The total force applied by the three people is:

This force is pushing toward the direction of the motion, while the frictional force

points in the opposite direction.
We can write Newton's second law applied to the block: the resultant of the two forces must be equal to the product between the block's mass and its acceleration

We know the mass of the block, m=500 kg, and the acceleration,

, so we can find the friction:
<span>As speed changes, wavelength changes and frequency remains constant.</span>
SONAR stands for "sound navigation and ranging,” and it is used to map and explore the ocean floor.
Answer:
A vacuum is an area without any air, like space. So sound cannot travel through space because there is no matter for the vibrations to work on it.