# Momentum and force

you are floating at rest in zero gravity. Your mass is 80 kg . You want to propel yourself in one direction by throwing a 10kg ball in the opposite direction . You throw it at 8 m/s. How much work you do?

A firehose shoots water at you at a rate of 30 kg/s. The water hits you at 20 m/ sec. Assuming half of the momentum of the water gets reflected back, what's the force of the water on you ?

Hello guys! I have no idea how to do these problems :( It dosent seem hard, but I just don't get it! Thank you!

## Answers and Replies

first question:
USE conservation of momentum to get the speed of the floating dude.
and than use the realtion between work and energy:
Work=Change in energy

D H
Staff Emeritus
Science Advisor
What are the relevant equations here?

1. There's a theorem regarding the relationship between work and energy. What is it?

2. What is the change in momentum in the water, per unit time? How would you convert this to force?

Well p=m•v. But I have two m's.

D H
Staff Emeritus
Science Advisor
We have a template for homework questions on purpose. The template asks you to supply the relevant equations and to show some work. Can you do that, please?

look at the system as a Whole!!!
If moementum=0 before, it has 2 be the same after! (no external force).
so momentum of body1 + momentum of body 2 equals...