# Homework Help: System of many particles

1. Dec 24, 2013

### omarMihilmy

While they are running the speed of the ship should remain the same?
How come the answers claim it is higher?

2. Dec 24, 2013

### Staff: Mentor

Think of it like a rocket. The rocket moves forward because it expels rocket fuel. So the passengers are like the rocket fuel as they run to the stern. Once they stop running then the ship speed will slow down to its prior speed. If the ran back to the front then then ship would slow even more and then when they stop would resume its original speed.

3. Dec 24, 2013

### omarMihilmy

So when they go to the back they are as if they are leaving the ship so the ship accelerates at a higher speed?

4. Dec 24, 2013

### Staff: Mentor

Yes and then when they stop it returns back to the original speed.

Another analogy would be to be on a long stationary sled on ice. You decide to run to one end and the sled moves in the other direction and then you fall off and the sled slides away.

5. Dec 24, 2013

### haruspex

I would have answered that when they stop the ship's speed will be less than when they started. That comes from assuming the ship delivers constant power, and that faster means more drag. When they started running the speed increased a little, but while they run the increased drag will bring the speed back somewhat to what it was before.
Another question that could have been asked is whether the ship's travel time will be more, less, etc.

6. Dec 24, 2013

### Staff: Mentor

Good point, I was thinking of an ideal system...

7. Dec 25, 2013

### omarMihilmy

The travel time while they are running to the back is less, but while they are running back. It should be greater or the same?

8. Dec 25, 2013

### haruspex

It depends. If the ship does not lose much of the extra speed while they're running then it could shorten the overall journey. But if it does lose much and the ship still has some way to go after they reach the back, the ship's overall travel time will have been increased.