1. Section 4
Universal Forces

2. Key Concepts What force can attract and repel?
What force holds the nucleus together?
What is Newton’s law of universal gravitation?

3. 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

4. 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.

5. Nuclear Forces
Two forces, the strong nuclear force and the weak nuclear force, act within the nucleus to hold it together.

6. 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.

7. Strong Nuclear Force Strong Force + + + + Electromagnetic

8. Weak Nuclear Force
An attractive force that acts only over a short range.
Responsible for radioactive decay of an atom

9. 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.

10. Gravitational Force x y x y y x
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.

11. 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.

12. 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
Motion of Moon

13. 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.