Physics Question... no, not my homework.

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TheLonelyPhoenix

TheLonelyPhoenix

Diamond Member
Feb 15, 2004
5,594
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So, let's say someone is standing in a sailboat and points a fan at the sails in an attempt to propel the boat forward. Assume all the wind generated by the fan hits the sails and no energy is lost, and he has some sort of infinitely long, weightless extension cord to power his fan with. There is no other force acting on the boat besides gravity and the buoyant force of the water around the boat.

Will the boat ever go anywhere?
 
Gunslinger08

Gunslinger08

Lifer
Nov 18, 2001
13,234
2
81
Originally posted by: TheLonelyPhoenix
So, let's say someone is standing in a sailboat and points a fan at the sails in an attempt to propel the boat forward. Assume all the wind generated by the fan hits the sails and no energy is lost, and he has some sort of infinitely long, weightless extension cord to power his fan with. There is no other force acting on the boat besides gravity and the buoyant force of the water around the boat.

Will the boat ever go anywhere?
Click to expand...

No, but it would if he just pointed it backwards from the rear of the boat. Air resistance ensures that some movement will take place.
 
drinkmorejava

drinkmorejava

Diamond Member
Jun 24, 2004
3,567
7
81
Yes, it will move people; do a force diagram on the boat and the person. For the person blowing, you would have a positive force x and a negative force x, which makes the net force of the person 0. However, the sail will also gain a positive force x which will give the boat a total net force of x (minus wind resistance and such). And if you doubt me, well, sucks for you; and btw, I know I was wrong in the plane/treadmill thread. I'd say it was a little late when I posted, except it's 1am now, but I'm not wrong here.
 
P

PurdueRy

Lifer
Nov 12, 2004
13,837
4
0
Originally posted by: drinkmorejava
Yes, it will move people; do a force diagram on the boat and the person. For the person blowing, you would have a positive force x and a negative force x, which makes the net force of the person 0. However, the sail will also gain a positive force x which will give the boat a total net force of x (minus wind resistance and such). And if you doubt me, well, sucks for you; and btw, I know I was wrong in the plane/treadmill thread. I'd say it was a little late when I posted, except it's 1am now, but I'm not wrong here.
Click to expand...

You're wrong again
 
Y

Yossarian

Lifer
Dec 26, 2000
18,010
1
81
Originally posted by: drinkmorejava
Yes, it will move people; do a force diagram on the boat and the person. For the person blowing, you would have a positive force x and a negative force x, which makes the net force of the person 0. However, the sail will also gain a positive force x which will give the boat a total net force of x (minus wind resistance and such). And if you doubt me, well, sucks for you; and btw, I know I was wrong in the plane/treadmill thread. I'd say it was a little late when I posted, except it's 1am now, but I'm not wrong here.
Click to expand...

you write as if you know what you're talking about but your conclusion is totally wrong.
 
W

wfbberzerker

Lifer
Apr 12, 2001
10,423
0
0
Originally posted by: drinkmorejava
Yes, it will move people; do a force diagram on the boat and the person. For the person blowing, you would have a positive force x and a negative force x, which makes the net force of the person 0. However, the sail will also gain a positive force x which will give the boat a total net force of x (minus wind resistance and such). And if you doubt me, well, sucks for you; and btw, I know I was wrong in the plane/treadmill thread. I'd say it was a little late when I posted, except it's 1am now, but I'm not wrong here.
Click to expand...

net force of the person is NOT zero. what is the positive force x for the person?
 
pray4mojo

pray4mojo

Diamond Member
Mar 8, 2003
3,647
0
0
Originally posted by: AbAbber2k
Originally posted by: pray4mojo
No thats like trying to lift yourself.
Click to expand...

What... you saying you can't do that? :confused:
Click to expand...

Pull yourself upwards on the collar of your shirt and see if you can lose your normal force.
 
E

Eeezee

Diamond Member
Jul 23, 2005
9,922
0
76
Originally posted by: pray4mojo
Originally posted by: AbAbber2k
Originally posted by: pray4mojo
No thats like trying to lift yourself.
Click to expand...

What... you saying you can't do that? :confused:
Click to expand...

Pull yourself upwards on the collar of your shirt and see if you can lose your normal force.
Click to expand...

Lifting yourself is different in a few important, obvious ways.

The fan acts as an external force. Let's make some assumptions, as we're supposed to do in physics

1) The friction between the water and the boat is zero (silly but okay)
2) The air coming out of the front side of the fan is moving faster than air coming out the back (this is true for fans, the motor does work to accelerate the air).
3) Gravity and the buoyant force have no horizontal components (a fair assumption for this example)
4) All energy created by the fan in the form of moving air impacts on the sail (assumption from the problem)

You are using a fan to generate moving air. This would create two forces: One force due to the change in momentum of the air and one force due to the air hitting the sail. These forces push in opposite directions; the air being pushed away by the fan would actually cause the boat to move backward whereas the air hitting the sail would cause the boat to move forward.

If we're assuming no energy loss, then the boat will not move at all. The force of the fan causing the boat to move backward will cancel with the force on the sail causing the boat to move forward.

As joshsquall stated, you can get the boat to move forward if you point the fan toward the rear of the boat. This would create only one force and therefore the boat would move forward. If we assume that the moving air travels around the world and hits the sail from the other direction, then the boat does not move at all even if you point the fan toward the rear of the boat.

The boat will not move.
 
E

Eeezee

Diamond Member
Jul 23, 2005
9,922
0
76
Since we're on the topic, was a consensus ever reached on the airplane problem? I don't feel like reading the thread.
 
Fritzo

Fritzo

Lifer
Jan 3, 2001
41,920
2,162
126
Originally posted by: joshsquall
Originally posted by: TheLonelyPhoenix
So, let's say someone is standing in a sailboat and points a fan at the sails in an attempt to propel the boat forward. Assume all the wind generated by the fan hits the sails and no energy is lost, and he has some sort of infinitely long, weightless extension cord to power his fan with. There is no other force acting on the boat besides gravity and the buoyant force of the water around the boat.

Will the boat ever go anywhere?
Click to expand...

No, but it would if he just pointed it backwards from the rear of the boat. Air resistance ensures that some movement will take place.
Click to expand...

Yep. The thrust the fan creates has to be greater than the air resistance, or it won't move.
 
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