There are other types of problems, in this section. Some of these consider an object that is moving, but it is moving on / in a body that is also moving. These can be called object in carrier problems.

examples of this type of problem include

  1. Boats / swimmers in rivers
  2. Planes / Birds flying through air
  3. object moving on an already moving surface.

Start it here

Now for you to do

calculate the time it takes a boat to travel downstream on a river river with flow down with river speed 3m/s and the boat having a max engine speed 10 m/s. How long does it take to travel back up ?

What about the same distances with no flow,

worked out

lets make the length of river 1km

when the boat goes with the flow it has a velocity of 10 m/s + 3 m/s = 13m/s

this means the time taken 1000m ÷ 13 m/s is 76.92s

when it goes against the current the actual velocity of the boat is now 10m/s - 3m/s = 7 m/s

this means the time is 1000m ÷ 7 m/s = 142.86

meaning the total journey time is 219.77s or 3m 39.77s

for the boat in still water 1000m up and back is 2000m and all done at 10m/s

therefore the time taken is 200s

So the overall effect of any roundtrip is lengthened by the relative motion.

Two more considerations, sketch the following on some paper

    1. for a boat crossing a river how does it get to the other side quickest?
    2. for a boat crossing a river how does it get to the other side in the shortest distance?

Imagine a lake quite, an unusual lake, lets not worry about that for now.

In this lake we will put all sorts of things,

at the end of the lake there is a river also quite odd but again we'll deal with it later.

If a boat has to travel from one side of the lake to the other, and it has an engine capable of v = 3.75m/s,

if the lake is 2km wide how long does it take the boat to cross the lake?

In minutes and seconds

What problems do we have with this question?

Conversion of units

A lake 2km wide, what is that?

The shortest distance, will be always be able to travel the shortest distance to places? think!

So this is easy

the oul flowing river need an ice-cream scenario

more applets on the river with the ice-cream, but in this one you can control quite a bit ...

mind you the key is that we know how to do this on paper

Relativistic velocity is something different to Relative Velocity


Due to relative velocity a foam cup shatters car window

A spitfire on take-off i hope i can get this in to class some time