Forces and Motion

Speed and Velocity

Description

Equation

velocity = distance__________time

v =  d—t

1.1

Just for Fun!

The next time you take a trip in a car, note the odometer reading and the time on the car’s clock or your watch (or your iPhone) when you leave and the time and odometer reading when you arrive. Calculate the average speed you have traveled.

Distance Traveled = last odometer reading – initial odometer reading

= 11,795 miles – 11,441 miles = 354 miles

Elapsed Time = final watch reading – initial watch reading

= 2:35 pm – 7:15  am = 7 hours 20 minutes

Speed = distance traveled / elapsed time

= d/t = 354 miles / 7.33 hours = 48.3 miles / hour

Note: 20 minutes out of 1 hr (60 min) is 20/60 = 2/6 = 1/3 = 0.33 hr

So Speed = change in distance / change in time. For this reason, all…ALL speeds are AVERAGE SPEEDS. You may have stopped for gas, stopped at a stoplight, spent time cruising down the highway and slowed down in a residential neighborhood. It all gets averaged together. You must always figure in a change in time and therefore, a change in distance, so the equation is just s = d/t.

Speed is a scalar quantity; that is, it only has a magnitude or size. Velocity, on the other hand, is a vector quantity. It has both magnitude (e.g. 50 miles per hour) AND direction (e.g. south). So you can say your average speed is 50 miles per hour but your average velocity would be 50 miles per hour to the south. In science we most commonly use velocity, so its equation is:

velocity =distance—————time v =  d— t

Police never give you a ticket for velociting because they don’t care which direction you are driving, just how fast you are going.