Homework Help: Basic mechanical dynamics

1. Mar 13, 2013

djstar

1. The problem statement, all variables and given/known data

Hi could some one point me in the right direction for both parts of these questions, im electrical biased so this is all new to me.

A model boat off mass 25 Kg is crossing a still lake at a constant speed, in a straight line. The engine is propelling the boat along with 30 force.

a) What is the size of the total friction force opposing the boat?
b) How much buoyancy force is the boat producing?

2. Relevant equations

F=ma ?????

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Mar 13, 2013

SteamKing

Staff Emeritus
a Sum Forces = mass * acceleration
b Archimedes Principle

3. Mar 13, 2013

djstar

for a) do i rearrange the formula and divide 30n/25kg = 1.2 m/s ?

for b) I now understand that the forces acting down must equal the forces acting up. does gravity come into this scenario?

4. Mar 13, 2013

SteamKing

Staff Emeritus
a) Don't be silly. The problem asks what is the magnitude of the friction force opposing the motion of the boat. Ask yourself, if the boat is travelling at a constant velocity (it does not matter what this velocity is), what forces are acting on the boat? What must their magnitudes be relative to one another? You are given the thrust produced by the engine (30 N).

b) Obviously, if the weight of water displaced by the boat is equal to the weight of the boat, according to Archimedes, gravity is involved.