Newton's Laws for Kids - First Law
A Simple Explanation of Principles of Motion and Inertia
Newton's First Law of Motion
Slide a block across a table. When you push the block, it moves; when you stop pushing, it stops. This observation led Aristotle and other ancient Greeks to think that a force must be continually applied to keep an object moving. This idea is however wrong.
To understand why, think about an air hockey table. When the air is turned off, you must push the puck to keep it moving. When the table is turned on, there is very little friction. The pucks keep sliding without constantly being pushed. Notice that they slide at a constant speed in a straight line until some force changes their motion. Also notice that if a puck is at rest, it will not start moving until something, a force, pushes it.
Building on work by Galileo, Newton realized that friction is a force acting to stop moving objects. We must push something to keep it moving because otherwise frictional forces would slow it down. If frictional forces could be completely eliminated, an object would continue to move at a constant velocity in a straight line until some other outside force changed the object's velocity. If there were no other forces, the object would continue in its constant velocity straight line motion forever.
Newton also realized that if an object is at rest it will remain at rest until some outside force acts on it. Whether an object is at rest or moving an outside force is needed to change it motion.
In physics the word velocity includes both how fast something is moving (its speed), and the direction in which it is moving. Changing the velocity can mean changing the speed or changing the direction. Any change in an object's velocity is an acceleration. Acceleration is not just speeding up. It includes speeding up, slowing down, and changing direction.
Newton discovered that an object with no outside forces acting on it will move in a straight line at a constant velocity. An object that is at rest (which is a constant velocity of zero) will stay at rest unless an outside forces acts on it. Basically Newton's first law says that an outside force is required to change an objects velocity (accelerate it) in any way. An objects resistance to changing its velocity is called its inertia, so this law is sometimes called the inertial law.
Statement of Newton's First Law
The official words for Newton's first law state that an object at rest will remain at rest unless acted on by an outside force and an object in uniform straight line motion will stay in that uniform straight line motion unless acted on by an outside force. Do not memorize the words; understand what they mean.
Changing an objects velocity (accelerating it) in any way requires an outside force acting on an object. Newton's second law tells us how much force is needed.
Further Reading
Newton's Laws for Kids - Overview
Newton's Laws for Kids - Third Law
Newton's Laws for Kids - Gravity
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