Physical Science 11.3 Speed and Velocity

Speed and Velocity Speed The ratio of the distance an object moves to the amount of time the object moves Units Measured in m/s or km/hr Average Speed Speed over the entire duration of a trip Instantaneous Speed Speed during a particular instant of time

Formula: Speed = Distance (d) Time (t) v = d / t Average Speed Formula: Speed = Distance (d) Time (t) v = d / t

Solving problems!! Ask yourself: What is the problem wanting me to solve for? Find and write the correct formula Substitute what your know into your formula Solve equation (Don’t forget units!!)

Average Speed Example #1: You travel 350 meters in 120 seconds. What is your average speed? 2.92 m/s Example #2: An object travels 100 meters in 4 seconds. What is the objects average speed? 25 m/s

Average Speed Example #3: A train is traveling 10 m/s in 240 seconds. How far did the train travel? 2,400 meters Example #4: How long did it take an object traveling at 25 m/s to go 2,000 meters? 80 seconds

How are instantaneous speed and average speed different? Instantaneous speed is measured at a particular instant in time Average speed is computed for the entire duration of a trip

Graphing Motion The slope of a line on a distance-time graph is speed The steeper the slope, the faster you are traveling The flatter the slope, the slower you are traveling

Graphing Motion Example #1 Each line (A and B) represent the motion of two objects. Which object is moving faster? Explain.

Graphing Motion Example #2 Which car has a faster speed: The big car or small car? Explain

Graphing Motion Example #3 Using this graph, what is the average speed of the Big Car and the average speed of the small car?

Different Motion Graph Examples

What is the slope of a distance vs. time graph? Speed!!!

Velocity Velocity: A description of both speed and direction of motion. Velocity is a vector Velocity has the same formula as speed, except we add a direction

Velocity Example #1 An automobile travels 2,500 m north along a straight road for 200 seconds. Calculate the velocity. Example #2 A jet liner passes over St. Louis travelling at 625 m/s, heading straight towards Kansas City, which is 235 meters away. How much time elapses before the aircraft passes over Kansas City if it maintains a constant velocity?

Looking at Graphs

Distance vs. Time Graphs Time always runs horizontally (the x-axis). The arrow shows the direction of time. The further to the right, the longer time from the start. Distance runs vertically (the y-axis). The higher up the graph we go, the further we are from the start.

What is the speed of the airplane? Speed = 1,200 km / 6 hrs = 200 km/hr Denver Phoenix

Different Slopes Speed = d / t = 0 km/2 hr = 0 km/hr Speed = d / t

What is the average speed? Below is a distance vs. time graph of my position during a race. What was my AVERAGE speed for the entire race? Average Speed = Total distance/Total time = 12 km/6 hr = 2 km/hr 12 km 6 hours

Question Below is a distance vs. time graph for 3 runners. Who is the fastest? Mr. HESS is the fastest. He completed the race in 3 hours

Each line represents a journey from home to the office. Here is another time-distance graph with 2 journeys represented on the same graph. Each line represents a journey from home to the office. The red line is Dave’s journey. The green line is Mike’s journey. 1000 m 500 m 5 10 15 1500 m How long after Dave does Mike leave? 3 seconds Distance After how many seconds does Mike catch up? 7 seconds How far from home are they then meet? 500 meters What is the speed of each person? Dave (Red): 70 m/s Mike (Green): 125 m/s Time (sec)

11.3 Assessment Questions Question #1 How are speed and velocity different? Velocity is speed with direction Question #2 What shows the speed on a distance-time graph? The slope

11.3 Assessment Questions Question #3 How can two or more velocities be combined? Vector addition

11.3 Assessment Questions Question #4 What is going on in each section of the following distance-time graph?