Section 4 Universal Forces Key Concepts What force

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

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 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.