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Question

A flatcar of mass m0=20 kg is moving to the right due to a constant horizontal force F=10 N. Sand spills on the flatcar from a stationary hopper. The velocity of loading is constant and equal to μ=1 kg/s.



Find the acceleration of the flatcar after t=20 s.

A
18 m/s2
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B
140 m/s2
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C
0
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D
5 m/s2
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Solution

The correct option is A 18 m/s2
Given, F=10 N
For a variable mass system where mass is added at a rate μ,
mdvdt=Fext+μu
where v = velocity of car, m0 = mass of car =20 kg and μ=1 kg/s
Here, m=m0+μt after t seconds
and relative velocity of mass added u=v
(in horizontal direction, sand has zero velocity)
(m0+μt)dvdt=Fμv

Rearranging the terms,
m0dv+μtdv=Fdtμvdt
m0dv+μ(tdv+vdt)=Fdt
Intergrating v from 0 to v and t from 0 to t:
m0dv+μ(tdv+vdt)=Fdt
m0v+μd(vt)=Ft
m0v+μvt=Ft
or v=Ft(m0+μt)=10t20+t

Now, differentiating both sides
dvdt=(m0+μt)FFt(μ)(m0+μt)2
a=m0Fm20(1+μtm0)2=10m0(1+μtm0)2
a=1020(1+1×2020)2
a=18 m/s2 after t=20 s

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