Why do canoes have a critical speed and why do jet planes fly high?

  • Thread starter Thread starter gunblaze
  • Start date Start date
  • Tags Tags
    Explanation
Join the discussion
Registration is free. Start your own thread to ask a follow-up.
Sign up Log in
9 replies · 2K views
gunblaze
Messages
187
Reaction score
0
Qn1: When paddling a canoe, one can attain critical speed with relatively little effort, but to go faster beyond this critical speed requires more effort. Why?

Qn2: Why do jet planes usually fly at altitudes above 10000m although it takes a lot of fuel to climb to that height?

Thx..
 
Physics news on Phys.org

Discussion

but how..? when u fly higher, u get lesser friction..?
 
Right. I may have the number wrong but when flying at the altitudes planes do on long flights (around 30,000 feet) the air is about 20% as thick as it is at sea level.
 
u mean air is less dense at high altitudes, therefore, by using F=pAv^2, we get a lesser drag force?
 
Yeah, the air density is much less up there. This also affects the plane's lift though.

Can you think of anything for the canoe?
 
Look at the force equation you gave above for drag resistance. Although that is the application for air, it is a simliar principle for fluids. What do you notice as velocity increases?
 
oOO.. I see. so as the speed of the canoe increases, the larger the drag force is on the canoe.

Whao, thanks guys!