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

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

Guest Feb 12, 2012

#1**0 **

avg. speed in km/h = distance / time

t: time

d: distance

t * 90km/h = d

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]

t = 3.25hours = 3hours + 15minutes

d = 292.5km

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

=>

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

admin
Feb 12, 2012