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logan3
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Homework Statement
A box of mass 72 kg is at rest on a horizontal frictionless surface. A constant horizontal force of magnitude F then acts on the box, accelerating it to the right. You observe that it takes the box 3.4 seconds to travel 13 meters. What is the magnitude of the force F?
m = 72kg
t = 3.4s
s = 13m
Homework Equations
v = s/t
a = v/t
F = ma
The Attempt at a Solution
My first attempt:
v = s/t = (13m) / (3.4s) = 3.8235m/s
a = v/t = (3.8235m/s) / (3.4s) = 1.1245m/s^2
F = ma = (72kg)(1.1245m/s^2) = 80.968N == 81N
My second attempt:
v_i = 0m/s
a = F/m
s = (v_i)t + 0.5 (F/m)t^2
=> F = 2sm / t^2 = (2(13m)(72kg)) / (3.4s)^2 = 161.93N == 162N
Which answer is correct and why is one method correct and the other not?
Thank-you
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