# Calculating the Magnitude of the Force

Once you have found the direction
of the force you need to calculate its MAGNITUDE. You can't directly
use the standard form of the Lorentz force law
since the values of Q, v and B would be very difficult to measure.
So you have to convert the equation in terms of things which are
easily measured:

F = (I t)(d / t)B simplify...

### F = IdB

The boat itself needs a reasonable force to accelerate
it. The following calculations are based on my construction of the boat.

Use F = I d B

I = 3 A (short circuit current between electrodes)

d = 2.7 cm (0.027 m) (distance between electrodes)

B = 0.2 T ( S.I. units: 0.2 N/(A*m) measured)

therefore force on water (force propelling the boat)
is:

F = (3.0 A)(0.027 m)(0.2 N/(A*m))

#### F = 0.0162 Newtons

It is only natural that students in grade 12 physics
don't have an intuitive idea of how much this force actually is,
so to put things into perspective you ask the question:

what mass would this be if the force is due to gravity
acting on that mass?

## What acceleration does this produce on the boat?

Boat has mass 0.470 kg

F = ma therefore a = F/m = (0.0162N)/(0.47kg) = 0.04
m/ s2

What speed will the boat be going after 30 s?

a = v/t therefore v = at = (0.04m/s2)(30s) = 0.0012
m/s = 1.2 mm/s

- Not fast but still noticeable!

Back to Main Menu