Sometimes our minds want to look at an object that is moving at an angle to a moving reference frame, like a boat trying to cross a river. Watch the following video of a timelapse of a ferry leaving the pier in Hong Kong.
Sometimes our minds want to look at an object that is moving at an angle to a moving reference frame, like a boat trying to cross a river. Watch the following video of a timelapse of a ferry leaving the pier in Hong Kong.
Source: Clément Bucco-Lechat. Wikimedia Commons
Looking back at the two-dimensional motion lesson, remember that you usually separate motion into the x-direction and the y-direction.
In problems like these, you would look instead at the direction of the current and the direction across the river. See another example by clicking on this link: The River Boat.
A common problem you will see is as follows:
If I point straight across the river, how far downstream will I land?
The animations below show a boat crossing a 224 m wide river.
The velocity of the boat (both magnitude and direction) is 4 m/s. Notice that the boat never lands directly across from the initial starting point.
To find the value of x, the distance the boat landed downstream:
Vacross = 4 m/s
Dacross = 224 m
d = vt or t = d/v
t = 224/4 = 56 seconds
Vcurrent = 5 m/s
ddownstream = x
t = 56 seconds
d = vt = (5) (56) = 280 m
Calculate the value of x, the distance downstream the boat landed.
