# Fan attached to a sled blowing at the sail

1. Mar 17, 2009

### cragar

1. The problem statement, all variables and given/known data
a fan is attached to a sled blowing air to the right at a sail
the sled is on ice and friction is neglected and the air around the sled is still
except the air moved by the fan.
will the sled move is so to the right or left.

2. Relevant equations

3. The attempt at a solution
i think the sled will move to the right becuase thats where the air hits the sail .
what do u guys think

2. Mar 17, 2009

### rl.bhat

Sail and fan are inside the sled.
The molecules of a gas inside a closed container, move with high velocity and collide with the container wall, but the container does not move. Why? What is Newton's first law of motion?

3. Mar 17, 2009

### cragar

newtons first law is inertia , k this is what i am thinking as the fan blade whacks
the air molecule it is given an intial momentum and then the air molecule hits the sail and bounces back thus changing direction and having a greater change in mometum
thus moving the sled to the right.

4. Mar 17, 2009

### rl.bhat

According to Newton's first law, body will not move unless we apply an external force on the body.
In your problem, no external force is acting on the sled.

5. Mar 17, 2009

### cragar

for every action there is an equal but oppostie reaction and when the air molecule hits the sail and bounces back there is an impulse in mometum greater than that of the original mometum of the air molecule. there fore it will move to the right so i ask u
when the fan intially hits the air molecule and when that molecule hits the sail and bounces back are those changes in momemtum the same .

6. Mar 17, 2009

### rl.bhat

In a railway compartment, if every passenger hits a ball to the front wall of the compartment, will the railway move forward?

7. Mar 17, 2009

### cragar

are these ball's suspended in the atmosphere

8. Mar 17, 2009

### rl.bhat

No. they may return back to the passenger or some where else. Even they may go out of the compartment.
In your problem, I presume sail is made up of cloth. In that case, when the fan is on, the sail can take a concave shape.

9. Mar 17, 2009

### cragar

i still would like u to explain how when the fan hits the molecule and from when it bounces off that their implueses are the same , and in ur case yes neglecting friction
the railroad car would move forward due to the impulse momentum theorem.

10. Mar 17, 2009

### rl.bhat

In a frictionless floor, sitting in the back seat of a car and kicking the front seat, the car will not move forward. Because the car and passenger constitute a single system. To move the system, force must be applied from outside the system.
Moving fan and sail together with the wind produced by the fan in the sled is not possible.

11. Mar 17, 2009

### cragar

k i knew the answer to this question before i asked it , we talked about this in my physics class and my professor said that the sled would move forward because of the impusle momentum theroem and he said this is a good question for physics instructors too see if they really know what they are doing , in ur case kicking the seat your body is part of the sled and in our case the air is not part fo the sled. so are u telling me that my professor with a ph.d in nuclear physics is wrong .

12. Mar 17, 2009

### mgb_phys

A simpler way to picture it.
1, Imagine a compessed air hose pointing backward = the sled will obviously move forward
2, Then imagine the hose has a loop in it so that coming out of the compressor the pipe goes forward but the nozzle still points back = no change, still fowad
3, Now imagine instead of the loop the jet points forward at a sail and the air bounces off and goes backward = exactly the same as 2

13. Mar 17, 2009

### cragar

so ur saying the sled will move

Last edited: Mar 17, 2009