20. The friction force that resists the motion between two surfaces that are
2 answers:
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42) The sailboat travels east with velocity 
, while the current moves south with speed 
. Since the two velocities are perpendicular to each other, he resultant velocity will be given by the Pytagorean theorem:
and the direction is in between the two original directions, therefore south-east. So, the correct answer is
D) 42 mph southeast
43) Since the light moves by uniform motion, we can calculate the distance corresponding to one light year by using the basic relationship between velocity, space and time. In fact, we know the velocity:
and the time is one year, corresponding to:
therefore, the distance corresponding to one light year is:
Therefore, the correct answer is D.
44) For the purpose of the problem, we can assume that the light travels instantaneously from the flash to us (because the distances involved are very small), so the time between the flash and the thunder corresponds to the time it took for the sound to travel to us.
The speed of sound is
And since the time between the flash and the thunder is t=3 s, the distance is
Therefore, the correct answer is A) 3/5 mile.
and the direction is in between the two original directions, therefore south-east. So, the correct answer is
D) 42 mph southeast
43) Since the light moves by uniform motion, we can calculate the distance corresponding to one light year by using the basic relationship between velocity, space and time. In fact, we know the velocity:
and the time is one year, corresponding to:
therefore, the distance corresponding to one light year is:
Therefore, the correct answer is D.
44) For the purpose of the problem, we can assume that the light travels instantaneously from the flash to us (because the distances involved are very small), so the time between the flash and the thunder corresponds to the time it took for the sound to travel to us.
The speed of sound is
And since the time between the flash and the thunder is t=3 s, the distance is
Therefore, the correct answer is A) 3/5 mile.