How do I solve these problems (kinematics)

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Homework Statement
She drives for 40.0 min at 80.0 km/h, 10.0 min at 75.0 km/h, and 30.0 min at 45.0 km/h

Determine the average speed for the trip



The attempt at a solution
I got 66.25khm by ((4*80)+(75)+(3*45))/8 which looks like its in the ballpark range but when I enter the answer for my online homework its incorrect.

So whats the proper way im suppose to solve this?
 

Answers and Replies

  • #2
SteamKing
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Homework Statement
She drives for 40.0 min at 80.0 km/h, 10.0 min at 75.0 km/h, and 30.0 min at 45.0 km/h

Determine the average speed for the trip



The attempt at a solution
I got 66.25khm by ((4*80)+(75)+(3*45))/8 which looks like its in the ballpark range but when I enter the answer for my online homework its incorrect.

So whats the proper way im suppose to solve this?
What fraction of an hour is 10 minutes?
 
  • #3
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I appreciate the answer the question but I would enjoy more on understanding how to do them myself

Other questions on my online homework I don't get
Two boats start together and race across a 70-km-wide lake and back. Boat A goes across at 70 km/h and returns at 70 km/h. Boat B goes across at 35 km/h, and its crew, realizing how far behind it is getting, returns at 105 km/h. Turnaround times are negligible, and the boat that completes the round trip first wins.

Boat A wins but by how much? What is the average velocity of the winning boat?
since the boat starts moving at 70km/h and returns at 70km/h how come its says im wrongs when I enter 70?
 
  • #5
SteamKing
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Why did you divide your calculation in Post #1 by 8 then?
 
  • #6
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Why did you divide your calculation in Post #1 by 8 then?


because they traveled for 80 minutes

I don/t know the proper equation to punch these numbers into.
 
  • #7
SteamKing
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because they traveled for 80 minutes

I don/t know the proper equation to punch these numbers into.
The basic equation to use is distance = rate × time

The only thing I can see that would be wrong with your original answer of 66.25 kph is that this figure has more significant figures than the original data. If you expressed it to 3 significant figures, that might be the difference between a correct result and an erroneous one.
 
  • #8
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The basic equation to use is distance = rate × time

The only thing I can see that would be wrong with your original answer of 66.25 kph is that this figure has more significant figures than the original data. If you expressed it to 3 significant figures, that might be the difference between a correct result and an erroneous one.
but my answer is correct right?
 
  • #9
SteamKing
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but my answer is correct right?
Yes, except for the sig figs.
 

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