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March 27, 2017

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Can someone please check my answers for the first three questions and help me with the last one?

1)A racecar accelerates from rest at +7.2 m/s2 for 4.1 seconds. How fast will it be going at the end of that time?

Answer: 29.5 m/s

2)A racecar starts from rest and is accelerated uniformly to +41 m/s in 8.0 s. What will be the car's displacement?

Answer: 328 m

3)A rubber ball strikes a wall at a velocity of +35 km/hr and rebounds with a velocity of -15 km/hr. The change in velociy is:

Answer: +50 km/hr

I need some help with this last question...

A boat going 5 m/s eastward is crossing a river that flows southward at 1 m/s. Find the resultant velocity of the boat.

  • Physics - ,

    1) right

    2) v= a t
    41 = a (8)
    a = 5.125
    x = 0 + 0 + .5 (5.125)(8)^2
    x = 164 meters
    I think you maybe forgot the 1/2 in (1/2) a t^2

    3)Vfinal = -15
    Vinitial = 35
    Vfinal - Vinitial = -15 -35 = -50km/hr

    4) I assume you mean the boat is HEADING east, NOT GOING east. To go east the pilot must steer somewhat upstream and I do not think that is what the teacher probably means
    If it is HEADED east it is going 5 m/s east and 1 m/s south
    the tangent of the angle south of east is 1/5
    so the angle south of east is tan^-1 (.20)

    which is 11.3 degrees south of east is the course made good.

    On a nautical chart which calls north zero and east 90 it would be 90 +11.3 = 101 degrees True

  • Physics - ,

    Oh another way to do number two is to say if it accelerated uniformly from 0 to 41, the average speed during the eight seconds would be 20.5 for the eight seconds.
    20.5 m/s * 8 s = 164 meters

  • Physics - ,

    Oh and the magnitude of the velocity in #4 is sqrt (1^2 + 5^2)
    the hypotenuse

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