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Acceleration physics

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1. A sprinter finishes the race with a velocity of 8.9 m/s. the sprinter accelerated to a stop at a rate of -2.7 m/s2. how long did it take the sprinter to come to a stop?

2. A cycllist starts from rest & reaches a velocity of 18 m/s southwest in 3.8 seconds. what was the cyclists acceleration?

3. a UFO is flying at a velocity of 45 m/s (E). if 5.9 seconds later it's velocity was 35 m/s (W) what was its acceleration? what assumption did you make?
4. a motorcycle & rider start from rest and reach a velocity of 50 km/h (E) in 2.7 seconds. what was the acceleration of the motorcycle in m/s2?
My answers
1. I don't get how to do it
2. 4.73 m/s2 (SW)
3. -1.69 m/s2 EW
4. 19.2 m/s2

I'm not sure i did these correctly so please help

  • Acceleration physics -

    Keep track of the units. We want time, which is (m/s) / (m/s^2)

    SO, divide the change in velocity by the acceleration used:

    -8.9 m/s divided by -2.7 m/s^2 = 3.30 s

    #2 ok

    The velocity change by 80 m/s in the W direction. This change took 5.9 seconds, so assuming constant acceleration, we have

    80/5.9 = 13.56 m/s W

    #4 the acceleration is

    50/2.7 km/hr/s = 18.518 km/hr/s
    But 1 km/hr = 1000/3600 m/s = 0.2777 m/s

    So, 18.518 km/hr/s = 5.14 m/s^2

  • Acceleration physics -

    OMG you r a life saver thank u so much! God bless thank u!!!!

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