# Motion and Forces

Determining the Net Force If you have been reading through Lessons 1 and 2, then Newton's first law of motion ought to be thoroughly understood. An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. In the statement of Newton's first law, the unbalanced force refers to that force that does not become completely balanced (or canceled) by the other individual forces. If either all the vertical forces (up and down) do not cancel each other and/or all horizontal forces do not cancel each other, then an unbalanced force exists. The existence of an unbalanced force for a given situation can be quickly realized by looking at the free-body diagram for that situation.

Motion: Introduction Motion is one of the key topics in physics. Everything in the universe moves. It might only be a small amount of movement and very very slow, but movement does happen. Don't forget that even if you appear to be standing still, the Earth is moving around the Sun, and the Sun is moving around our galaxy. The movement never stops. Motion is one part of what physicists call mechanics. Average vs. Instantaneous Speed During a typical trip to school, your car will undergo a series of changes in its speed. If you were to inspect the speedometer readings at regular intervals, you would notice that it changes often. The speedometer of a car reveals information about the instantaneous speed of your car.

The Energy Story - Introduction Energy is one of the most fundamental parts of our universe. We use energy to do work. Energy lights our cities. Energy powers our vehicles, trains, planes and rockets. A bowling ball and feather fall in world's biggest vacuum chamber You probably already know how this one ends, but that doesn't make watching it play out any less spectacular. [optional caption text here] Image: [name here]/Shutterstock

The Meaning of Force A force is a push or pull upon an object resulting from the object's interaction with another object. Whenever there is an interaction between two objects, there is a force upon each of the objects. When the interaction ceases, the two objects no longer experience the force. Forces only exist as a result of an interaction. Contact versus Action-at-a-Distance Forces For simplicity sake, all forces (interactions) between objects can be placed into two broad categories: Galileo Drops the Ball - Virtual Experiment In around 1590 Galileo Galilei (1564-1642) climbed up the Leaning Tower of Pisa and dropped some balls to the ground. Two balls of different masses, but of similar shape and density that were released together hit the ground at the same time. Until then it was commonly believed that heavy things fall faster than light things. Many people still believe this, and casual observation of everyday phenomena often does tend to confirm this view. If you drop a brick and a feather at the same time the brick will probably hit the ground first.

Matter: Chemical vs. Physical Changes It is important to understand the difference between chemical and physical changes. Some changes are obvious, but there are some basic ideas you should know. Physical changes are usually about states and physical states of states. Chemical changes happen on a molecular level when you have two or more molecules that interact.

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