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## Homework Statement

http://www.physics.umd.edu/lecdem/outreach/QOTW/arch3/a046.htm

According to the site, when a fan is blowing air into a sail, the entire cart system remains motionless.

Why doesn't the following approach work:

Since there are no net external forces acting on the entire cart, momentum is conserved. The air particles are bombarding the sail and are bouncing back with a momentum of p_f. Since momentum is conserved, the cart must then move in the opposite direction with a momentum of p_0+p_f. Because the sail is attached to the car, the car itself moves in the direction in which the fan is blowing.

What's wrong with this?

Also, in another example where a person throws tennis balls leftward at a rigid surface that is perpendicularly attached to a cart, the cart itself moves leftward.

What is the difference here?

Thanks

## Homework Equations

p_i = p_f

[tex]F_{net}\Delta t=m\Delta v[/tex]

## The Attempt at a Solution

(Essentially given in part 1)

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