# Can anyone help provide an explanation?

#### gunblaze

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..

Related Introductory Physics Homework News on Phys.org

Hint: Friction

#### gunblaze

yup.. around there. Friction and the drag force

#### gunblaze

but how..? when u fly higher, u get lesser friction..?

V

#### vsage

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.

#### gunblaze

u mean air is less dense at high altitudes, therefore, by using F=pAv^2, we get a lesser drag force?

#### whozum

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?

#### gunblaze

no.. that's really what i don't know

#### whozum

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?

#### gunblaze

oOO.. I see. so as the speed of the canoe increases, the larger the drag force is on the canoe.

Whao, thx guys!

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving