Relating speed to distance & time

AVERAGE SPEED If you drove 80km and it took you 1 hour to do it, most people would say that your average speed was 80km/h. Average speed does not take into account the fact you sped up to pass cars or slowed down to watch some cows! AVERAGE SPEED is the total distance divided by the total time it took to go that distance.

FORMULA V = average speed ∆d = the “change in” distance ∆d = d2 – d1 d1 is often zero ∆t = the “change in” time ∆t – t2 – t1 t1 is often zero

UNITS FOR QUANTITIES QUANTITY SYMBOL UNIT Distance ∆d Millimeter Centimeter Meter Kilometer mm cm m km Time ∆t Second Minute Hour Day Year s min h d a Speed V Meters per second Kilometers per hour m/s km/h

INSTANTANEOUS SPEED INSTANTANEOUS SPEED is the speed of an object at a particular “instant” in time. EXAMPLES: At a red light the instantaneous speed of a car should be 0km/h. Passing a sign on a highway, each car would have slightly different instantaneous speeds. SPEEDOMETERS and RADAR GUNS are both tools that measure instantaneous speed that have improved road safety.

CONSTANT SPEED When speed remain the same over a period of time it is called CONTSTANT SPEED or UNIFORM MOTION. EXAMPLES: Cars driving in cruise control Walking on a treadmill at the same pace

EXAMPLE PROBLEMS Jack skateboards to school, a total distance of 4.5km. He has to slow down at intersections but overall the trip takes 0.62h. What is Jack’s average speed? Δd = 4.5km V = ? Δt = 0.62h

2. Sue is trying to figure out how long it will take her to ride her bike to the beach. She knows the distance is 45km and that she usually averages about 20km/h including slowing down for hills. How long will the trip take her? Δd = 45km V = 20km/h Δt = ?

Tom likes to walk the trails at a local nature park Tom likes to walk the trails at a local nature park. His average walking speed is 7.1km/h. If he walks for 2.3hours how far will he have gone in total? Δd = ? V = 7.1km/h Δt = 2.3h

CONVERSIONS TO KNOW CONVERT into UNIT OPERATION m km Divide by 1000 Multiply by 1000 sec min Divide by 60 Multiply by 60 h H Divide by 3600 Multiply by 3600

IN CLASS ASSIGNMENT & HOMEWORK Complete worksheet!!