Gabe M.

asked • 11/24/17

How 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,

Arturo O.

The formula
 
d = v0t
 
is valid only if the speed is constant.  This problem has acceleration, so the speed is not constant.  See my solution below.
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11/24/17

1 Expert Answer

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Arturo O. answered • 11/24/17

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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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