# Calculating Thrust and Resistance

## Homework Statement

i built a boat for physics class with an electric motor. we are asked to calculate the thrust of the motor and the resistance of the hull using Newton's second law of motion. i conducted an experiment (to collect data in order to find resistance) that required a string attached to the front of the boat in a trough of water (which was placed up off the ground) with the motor turned off. the string had a 20g mass on the end of it. when we let the mass fall to the ground, it took 2.35s to fall .88m. i conducted another experiment (to collect data in order to measure thrust) and tied the string to the back end of the boat with the motor turned on. there was a 20g mass tied on the end of the string again. we measured a .1m length in the trough and found out that the boat moved .1m backwards in 1.33s. Key information: the boat has a mass of .154Kg

## Homework Equations

d= Vi X t + 1/2 A t(squared)
Fg= mg
F=ma
F = Fapplied + Ffriction??????
probably others but im unsure

## The Attempt at a Solution

FIRST EXPERIMENT TO FIND RESISTANCE
calculated the force of gravity acting upon the mass ---> .196N
calculated the acceleration of the mass falling .88m in 2.35s ---> .32m/s(squared)

SECOND EXPERIMENT TO FIND THRUST
calculated the force of gravity acting upon the mass ---> .196N
calculated the acceleration of the mass falling .1m in 1.33s --->.113m/s(squared)

WHAT SHOULD MY NEXT STEPS BE? thanks so much

## Answers and Replies

ehild
Homework Helper
Draw a picture of both experiments and show what forces act on the boat and stone. These forces determine the acceleration of the whole system and you can find the resistive force in the first case and the difference between thrust and resistive force in the second case.

ehild