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July 31, 2014

July 31, 2014

Posted by **Physics** on Saturday, June 20, 2009 at 1:02pm.

A car traveling 90 km/h is 100 m behind a truck traveling 75 km/h. How long will it take the car to reach the truck?

- physics -
**DrBob222**, Saturday, June 20, 2009 at 2:29pmTry this.

car|--0.100 km--|-------d---------|

t r u c k xxxxxx|-------d---------|

The truck travels a distance, d km, while the car travels d + 0.100 km.

distance = rate x time. The time must be equal so

time = distance/rate

time for truck = d/75

time for car = (d+0.100)/90

time = time

d/75 = (d+0.100)/90

solve for d and I get 0.5 km. So the truck travels 0.5 km and the car travels 0.6 km. How long does that take.

d = r*t

t = d/r

t = 0.5/75 = 0.00667 hr or

t = 0.6/90 = 0.00667 hr.

You can convert 0.00667 hr to minutes, then to sec by

0.00667 hr x (60 min/hr) x (60 sec/min) = I get 24 seconds but check my work.

- physics -
**DrBob222**, Saturday, June 20, 2009 at 3:07pmThere is another way to do this which is a little shorter.

Note that the difference between 90 km/hr and 75 km/hr is 15 km/hr. So we ask ourselves how long it takes to cover that additional 0.1 km at the rate of 15 km/hr. That is t=d/r = 0.100/15 = 0.00667 hr. Voila!

- physics -
**Physics**, Saturday, June 20, 2009 at 3:28pmThank You

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