Homework Help: physics- what did i do wrong

Posted by jeff on Wednesday, August 29, 2007 at 4:00pm.

If the bus is 100 meters ahead of the car and the bus is travling at 65 km/hr and the car at 78km/hr, how long will it take the car to catch the bus?

I used the following formula, but was informed my answer was wrong.

100m + (t) 65km/hr = 0 + (t)78km/hr

and solved for (t).

t = 100m / (78km/hr - 65km/hr)

I converted km/hr to m/sec and did the math and got

t = 100m / (21.668 m/sec - 18.057 m/sec)

t = 100m / 3.611 m/sec

t = 27.693 seconds

What did I do Wrong?

physics- what did i do wrong - bobpursley, Wednesday, August 29, 2007 at 4:25pm
The only thing I see is significant digits. There is no way you can have the answer in five sig digits, given the input data.
physics- what did i do wrong - DrBob222, Wednesday, August 29, 2007 at 4:40pm
If you leave it in hr, the time for the car to catch the bus is 0.00769 hours.
The bus will travel 0.00769*65 = 0.5 km
The car will travel 0.00769*78 = 0.6 km
The difference is 0.1 km which is the 100 meter separation at th beginning. So the answer MUST be right, at least worked the right way, but the teacher may object to the digits. Perhaps 28 seconds?

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