# Homework Help: Propulsive force

1. Jan 16, 2012

### The-alexandra

1. The problem statement, all variables and given/known data
hello, I don't understand what is the propulsive force and how I can develop this equation? apply to kayak

What is the propulsive force??
Which is the propulsive force in kayak?? generate for the paddle

thanks for hour help

2. Relevant equations

3. The attempt at a solution
the propulsive force is the lift force??

2. Jan 16, 2012

### I like Serena

Hi The-alexandra!

When you put your paddle in the water, and pull, you apply a horizontal force to the water.
In response the water pushes the paddle forward.
This is the propulsive force.

This is not lift force.
Usually we speak of lift force when talking about an airplane, which is lifted up by a vertical force.

3. Jan 16, 2012

### The-alexandra

hi.. "I like serena"

I'm trying to write the physics equations in kayaking.. this is all my problem...
so, the propulsive force have the same valuer that the horizontal force in the center of mass of the paddle buts its negative??

thanks so much.. one more time..

have a good day..

4. Jan 16, 2012

### I like Serena

The propulsive force is at the center of the part of the paddle that is in the water.

Its direction is in the direction that the kayak is propelled.
I would tend to call this positive.
The force that the paddler exerts is in the opposite direction (which would then be negative).

Be well!

5. Jan 16, 2012

### The-alexandra

HI

Last edited: Jan 16, 2012
6. Jan 16, 2012

### I like Serena

It looks like a nice piece of work! :)
What is the purpose if I may ask?

Since you asked me to critique your work, I'll try to suggest a couple of improvements.

Newton's third law states: action = -reaction.

Where the paddler and the boat interact, this law must hold.
In particular this means that the force of the seat on the paddler must be equal and opposite to the force of the paddler on the seat.
What I mean, is that in your equation (2) en (3) $F_{seat}$ should have opposite signs.

Same thing for $F_{footbar}$.
In equation (3) this is positive, but in equation (2) it should be negative.

You have $F_{dragAir}$.
I'm assuming that is the drag of the air on the paddler.
But this force does not act on the boat, but only on the paddler.

In your kayak stroke (equation 5), you should see only the so called "external" forces.
The forces on the footbar and the seat are not external forces.
They (should) cancel when you add equations (2) en (3) together.
This would leave you with only $F_{paddle}$ and $D$, which are the external forces.

Good luck!

7. Jan 16, 2012

### The-alexandra

ok, thanks for all your ideas.. the work is for a bio-mechanics project.. so I try to do something good.. thanks for your help, I work .. and I will come back
bye