An unbalanced force is required to make a resting object move or a moving object change speed or direction
Answer:
See explanation.
Explanation:
If both stars explode in simultaneously in the <em>your </em>frame of reference then obviously you will see the two flashes simultaneously, and therefore, the time difference between the events would be zero.
If however, the stars exploded simultaneously in their frame of reference, then you would not observe the flashes simultaneously. Then the time difference between the events will not be zero, rather, you will observe star B exploding first and star A after.
(a) 646.9 Hz
The formula for the Doppler effect is:

where
f = 600 Hz is the real frequency of the sound
f' is the apparent frequency
v = 345 m/s is the speed of sound
is the velocity of the observer (zero since it is stationary at the station)
is the velocity of the source (the train), moving toward the observer
Substituting into the formula,

(b) 20.1 m/s
In this case, we have
f = 600 Hz is the real frequency
f' = 567 Hz is the apparent frequency
Assuming the observer is still at rest,

so we can re-arrange the Doppler formula to find
, the new velocity of the train:

and the negative sign means the train is moving away from the observer at the station.
The net force on the center of the rope is C. 14 N West
Explanation:
We have two forces acting on the rope:
- The force applied by Jack, 82 N, towards East
- The force applied by Tony, 96 N, towards West
Taking East as positive direction, we can rewrite the two forces as

where the second force is negative since its direction is west.
The two forces act along the same line, so we can calculate their resultant by simply calculating their algebraic sum. Therefore:

And the negative sign tells us that the direction is West.
Learn more about forces:
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The time (t) it will take for the train to reach to Grenoble is the quotient when the distance (d) is divided by the speed (s). That is,
t = d / s
Substituting the known values,
t = 480 km / 180 km/h
t = 2.667 h
Thus, it will take approximately 2.7 h to reach Grenoble. The answer is letter D.