# Help with momentum

#### greenseeking

7+ Year Member
Hi,
This is from the Skateboarding Passage from the Berkeley Review. I am wondering if anybody could explain this to me!

Scenario: Student I and II are facing each other so that their skateboards are touching. Student I and his skateboard has mass 50kg and student 2 and his skateboard has mass 40kg. They push each other with a force of 50N.

Student I modifies skateboard so that he puts a sail on the front of the skateboard and puts a small fan on the back of the skateboard so that the fan blows air toward the sail. Student 2 puts a fan on the back of her skateboard so that the fan blows air behind the skateboard.

Question: Which skateboard setup generates net motion?
Answer: Only Student II's set up could move.
Back of the book Says that the fan blows air onto the sail and air pushes back on the fan. The air that gets to the sail pushes the sail forward, but the sail responds by pushing back on the air. So wouldn't the net movement be going backwards according to the book's defination?
My question is, why doesn't Student I's setup move? When I think of the sailboat, the wind blows into the sail, which propels the boat forward... right?

OP
G

#### greenseeking

7+ Year Member
Oops figured it out. The fan is actually attached to the skateboard so the forward motion would cancel out with the backward motion due to the conservation of momentum! I'll leave this post here in case anybody needs it in the future.