Frames of Reference

• An object’s motion is described relative to a frame of reference. (Imagine yourself describing the motion of a car to someone standing on the sidewalk; you’d use the sidewalk as your frame of reference.)
• A frame of reference is a set of objects that are fixed in space. (The sidewalk, the trees, the buildings all make up a fixed frame of reference for the car.)
• The most common frame of reference is the Earth. (We tend to think of ourselves as stationary and describe everything else’s motion relative to us.)
• Other frames of reference include the sun, the moon, and the stars. (When describing the motion of a spacecraft, for example, we might use the sun as our frame of reference.)

Motion in a Straight Line

• Motion in a straight line is one-dimensional motion. (Imagine a car moving down a straight road; its motion can be described entirely by its position along the road.)
• The displacement of an object is the distance between its initial and final positions. (If a car starts at point A and ends at point B, its displacement is the distance between A and B.)
• The velocity of an object is the rate at which its displacement changes. (If a car travels 100 miles in 2 hours, its average velocity is 50 miles per hour.)
• The acceleration of an object is the rate at which its velocity changes. (If a car starts from rest and reaches a speed of 60 miles per hour in 10 seconds, its acceleration is 6 miles per hour per second.)

Uniform Motion

• Uniform motion is motion with constant velocity. (A car driving at a steady 50 miles per hour is in uniform motion.)
• The acceleration of an object in uniform motion is zero. (If a car’s velocity is constant, its acceleration is zero.)
• The displacement of an object in uniform motion is equal to the product of its velocity and the time interval. (If a car travels at 50 miles per hour for 2 hours, its displacement is 100 miles.)