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# Speed. Uniform Straight-Line Motion Suppose you are driving on a nearly empty freeway. You look at the speedometer and notice that the car's speed doesn't change much. If the car neither slows down nor speeds up, it is traveling at a constant speed. You can measure the speed over any distance int...

# Speed. Uniform Straight-Line Motion Suppose you are driving on a nearly empty freeway. You look at the speedometer and notice that the car's speed doesn't change much. If the car neither slows down nor speeds up, it is traveling at a constant speed. You can measure the speed over any distance interval. This motion, done under constant speed, is called uniform straight-line motion. ## Speed To describe how fast something is moving, you need to know how far it travels in a given period of time. Speed is the distance an object travels per unit of time. In SI units, the unit of speed is meters per second (m/s). ## Calculating Speed Any change over time is called a rate. If distance is considered as a change in position, then speed is the rate of change in position. Speed can be calculated using this equation: Speed = $\frac{distance}{time}$ In SI units, distance is measured in meters and time in seconds, so the SI unit for speed is meters per second (m/s). Sometimes, it's more convenient to express speed in other units, such as kilometers per hour (km/h). 1 km/h = 3.6 m/s ## Graphing Motion The motion of an object over a period of time can be represented on a distance-time graph. Time is plotted on the horizontal axis, and distance on the vertical axis. If the object moves with a constant speed, the increase in distance over equal time intervals is the same. Consequently, the line representing the object's motion is a straight line. From the graph of uniform straight-line motion, we understand that this motion has only one speed that remains constant throughout the motion. We say that this speed is constant (v = constant). ## Calculating Distance