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# help with homework please! speed/time

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Hi guys i know this is a maths website but hope you can help me with this physics homework
im having so much trouble with this question:

Caleb takes a trip in his car. He drives at a constant speed of 90.0km/h except for a 30 minute rest stop. His average speed for the trip is 78 km/h.
a: How much time does the whole trip take?
b: How far does Caleb travel?

i know how to work these out but i need at least a time/distance to work it out.
someone help?
thanks so much
Feb 12, 2012

#1
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avg. speed in km/h = distance / time

t: time
d: distance

NOTphysicsnerd:

He drives at a constant speed of 90.0km/h except for a 30 minute rest stop.

t * 90km/h = d

NOTphysicsnerd:

His average speed for the trip is 78 km/h.

avg speed = d / (t + 30min) = 78km/h

equations:
t * 90 = d
d / (t + 0.5) = 78

[input]solve( t * 90 = d, d / (t + 0.5) = 78 )[/input]
or solve on paper..
d / (t + 0.5) = 78
=> d = 78 * (t+0.5)
t * 90 = d = 78 * (t+0.5)
t * 90 = 78*t + 78/2
t * 90 - 78*t = 78/2
t * (90 -78) = 39
t * 12 = 39
t = 39/12 = 13/4 = 3.25
=> d = 13/4*90 = 292.5

result check:
[input]3.25hour*90km/h in km[/input]
[input]292.5km/78km/h-30min in std[/input]

NOTphysicsnerd:

a: How much time does the whole trip take?

t = 3.25hours = 3hours + 15minutes

NOTphysicsnerd:

b: How far does Caleb travel?

d = 292.5km
Feb 12, 2012
#2
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wow thank you so much!!
Feb 13, 2012