Awoman is riding a bicycle at 18.0 m/s along a straight road that runs parallel to and right next to some railroad tracks. she hears the whistle of a train that is behind. the frequency emitted by the train is 840 hz, but the frequency the woman hears is 778 hz. take the speed of sound to be 340 m/s. what is the train’s speed?

Explanation:

We need to use the doppler effect equation:

Where:

In this case, we can assume that the observer is moving away from the source and the source is moving towards the observer, then:

Isolating

 (1)

Replacing the data provided in (1)

The minus in the result is actually telling us that the source is really moving away from the observer.

B, A, E, D, C

Explanation:

If the arrow points to the right, then the force is positive.

If the arrow points to the left, then the force is negative.

Also, remember that the force does not relate (necessarily) to the mass, so the different blocks having different masses is not an important factor in this problem.

Let's find the force in each block.

A)

Here we have a positive force of 3N and a negative force of 1N

Then the net force (the sum of all the forces) is:

F = 3N  - 1N = 2N

B)

Here we have two positive forces, one of 1N and other that I cant see, I guess that it is 3N

Then the net force is:

F = 1N + 3N = 4N

C)

Here we have two negative forces of 2N

Then the net force is:

F = - 2N + ( - 2N) = -4N

D)

Here we have a single force of 2N that is negative, then the net force is:

F = -2N

E)

Here we have a negative force of 3N and a positive force of 2N

then the net force is:

F = -3N + 2N = -1N

Then, the order of greatest to least (from positive to negative) is:

B, A, E, D, C