the nucleus together?
• What is Newton’s law of universal
gravitation?
• Four different forces exist throughout the
universe.
• These forces are known as universal
forces.
• The four universal forces are the:
electromagnetic, strong nuclear,
weak nuclear, and gravitational
Electromagnetic Forces
• Electromagnetic force is associated with
charged particles.
• Electric force and magnetic force are the
only forces that can both attract and repel.
• Electric Forces: Electric forces act between
charged objects or particles such as electrons
and protons.
• Magnetic Forces: Magnetic forces act on
certain metals, on the poles of magnets, and on
moving charges.
Nuclear Forces
• Two forces, the strong nuclear force
and the weak nuclear force, act within
the nucleus to hold it together.
Strong Nuclear Force
• A powerful force of attraction that acts only
on the neutrons and protons in the
nucleus, holding them together.
• Although this force acts over only
extremely short distances, it is 100 times
stronger than the electric force of repulsion
at these distances.
Strong Nuclear Force
Strong Force
+
+
Electromagnetic
Force
Electromagnetic
Force
+
+
Weak Nuclear Force
• An attractive force that acts only over a
short range.
• Responsible for radioactive decay of an
atom
Gravitational Force
• an attractive force that acts between any
two masses.
• Newton’s law of universal gravitation
states that every object in the universe
attracts every other object.
• Earth exerts a force on an apple, and the
apple exerts an equal force on Earth.
Gravitational Force
x
y
x
y
x
y
Gravitational force of attraction of mass X by mass Y
Gravitational force of attraction of mass Y by mass X
Gravitational force depends upon mass and distance. A
Two masses, X and Y, attract each other. B The larger
mass of X results in a larger gravitational force. C
Increasing the distance between the masses significantly
reduces the gravitational force.
Gravity Acts Over Large
Distances
• The gravitational force between two
objects is proportional to their masses and
decreases rapidly as the distance between
the masses increases.
• Gravity is the weakest universal force, but
it is the most effective force over long
distances.
The Earth, Moon, and Tides
• Earth’s gravitational
attraction keeps the moon
in a nearly circular orbit
around Earth.
• Centripetal force is a
center-directed force that
continuously changes the
direction of an object to
make it move in a circle.
• The gravitational pull from
the moon produces two
bulges in Earth’s oceans.
• The Tides
Earth
gravity
Moon
Reviewing Concepts
• 1. Which universal force can repel as well
as attract?
• 2. Which universal force acts to hold the
nucleus together?
• 3. State in your own words what is mean
by Newton’s law of universal gravitation.