Conservation of Momentum of a skydiver

1. Jul 19, 2009

James98765

I reading a physics book on my own right now and I appear to be stuck. The problem is as follows:

A 10 m long glider with a mass of 680 kg (including the passengers) is gliding horizontally through the air at 30 m/s when a 60 kg skydiver drops out by realesing his grip on the glider. What is the glider's velocity just after the skydiver lets go?

The problem is dealing with the law of conservation of momentum. I feel like I understand the concept except this particular problem is confusing me. I have defined the system as glider + skydiver. The problem is that gravity is an external force acting on the system so momentum shouldn't be conserved. I don't quite know how to handle this. If anyone could help me that would be great. Thanks a lot!

-James

2. Jul 19, 2009

Staff: Mentor

Forget about gravity. Just worry about the "moment" when the skydiver drops. Hint: How does the momentum of the skydiver change when he lets go?

3. Jul 19, 2009

James98765

That's a good suggestion but I still don't understand one thing. Inorder for there to be a change in momentum their must be an impulse. Because the glider is supposed to gain vertical momentum, wouldn't there have to be a vertical impulse acting on the glider. I don't see how the skydiver letting go could cause I vertical impulse.

4. Jul 19, 2009

Staff: Mentor

Why do you think the glider is supposed to gain vertical momentum?

5. Jul 19, 2009

James98765

It seems to make sense. If the skydiver is gaining downward vertical momentum shouldn't the glider have to gain upward vertical momentum due to the law of conservation of momentum. I guess that the momentum increase of the skydiver in the down direction is due to an external force (gravity). So that would mean that the glider wouldn't neccessarily have to accelerate upward. Does the glider change momentum in the horizontal direction? If so, where is the impulse in the horizontal direction causing the change?

6. Jul 19, 2009

Staff: Mentor

Exactly.
What makes you think there's a change in horizontal momentum?

7. Jul 19, 2009

James98765

I think I am coming to the conclusion that the glider doesn't change velocity at all because there is no impulse acting on it. I can't check my answer because it isn't in the book but the conclusion seems valid.

8. Jul 19, 2009

Staff: Mentor

You are correct. Neither glider nor skydiver change velocity at the instant he lets go.

9. Jul 19, 2009

James98765

Okay thanks a lot for your help! I think I fell for a trick question from my text book. I wasn't expecting that. Thanks again! You lead me to a conclusion perfectly!

-James