Motion 4.1 Speed and Velocity

4.1 Position, Speed and Velocity The term speed describes how quickly something moves. To calculate the speed of a moving object divide the distance it moves by the time it takes to move.

4.1 Position, Speed and Velocity The units for speed are distance units over time units. This table shows different units commonly used for speed.

4.1 Average speed When you divide the total distance of a trip by the time taken you get the average speed. On this driving trip around Chicago, the car traveled and average of 100 km/h.

4.1 Instantaneous speed A speedometer shows a car’s instantaneous speed. The instantaneous speed is the actual speed an object has at any moment.

Example Problem: A person runs 100 meters in 12 seconds. What is his speed in m/s? Start by writing the equation: Speed = distance / time or S = d / t Write what info you know d = 100 m t = 12 s S = ? Sub in the info you know S = 100 m / 12 s Calculate S = 8.3 m/s In correct sig figs: S = 8 m/s

Calculating Speed From a Graph You can obtain distance and time from the graph. Sub into equation and solve. Ex: S = d/t S = 50 m / 10 s S = 5 m/s

4.1 Velocity We use the term velocity to mean speed with direction. Velocity is usually defined as positive when moving forward (to the right from an outside observer), and negative when moving the opposite direction (to the left to an outside observer).

Using the formula with velocity gives you a change of position instead of distance.

EX: What is the velocity of a swimmer who swam north 110 m in 72 sec EX: What is the velocity of a swimmer who swam north 110 m in 72 sec.? v = 110m + North = 1.5 m/s North 72 s We use velocity to predict position. EX: If we know how fast and in what direction something is moving, we can predict where it will end up.

Distance-Time Graphs for Motion of Three Cars