# Relating Homework problems to Newton's laws

• AttilaTheNun
In summary, the conversation is about a new member asking for help with physics problems on the forum. The problems relate to fluid mechanics and the member is having trouble applying Newton's three laws of motion. The members on the forum offer to explain the problems and formulas needed. Some of the problems include calculating the pressure needed to pump water up a tall building and determining the maximum mass of a balloon filled with hot air. The conversation also touches on the concept of suction and its limitations.
AttilaTheNun
Hi guys,

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).

Not to worry AttilaTheNun.These can be solved easily.

At least for your second question apply the formula P=DENSITY*g*H, this should give you the pressure required.
You have density of water as 1, g=accln. due to gravity=10m/s^2, H=20m
=>Pressure=200 Pascals
Yes water could also be sucked up.
One cannot suck a fluid more than the height after which pressure of liquid column balances atmospheric pressure,i.e DENSITY*g*H=atmospheric pressure=104880 N/m^2.

Last edited:
Thanks.

Anyone have some ideas?

Last edited:
For your third question balance mass of balloon with buoyant force of air.
Dair(not heated)*g*Volume of balloon=mass of balloon*g

=>Dair*Volume of balloon=mass of balloon

we get maximum mass nearly 5Kg.

## 1. How does Newton's first law relate to homework problems?

Newton's first law, also known as the law of inertia, states that an object at rest will remain at rest and an object in motion will remain in motion at a constant velocity unless acted upon by an external force. In homework problems, this law can be applied to situations where an object is either at rest or in motion and the problem involves analyzing the forces acting on the object and determining its resulting motion.

## 2. How can Newton's second law be used to solve homework problems?

Newton's second law, also known as the law of acceleration, states that the acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to its mass. In homework problems, this law can be used to calculate the acceleration of an object when given the net force acting on it and its mass, or to determine the net force required to produce a certain acceleration.

## 3. In what types of homework problems is Newton's third law relevant?

Newton's third law, also known as the law of action and reaction, states that for every action, there is an equal and opposite reaction. This law is relevant in homework problems involving interactions between two objects, such as collisions or objects connected by a string or spring. It is important to consider both the action and reaction forces in these types of problems.

## 4. Can Newton's laws be used to solve problems involving non-rigid bodies?

Yes, Newton's laws can be applied to solve problems involving both rigid and non-rigid bodies. However, in the case of non-rigid bodies, additional considerations such as deformations and changes in shape may need to be taken into account when applying the laws.

## 5. How can Newton's laws be used to analyze the motion of objects on an inclined plane?

Newton's laws can be used to analyze the motion of objects on an inclined plane by breaking down the forces acting on the object into its components along the plane and perpendicular to the plane. This allows for the application of Newton's second law in the direction of motion and the use of trigonometric functions to solve for the acceleration and motion of the object.

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