# I need help wih 2 simple questions please someone help

1. Jan 14, 2004

### nocturnalwun

Consider a boat that travels at 10 km/hr in still (non-moving) water. If the boat travels in a river that flows at a rate of 10 km/hr, what will be its velocity relative to the shore when it heads directly upstream (against the current)? What happens when it heads downstream (with the flow)? Explain

Suppose the boat is oriented at right angles to the water flow and begins moving. Using vector components and vector addition rules, how would you go about finding the direction in which the boat will move relative to the shore? I am interested in the process, not the numerical answer.

This is part of a test I have to take, but i have not been able to purchase the text book required for the class yet and there is no way for me to figure out how to do these problems

2. Jan 15, 2004

### HallsofIvy

Staff Emeritus
Think about walking upward on a "down" escalator. It is possible to walk upward at exactly the speed the escalator is moving so that you are not going up or down. On the other hand, if you walk downward while the escalator is moving down, you speed adds to that of the escalator. If your boat moves 10km/hr relative to the water and the water is flowing at 10 km/hr then the speeds add (boat going downstream) and subtract (boat going upstream).
In particular, if the boat moves upstream, relative to the water, at exactly the same speed the water flows relative to the bank, then the boat will have speed 0 relative to the bank.

If the boat is moving at right angles to the flow of the river, then you can think of this as a right triangle. Draw a leg of the triangle across the river with length the same as the speed of the boat. Draw a leg of the triangle in the direction of flow (at right angles to the first leg) with length the same as the speed of the river. The length of the hypotenuse will give the speed of the boat relative to the bank. Do you know how to find that? Do you know how to find the angles in the triangle?