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,

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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.

v

_{2}^{2}- v_{1}^{2}= 2ad ⇒a = (v

_{2}^{2}- v_{1}^{2})/(2d)v

_{1}= 0 [started from rest]v

_{2}= 11 m/sd = 25 m

a = (11

^{2}- 0^{2})/[2(25)] m/s^{2}= 2.42 m/s^{2}F = ma = (99 kg)(2.42 m/s

^{2}) = 239.58 NGabe 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.

_{0}t11/24/17