Science Chapter Three Section Five "Gravity-A Familiar Force"

Gravity is the force that exerts a force on every other object in the universe. An example of this cold be the gravitational force of attraction between your hand and your pencil. When you let go of the pencil it doesn't stay in your hand because the gravitational force of Earth is much stronger that the gravitational force of your hand. On Earth gravity exerts a downward force on your body.

Depending on the mass of objects and how far away they are from one another will determine the amount of gravitational force there is. That is why a pencil will drop to the Earth when you let it go. The Earth is larger than your hand so the pencil is drawn to the Earth instead of your hand. 

The closer an orbiting object is to Earth the greater gravitational force it will have. The farther away it is, the less effect gravity has on it. The amount of gravitational force between two objects depends on their mass and distance.

The measure of the force of gravity on an object is the object's weight. When you hear or read the term weight it is used in reference to the gravitational force between Earth and a bod at Earth's surface. It is not the same as mass. Weight is a force and is measured in Newtons.

A person who weighs 480 Newtons on Earth would only weigh about 80 Newtons on the moon because the Earth has more mass than the moon so it has more of a gravitational force. That same person would have the same mass whether they were on the moon or on Earth.

Scales use the principle of balanced forces to measure how uch something weighs.

Science Section 3-4 "Connecting Motion with Forces"

A force is a push or a pull one body exerts on another. Examples of force; jump, open or close a door, picking up a pencil

Force does not always change velocity. Forces that are equal in size and opposite in direction are balanced forces.

A net force on an object always changes the velocity of the object. Objects accelerate in the direction of the greater force. A net force acting on an object will change its speed, direction, or both.

Inertia is the tendency of an object to resist any change in its motion. If an object is moving, it will keep moving at the same speed and in the same direction unless an unbalanced force acts on it. The more mass an object has, the greater its inertia is.

Sir Isaac Newton (1642-1727) was able to state laws that describe the effects of forces. Newton's first law of motion is that an object moving at a constant velocity keeps moving at that velocity unless a net force acts on it. If an object is at rest, it stays at rest unless a net force acts on it.  This law is sometimes called the law of inertia.

Friction is the force that opposes motion between two surfaces that are touching each other. The amount of friction depends on two factors- the kinds of surfaces and the force pressing the surfaces together.

Science Section 3-3:  A Crash Course in Safety

-Through experiments called crash tests, and by studying actual car collisions, scientists have learned what happens to people in accidents.

-Researchers put lifelike dummies in cars and crash the cars into each other or into crash barriers.

-When a car traveling about 50 k/h collides head on with something solid, the car crumples, slows down , and stops within approximately 0.1 second,

-any passenger not wearing a set belt continues to move forward at the same speed the car was traveling because of inertia.

-The person putting on a seat belt will be attached to the car and will slow down as the car slows down.

-The force needed to slow a person from 50 km/h to zero in 0.1 second is equal to 14 times that person's weight.

-The belt gives a little as it restrains the person, increasing the time it takes.