4.05 Determine factors that affect motion including: Forces and Motion 4.05 Determine factors that affect motion including: • Force • Friction. • Inertia. • Momentum

Position and Direction The motion of any object can be described by its position and how that position changes over time. If an object moves slowly, its position will also change slowly.

Position is the location of an object. You can describe the motion of an object by describing its original position and its final position. Motion can also be described using the directions north, south, east, and west. All motions can be described in terms of position and direction.

Position and direction are very helpful for describing motion Position and direction are very helpful for describing motion. However, motion also occurs over time. Speed is a measurement of distance over time. Velocity measures distance over time but also includes direction. For example, a soccer ball that moves 20 meters in 2 seconds is moving with a speed of 10 meters per second. The velocity of that ball is 10 meters per second east.

Describing Motion Scientists use many tools to measure motion. A compass will measure the direction of a motion. A meterstick will measure position. A stopwatch will measure time. Scientists can then calculate speed by dividing distance by time.

What is the speed of a baseball that moves 40 meters in 5 seconds? 8 meters per second. Why is a compass a useful tool for describing motion? It describes direction.

Forces Change Motion A force is a push or pull on an object. Forces cause objects to move or to stop moving. There are many kinds of forces. The motion of any object can be understood by looking at all of the forces that are acting on that object. Nothing can move without a force being applied to it.

The mass of an object tends to make the object resist being set into motion. That’s why objects with more mass are set into less rapid motion by a certain amount of force. The tendency of an object to resist a change in its state of motion is called the object’s inertia.

Galileo, an Italian scientist, figured out how inertia affects the motion of objects. He observed that pendulums swing back and forth to the same height. He reasoned that a ball rolling down one ramp and up another would roll to the same height on any ramp.

Just as it takes a force to set an object in motion, it also takes a force to slow or stop a coasting object. Without any such force, the object will coast forever in a straight line. Friction opposes the motion of one object moving past another. If the friction is taken away, no force is needed to maintain motion at a steady rate. An object’s inertia is all that is needed to keep it moving.

Forty-five years after Galileo died, Sir Isaac Newton published a complete description of the concept of inertia. This is Newton’s first law of motion: Objects at rest remain at rest and objects traveling at a steady rate in a straight line continue that way until a force acts on them.

Newton’s first law of motion -- the law of inertia -- tells us that the state of motion of an object does not change until a force is applied to it. That means, if an object is traveling at a steady rate in a straight line, it will continue to do so until a force is applied to it. Newton’s law also means that if the object is sitting at rest, it will continue to be at rest until a force is applied to it.

An object’s velocity changes if either its speed or its direction changes. There are two forces that can affect an object’s velocity: 1. Friction, a force that opposes the motion of one object moving past another. 2. Gravity, keeps the Earth moving in a circular path about the sun.

As long as an object travels in a straight line at a steady speed, its velocity is constant. Without the force of gravity, Earth would fly off in a straight line into deep space.