
Gabe M.
asked 11/24/17How would I find acceleration from distance and velocity?
My question is: a 99 kg water skier floating in a lake is pulled from rest to a speed of 11 m/s in a distance of 25 m. What is the net force exerted on the skier, assuming his acceleration is constant?
I obviously need to find acceleration first, so I can do f=ma. I did d=v0t, with 25=11t, which led me to 25/11=t. However, when I plugged this into f=ma as f=99(4.4), I got 435.6, which doesn't make sense contextually. What am I doing incorrectly?
Thanks in advance,
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1 Expert Answer

Arturo O. answered 11/24/17
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Since you are given the travel distance and the initial and final speeds, use the appropriate kinematic relation to find acceleration, and then get force from the mass and acceleration.
v22 - v12 = 2ad ⇒
a = (v22 - v12)/(2d)
v1 = 0 [started from rest]
v2 = 11 m/s
d = 25 m
a = (112 - 02)/[2(25)] m/s2 = 2.42 m/s2
F = ma = (99 kg)(2.42 m/s2) = 239.58 N
Gabe M.
When would I determine when to use this equation, v2^2 - v1^2 = 2ad, as opposed to .5(a)t^2?
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11/24/17

Arturo O.
It depends on what information is given in the problem. If time was given, you may be able to work with 0.5(a)t^2. But no time is given in this problem, only distance and the initial and final speeds. Based on this, you pick the kinematic equation that has these 3 quantities in it, plug in the numbers, and solve for the 4th quantity. Have you worked with the kinematic equations in your physics course?
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11/24/17
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Arturo O.
11/24/17