1. Universal Forces Section 3.5

2. What do we know? What force keeps the Earth moving around the sun?
What do lightening and static cling have in common?

3. Common Forces Give a reason why each of the following happens.
A ball that rolls off the table falls to the floor.
A cabinet door with a magnet on the latch stays closed.
Clothes from the dryer cling to each other.

4. Universal Forces Forces act upon you no matter where you are.
Even if you traveled to a distant planet, four of the universal forces that you feel on Earth would be present there.
Universal forces:
Gravitational
Electromagnetic
Strong Nuclear
Weak Nuclear
Table 3.1 on page 73 shows the strengths of these universal forces.

5. Gravitational Force
Objects of any size are pulled toward each other by gravitational force.
Don’t notice it between small objects because it is the weakest of the universal forces.
Strength of the gravitational force depends on the amount of mass in an objects and the distance between the objects.
Planets, stars, and galaxies are so large that their gravitational force affects other objects in space.

6. Gravitational Force in Space
Gravity keeps the planets in orbit around the sun.
It also keeps the stars in orbit in their galaxies.
Gravitational force of the Earth on the Moon keeps the Moon in its orbit around the Earth.
Gravitational force of the Moon on the Earth causes the ocean tides to rise and fall.

7. Gravity and the Solar System
Newton proposed that the sun exerts a gravitational force on the Earth and its Moon, but also on the other planets.
Because of this gravitational force, the Earth and other planets in the solar system orbit around the sun.
Each planet moves in an elliptical orbit around the sun.

8. What do you see? Look at Figure 3.17 on page 74:
Which planet’s surface gravity is more than twice as great as the Earth’s gravity?
Which planets are between Earth and the sun?
Which planets are beyond Earth?
What would happen if a spaceship came very close to a planet? Why?
On what objects do the planets exert a gravitational force like the sun does on them?

9. Law of Universal Gravitation
Newton’s law of universal gravitation states that every object in the universe attracts every other object.
Gravitational force between two objects depends on the mass of each object and the distance between them.
Gravitational force between ordinary objects on Earth is extremely small (too small to measure).
Gravitational force between earth and moon is easily measured because they both have a large mass.
Gravitational force is also affected by distance. (Saturn doesn’t affect gravitational pull on your textbook because its far away.)
But Earth’s gravitational pull is very noticeable.

10. The Earth-Moon System
Newton realized that the gravitational force also applies to the moon.
Newton also predicted that objects on the moon weigh less than they do on Earth due to the fact that the moon is 81 times less massive.
Earth and Moon are affected by each other’s gravitational force.
As the moon orbits the Earth, the Moon pulls on surface water of the Earth.
Gravitational pull of the Moon causes the ocean waters nearest the moon to bulge (tides).

11. What do you think? Looking at Figure 3.18 on page 76:
Why are the tides stronger when the sun and the moon are in line?
What causes the neap tides?

12. Electromagnetic Forces
Electromagnetic forces are really two different forces that are closely related:
Electric Force
Magnetic Force
Both of these forces may attract or repel.

13. Electromagnetic Forces
Electric Force:
Exists between charged particles.
Objects with opposite charges attract; objects with like charges repel.
Much stronger than gravitational force.
You are constantly affected by this force.
Causes some objects to come together and some objects to stay apart.
Holds together the particles that make up matter.
Causes static, creates lightening, and creates electricity.

14. Electromagnetic Forces
Similar to electric force.
Acts between two magnets
Attracts or repels that same as an electric force.
Magnetic force and electric forcer interact in motors and other devices.
For example, the carbon granules in the receiver of your telephone change the sound of your voice into electric signals. These electric signals travel through the wires from your home to that of your friend where a magnet in your friend’s telephone receiver changes the electric signals back into the sound of your voice.

15. Nuclear Forces All matter is made up of tiny particles called atoms.
Atoms are made up of 3 subatomic particles: proton, neutron, and electron. Protons and Neutrons are in the nucleus of the atom.
Nuclear Forces- forces that act on the nucleus.

16. Nuclear Forces Strong Nuclear Force: Holds the nucleus together.
Without this force, protons and neutrons would fly apart.
These forces are very strong.
Acts only over extremely short distances.
If the forces are broken, huge amounts of energy are released, which can be used to produce electricity or a nuclear explosion.

17. Nuclear Forces Weak nuclear forces:
Slightly weaker than the strong force.
Acts over a short range, but affects all particles, not just protons and neutrons.
Can cause a neutron to decay into a proton, an electron, and a third particle called an antineutrino.
All particles that decay in this way are called radioactive.
Measurement of radioactive decay can be used to reveal the age of a fossil.
Some kinds of nuclear decay release harmful radiation that can affect people and the environment.

18. What have we learned? Name the four universal forces.
Explain the roles of the universal forces when you heat food in an electric oven.
State why the gravitational force is important to every object in the universe.
List some activities taking place in your classroom. Classify each activity according to the universal force that applies to it.