# Homework Help: Force, maybe displacement

1. Jul 16, 2009

### dance_sg

1. The problem statement, all variables and given/known data
At an altitude of 315 m above a lake, a hot-air balloon pilot drops a 2.50 kg sandbag. If the bag comes to a stop 4.50 m below the surface of the lake, what force was applied by the water on it?

2. Relevant equations
f=ma, d=df-di

3. The attempt at a solution
im kinda completely lost about what to do for this question, so i tried just multiplying the mass by gravity. i could also try finding displacement , but i dont know what to do after if i did that.

2. Jul 16, 2009

### astropi

You're on the right track. Here's a hint: what is impulse?

3. Jul 16, 2009

### dance_sg

uh to act suddenly?

4. Jul 16, 2009

### astropi

Very funny...
Now, what makes a bigger splash? A rock dropped from 1 meter, or from 1 km? Gravity is a constant, right? So then, what could account for the force. I've given you some hints, time for you to do a bit of reading :)

5. Jul 16, 2009

### rock.freak667

If you hold something up at some height in the air. It has a certain type of energy. If you drop it, it turns into another type of energy which is only due to its motion. Work is being done on the sandbag by the water. Do you know an expression for work done on an object? Can you form a relation between the first type of energy and the work done on the object? (Think conservation of energy)

6. Jul 16, 2009

### astropi

Yes, you can do it using conservation of energy as well. I personally think doing it the impulse way is easier :)

7. Jul 16, 2009

### Redbelly98

Staff Emeritus
The problem statement tells us the distances, rather than the times, it takes the mass to fall and then come to a stop. That suggests that energy & work is a better way (i.e. more direct and easier) to solve this than using impulses.