- #1

AttilaTheNun

- 11

- 0

I realize this can be a rather uncanny occurance when a new member's first post is to ask for physics help

on his homework but I've been lurking around the forum here for a while and am now in a really tight spot

and thus had to get over myself and finally make an account.

Written below are four problems from my problem set that I am having difficulty on.

They relate mostly to fluid mechanics.

Essentially, I am having trouble relating the very basics (Newton's three laws of motion) to the problems below.

In other words, the professor has gotten much further in the course than my understanding has allowed.

I like to trully understand every step involved in solving a problem so that I could theoretically explain everything from scratch.

If you could write out the formulas I need and explain the principles (related back to Newton's

laws of motion if possible) I apply to find each of the variables I would really appreciate it.

I realize this is a lot of material so I encourage people to pick and choose which problem(s) you wouldn't mind taking

the time to explain. Perhaps one that is fresh in your memory or won't require you to spend an

absorbitant amount of time explaining.

Thanks guys, I really appreciate any and all help.

## Homework Statement

**1.**A frisbee of radius r and mass m is thrown through the air. If the distance air

goes over the top as it flies is about 10 percent more than the distance it goes underneath, estimate

how fast it has to fly so that its lift balances the force of gravity.

**Bonus:**What is it about spinning that help a frisbee fly well?

(Hint: What does a spinning frisbee have that might be conserved?)

**2.**How much pressure must be applied at the bottom of a building

that's 20m tall in order to get water up to the top? Could water be sucked up or

would one actually have to pump from the bottom? Explain briefly.

**Bonus:**How high are the tallest trees? Could simple suction from

the top be enough to pull water up that high?

**3.**Air at room temperature (20C) has a density of (density) = 1:2041kg=m3.

Air heated to 35C has a density(heated) = 1:1455kg=m3.

What is the maximum mass that a 1m radius uninflated balloon could be so that when filled

with 35C of hot air it would rise (lifting the hot air and the balloon itself)?

Give an answer in symbols (in terms of density(initial) and density(heated))

**Bonus:**Air tends to get cooler as one goes higher, but it also gets thinner.

What can you say about the air when the balloon stops rising? (This is a bit

open-ended

**4.**A venturi pump or aspirator is a cheap device to get suction

with no moving parts other than a fluid! These things are used by chemists all the time and can

be fastened onto any convenient tap.

Suppose one has a 1cm diameter water faucet with water coming out out at 1m/s and an ambient pressure

of 1atmosphere going into a tube that has a narrow section 1mm in diameter before it opens back out to 1cm.

How much less than one atmosphere pressure could be obtained by attaching a tube

to the narrow bit of the pump and using it forsuction? Neglect the effects of gravity

for this (just rely on the water beingpushed out of the tap).